THE HUBBLE DEEP FIELD OBSERVATIONS

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关于太空的英文作文50字左右Space is a vast and mysterious realm that has captured the imagination of humankind for centuries. Its vastness is unfathomable, with countless stars, planets, and galaxies stretching out in all directions. The study of space, known as astronomy, has yielded a wealth of knowledge about the universe we inhabit.From the early observations of astronomers like Galileo and Copernicus to the modern marvels of space telescopes like Hubble, our understanding of space has undergone a dramatic transformation. We have learned about the life cycles of stars, the formation of planets, and the existence of black holes. Space exploration has also played a crucial role in expanding our knowledge. Human missions to the Moon, and robotic missions to other planets, have provided invaluable insights into the nature of our solar system.The vastness of space also presents numerouschallenges and opportunities. The harsh conditions of outer space, including extreme temperatures, radiation, and a lack of atmosphere, require innovative engineeringsolutions for space exploration. As we push the boundaries of human knowledge, the exploration of space will continue to yield new discoveries and inspire generations to come.The mysteries of space continue to fascinate and inspire us. From the Hubble Ultra Deep Field image, which revealed thousands of distant galaxies, to the discovery of exoplanets orbiting distant stars, space exploration has captured our imagination and fueled our curiosity about the universe we inhabit.As we continue to explore space and unravel its secrets, we not only expand our knowledge but also gain a deeper appreciation for the vastness and wonder of the cosmos.。

The Nature of Compact Galaxies in the Hubble Deep Field (II) Spectroscopic Properties and I

a rXiv:as tr o-ph/97401v11Apr1997The Nature of Compact Galaxies in the Hubble Deep Field —II:Spectroscopic Properties and Implications for the Evolution of the Star Formation Rate Density of the Universe.1,2,3Rafael Guzm´a n,Jes´u s Gallego,David C.Koo,Andrew C.Phillips,James D.Lowenthal 4,S.M.Faber,Garth D.Illingworth &Nicole P.Vogt University of California Observatories /Lick Observatory,Board of Studies in Astronomy and Astrophysics,University of California,Santa Cruz,CA 95064Received ................;accepted ................To be submitted to Astrophysical JournalWe present a spectroscopic study of51compactﬁeld galaxies with redshifts z<1.4 and apparent magnitudes I814<23.74in theﬂankingﬁelds of the Hubble Deep Field.These galaxies are compact in the sense that they have small apparent half-light radii(r1/2≤0.5 arcsec)and high surface brightnesses(µI814≤22.2mag arcsec−2).The spectra,taken at the Keck telescope,show emission lines in88%of our sample,and only absorption lines in the remaining12%.Emission-line proﬁles are roughly Gaussian with velocity widths that range from the measurement limit ofσ∼35km s−1to150km s−1.Rest-frame [OII]λ3727equivalent widths range from5˚A to94˚A,yielding star formation rates(SFR)of ∼0.1to14M⊙yr−1.The analysis of various line diagnostic diagrams reveals that∼60%of compact emission-line galaxies have velocity widths,excitations,Hβluminosities,SFRs, and mass-to-light ratios characteristic of young star-forming HII galaxies.The remaining 40%form a more heterogeneous class of evolved starbursts,similar to local starburst disk galaxies.Weﬁnd that,although the compact galaxies at z>0.7have similar SFRs per unit mass to those at z<0.7,they are on average∼10times more massive.Our sample implies a lower limit for the global comoving SFR density of∼0.004M⊙yr−1Mpc−3at z=0.55, and∼0.008M⊙yr−1Mpc−3at z=0.85(assuming Salpeter IMF,H0=50km s−1Mpc−1, and q0=0.5).These values,when compared to estimates for a sample of local compact galaxies selected in a similar fashion,support a history of the universe in which the SFR density declines by a factor∼10from z=1to today.From the comparison with the SFR densities derived for magnitude-limited samples ofﬁeld galaxies,we conclude that compact emission-line galaxies,though only∼20%of the generalﬁeld population,may contribute as much as∼45%to the global SFR of the universe at0.4<z<1.Subject headings:galaxies:formation—galaxies:compact—galaxies:evolution—galaxies:fundamental parameters—cosmology:observations1.IntroductionFaint compact galaxies are relevant to observational cosmology because they serve to constrain several proposed explanations of the abundance of faint blueﬁeld galaxies(see reviews by Koo and Kron1992;Lilly1993;Ellis1996).These include models with large populations of low-luminosity dwarfs at low redshifts(Phillipps&Driver1995);bursting dwarfs at z<1that have faded or disappeared by today(Cowie,Songaila&Hu1991; Babul&Ferguson1996);low-luminosity AGN’s(Tresse et al.1996);or smaller pre-merger components(Guiderdoni&Rocca-Volmerange1990;Broadhurst et al.1992;Kauﬀmann et al.1993;Cole et al.1994).Faint compact galaxies are also likely to include compact narrow emission-line galaxies(CNELGs),which are starbursts at moderate redshifts that have been proposed to be progenitors of today’s spheroidal galaxies like NGC205(Koo et al.1994,1995;Guzm´a n et al.1996).Given their likely starburst nature,faint compact galaxies may be major contributors to the global star formation rate(SFR)density already found to increase with lookback time to at least redshift z∼1(Cowie et al.1995,Lilly et al.1996).Most compact galaxies at moderate redshifts yield little morphological information, even in HST images.Their integrated spectra are thus particularly valuable in providing information on their galaxy type,kinematics,and stellar content,as well as the physical conditions of the ionized gas and star formation activity.In this pair of papers,we study the properties of a sample of51faint compact galaxies in theﬂankingﬁelds of the Hubble Deep Field(HDF;Williams et al.1996).The global properties of our sample are described by Phillips et al.(1997;hereafter Paper I),whoﬁnd that the majority of faint compact galaxies tends to have colors,luminosities,half-light radii,surface brightnesses,and mass-to-light ratios consistent with those of local vigorously star-forming galaxies.In this paper,we focus our analysis on their spectroscopic properties.The wide spectral range(4000-9000˚A)and good resolution(∼4˚A FWHM)of our survey oﬀer signiﬁcant advantages for a spectral analysis of compactﬁeld galaxies at intermediate redshifts.Most spectroscopic surveysof faint galaxies do not cover the entire optical wavelength range,hence emission-line studies have been restricted mainly to the[OII]λ3727doublet(e.g.,Broadhurst et al.1988; Colless et al.1990;Glazebrook et al.1995).At z<0.7,we can reliably measure not only[OII]λ3727but also[OIII]λ4959,[OIII]λ5007,and Hβ.Flux ratios among these lines provide powerful diagnostics to discriminate among diﬀerent classes of emission-line galaxies (Baldwin,Phillips&Terlevich1981;Veilleux&Osterbrock1987).For example,a recent study of line ratios forﬁeld galaxies at z<0.3suggests that at least8%are active galaxies such as Seyfert2or LINERS(Tresse et al.1996).A second major distinguishing feature of the present study is the inclusion of velocity width measurements from emission lines. Internal velocities,as inferred from the motions of the ionized gas,have proven to be very useful in understanding the nature of distantﬁeld galaxies and assessing their evolutionary state(Koo et al.1995;Forbes et al.1996;Guzm´a n et al.1996;Vogt et al.1996;Rix et al. 1997).Together with other spectroscopic parameters such as the excitation of the ionized gas,Hβluminosity,and current SFR,the new velocity widths improve discrimination among diﬀerent classes of emission-line galaxies and provide a more reliable comparison to potential local counterparts.This paper is organized as follows.In Section2we describe brieﬂy our sample selection and spectroscopic observations.In Section3we describe the emission-line measurements. In Section4we study the nature of the compact emission-line galaxies.In Section5we derive the comoving SFR density at0.4≤z≤1.0for our sample and compare our results to previous observations and model predictions.The main results of this paper are summarized in Section6.Throughout this paper we adopt H0=50km s−1Mpc−1and q0=0.05,unless otherwise stated.Given these parameters,L⋆(M B∼−21)corresponds to I814∼21and1′′spans9kpc at a redshift of z=0.7.This project is part of the DEEPprogram(Koo1995).2.Description of the sample and observationsThe galaxy sample was selected from I814HST images of theﬂankingﬁelds around the HDF(Williams et al.1996).A full description of the sample selection,spectroscopic observations,and measurements of photometric parameters is given in Paper I.Brieﬂy, these galaxies were chosen to have I814≤23.74,half-light radius r1/2≤0.5arcsec,and average surface brightness within the half-light radiusµI814≤22.2mag arcsec−2.Hereafter we refer to this sample as compact galaxies.Stellar-like objects with r1/2≤0.16arcsec were rejected.Spectra were obtained using the Low-Resolution Imaging Spectrograph(Oke et al.1995)at the W.M.Keck Telescope in UT April22-24,1996.The seeing was typically ∼0.8arcsec.With a slitwidth of1.1arcsec,a600l/mm grating yielded an instrumental resolution of∼4˚A FWHM at1.26˚A pix−1.However,since our objects have r1/2≤0.5 arcsec,the eﬀective resolution is estimated to be∼3.1˚A(see Paper I).Total exposure times were3000s at each of two gratings setups(blue and red).The total spectral range is∼4000-9000˚A,with the exact range depending on the position of the target on the mask.The spectroscopic reduction included the usual corrections for bias,dark current,ﬂatﬁeld,and cosmic rays as well as wavelength calibration and background sky subtraction. Noﬂux calibration was attempted.Theﬁnal one-dimensional spectra were produced by coadding the central6pixels(1.3arcsec)for each object.The analysis presented in this paper refers to the sub-sample of51compact galaxies with measured V−I colors and redshift identiﬁcations with z<2.We have also excluded one nearly stellar-like object (iw30556at z=0.960)with broad MgIIλ2795,2803emission lines(rest-frame FWHM ∼23˚A)similar to those found in low-luminosity QSOs or Seyfert galaxies(see Appendix to Paper I).Figure1showsﬁve representative spectra.Of the51compact galaxies,6(or12%)show absorption-line spectra characteristic of elliptical and S0galaxies(Figure1a).The major features displayed in these spectra are blended stellar absorption lines in the continuum dominated by K-giant stars,including the4000˚A Ca H+K break,G-band,Mgb and Na D features.Except for iw41391,which shows marginal emission in[OII]λ3727, there is no evidence for any nebular emission within the observed wavelength range.The remaining45galaxies(88%)exhibit prominent nebular oxygen and/or Balmer emission lines,and blue continua characteristic of vigorous star-forming systems or narrow-line active galactic nuclei.A large fraction have spectra that resemble those of star-forming HII regions(Figures1b and1c),showing narrow lines and a wide range in excitation as evidenced by the[OIII]λ5007/Hβﬂux ratio.Particularly interesting are the spectra of seven galaxies at z>0.7showing strong[OII]λ3727lines,and a strong continuum bluewards of [OII]λ3727with clear FeIIλ2600and MgIIλ2796absorption features(Figure1d).These features are characteristic of extreme local starburst galaxies such as NGC2415or NGC 5253,which are undergoing a very recent violent episode of star formation(Kinney et al. 1996).Two other galaxies,also at z>0.7,have similar blue continuum but show unusually narrow MgIIλ2796emission(restframe FWHM∼3.5˚A)with P-Cygni line proﬁles,as well as strong[OII]λ3727and[NeIII]λ3869,and weak narrow Hγand Hδemission lines(Figure 1e).Although the narrow lines argue against nuclear activity in these objects,we have not been able toﬁnd similar spectral characteristics in the UV spectra of local starbursts.A more quantitative analysis of the general spectroscopic properties of the emission-line compact galaxies is presented in Section4.3.Emission-line MeasurementsPaper I shows that the redshift distribution of compact galaxies parallels that of typical ﬁeld galaxies in the HDF and is roughly bimodal with peaks around z∼0.5and z∼0.9. We thus divide our objects into‘intermediate-z’(26emission-line galaxies at z<0.7)and ‘high-z’(19emission-line galaxies at z>0.7)samples.The main spectral features seenin the intermediate sample are:[OII]λ3726,3729,Hβ,[OIII]λ4959,and[OIII]λ5007.Four objects at z<0.4also exhibit other emission lines such as HeIλ5876,Hα,[NII]λ6548,6583, and[SII]λ6717,6731.For the high-z sample(z>0.7),the[OII]doublet is generallythe only major feature that can be reliably measured.The emission-line measurements described below refer to the strongest features most commonly observed in our spectra, i.e.,[OII]λ3726,3729,Hβ,and[OIII]λ5007(hereafter referred to as[OII],Hβand[OIII], respectively).3.1.Equivalent WidthsEquivalent widths(EW)were measured byﬁtting a Gaussian function to the emission-line proﬁles using the SPLOT program in IRAF5.The continuum levels and the range over which theﬁts were performed were set interactively,with repeat measurements made in diﬃcult cases.The eﬀective instrumental resolution of3.1˚A resolves the[OII]doublet in ∼25%of the spectra.In all cases,the standard deblending routine within SPLOT was used toﬁt both components.The FWHM for the two-Gaussianﬁt were forced to be equal,and the distance in wavelength between the two centroids wasﬁxed to the theoretical value(2.75×(1+z)˚A).All lines,whether double or single,were generally well-ﬁtted by Gaussian proﬁles.EW’s were measured from direct integration of theﬂux given by the Gaussianﬁt in the rest frame.No correction for stellar absorption was made to the measured EW of Hβ.This correction amounts typically to∼2-5˚A for HII galaxies and spiral galaxies(Tresse et al.1996;Kennicutt1992).For the[OII]doublet,we coadded theﬂux of each line to give a single measure of EW that can be directly compared to that commonly measured at lower spectral resolution.All EW measurements were derived independently using software designed by one of us(ACP),which directly integrated theﬂux in the paring both techniques,we estimate that the average uncertainty of our measurements is∼15%.The histograms of EW[OII]in the rest-frame for the intermediate and high-z samples are shown in Figure2a.These values range from5˚A to94˚A with an average of43˚A(rms=24˚A). Note that there is no signiﬁcant diﬀerence in the distribution of EW[OII]between both redshift ing the two-sided Kolmogorv-Smirnoﬀtest,the probability is∼82%for the intermediate-and high-z samples to be drawn from the same parent distribution.3.2.Velocity WidthsVelocity widths(σ)were characterized as the rms velocity dispersion of the Gaussian ﬁt to a given line with rest-frame wavelengthλi,corrected for redshift and instrumental resolution,i.e.:σi= 2.35λi(1+z)Most of the emission-lines in our spectra do not deviate signiﬁcantly from the Gaussian ﬁt.These proﬁles are consistent with those measured in high resolution,high signal-to-noise spectra of a similar sample of compact,narrow emission-line galaxies(Koo et al.1995; Guzm´a n et al.1996).We estimate that the lowestσi value that can be reliably measuredwith our instrumental resolution is∼35km s−1(at a90%conﬁdence level).Individual σi measurements were assigned a quality parameter Q related to the signal-to-noise ratio (SNR)per˚A of each line.We adopted Q=1for SNR≤20,Q=2for20<SNR<40,and Q=3for SNR≥40.A total of48objects have at least oneσi measurement with Q> 1.No signiﬁcant systematic diﬀerence amongσi measurements from various lines of the same object was found,although we note that the velocity widths derived from[OII]are ∼15%±8%higher than those values derived from[OIII]and Hβ.From the variance among diﬀerent line measurements for the same galaxy,the typical uncertainty of an individualσi measurement is∼20%.The quality code was used to derive aﬁnalσvalue as the weighted average of the values for all available emission lines given by the expression:σ= i Q iσi3.3.Excitation and HβluminositiesA useful indicator of the physical conditions of the ionized gas is the[OIII]λ5007to Hβﬂux ratio(so-called excitation).The proximity in wavelength between Hβand [OIII]ensures that any extinction correction in this ratio is small.Unfortunately,for some galaxies at z<0.7,one or both lines lie close to strong sky emission lines which prevented reliable measurements.In total,[OIII]λ5007/Hβcould be measured for only24galaxies, mostly at z≤0.7.For28galaxies we have also derived Hβluminosities from the measured rest-frame equivalent widths EW Hβand absolute B magnitudes(see Terlevich&Melnick 1981).In principle,these luminosities need to be corrected for internal extinction and stellar absorption.For local star-forming galaxies,these two corrections typically amount to∼0.8dex and∼0.1dex,respectively.Since their size is very uncertain for our sample,we will not apply any such corrections to the observed Hβluminosities of compact galaxies. Comparison with local samples will be made using only un-corrected values for the nearby galaxies.3.4.Star Formation RatesWe have estimated the SFR based on EW[OII].Metallic nebular lines like[OII]are aﬀected by the physical conditions of the ionized gas(e.g.,excitation and metallicity),and the transformation from SFR estimated this way to SFR from Hαﬂuxes(the best SFR tracer)is not straightforward.Previous studies for local emission-line galaxies by Gallagher et al.(1989)and Kennicutt(1992)give expressions for such transformations that diﬀer by a factor∼5.Gallagher et al.studied a sample of nearby blue irregulars,while Kennicutt used a sample of nearby galaxies covering all disk galaxy types.The disagreement between the two calibrations may reﬂect the diﬀerence in extinction and reddening between irregulars and spirals,and the diﬀerent IMF and stellar models used by Kennicutt and Gallagheret al.(Kennicutt1992).Other factors that may contribute to the observed diﬀerence are the possible stronger contribution of extended diﬀuse ionized gas in irregulars,or variations in the sampling of the disk for the two galaxy types.In order to estimate the SFR using EW[OII],we have derived our own transformation between Hαand[OII]ﬂuxes using a sample of local emission-line galaxies that best resembles the typical characteristics of our sample.The derivation of such transformation is described in detail in the Appendix.Theﬁnal expression to estimate the SFR as a function of the observed EW[OII]for compact galaxies is:SF R(M⊙yr−1)≈2.5×10−12×10−0.4(M B−M B⊙)EW[OII]This estimate is∼3times smaller than that derived by Kennicutt,and∼1.5times larger than that obtained by Gallagher et al.Absolute blue magnitudes for our sample are listed in Paper I.Figure2c shows the histograms of SFR for the intermediate-and high-z samples. While compact galaxies at z<0.7have SFRs<3M⊙yr−1,those at higher redshifts have SFRs that span a large range from2to14M⊙yr−1.Since the distribution in EW[OII]is very similar for both samples(Figure2a),the observed diﬀerence in SFRs mainly reﬂects the fact that we are selecting more luminous galaxies at higher redshifts.The median luminosities in the intermediate and high redshift samples are M B=-19.4and-21.0, respectively(see Paper I).In other words,the average SFR per unit luminosity is similar for the intermediate-and high-z samples.3.5.The DataA complete listing of the emission-line data is given in Table1.Column(1)lists the galaxy identiﬁcation.Columns(2)and(3)list the apparent I814magnitudes and redshifts given in Paper I.The rest-frame equivalent widths of[OII],Hβ,and[OIII]in˚A are listed in columns(4),(5)and(6),respectively.Column(7)lists the excitation.Velocity widths are listed in column(8)in km s−1.Hβluminosities in erg s−1,uncorrected for extinction, are listed in column(9).Star formation rates in M⊙yr−1are listed in column(10).Finally, in column(11)we list the spectral type assigned in Section4below.4.Spectroscopic Properties of Compact Emission-Line GalaxiesWe investigate the nature of the faint compact galaxies by comparing their spectroscopic properties in various diagnostic diagrams with diﬀerent types of emission-line galaxies. First,we focus the analysis on two well-known diagrams:excitation vs.luminosity,and Hβluminosity vs.velocity width.Together,these diagrams provide insight into the physical characteristics of the ionized gas(e.g.,metallicity and internal motions)as well as the strength of any starburst6.These plots,however,are useful only for compact galaxies at z<0.7,for which Hβand[OIII]lie within the observed wavelength range.For galaxies at higher redshift,[OII]is generally the only major feature that can be reliably measured. Since[OII]is a good tracer of the SFR,the EW[OII]vs.luminosity diagram providesa useful tool to study the star formation characteristics of both the intermediate-andhigh-z galaxy samples.We also introduce a new diagram:SFR per unit mass vs.mass. This plot discriminates among various types of star-forming galaxies based solely on their star formation activity,independently of their luminosity.Finally,we propose a broad classiﬁcation scheme of the compact galaxy sample based on this analysis.4.1.The Excitation vs.Luminosity DiagramThe spectroscopic properties of narrow emission-line galaxies are generally characterized using line-ratio diagnostic diagrams(Baldwin,Phillips&Terlevich1981;Veilleux& Osterbrock1987).Figure3shows the[OIII]/Hβvs.M B diagram for the28compact galaxies with measured[OIII]/Hβratio(squares).All but one(ﬁlled square)belong to the intermediate sample(z<0.7).Note that our magnitude limit prevents us from observing galaxies fainter than M B∼−17at z>0.4(see Paper I).For comparison,we also plotHα-selected emission-line galaxies from the UCM local survey(Gallego et al.1997)and a sample of compact narrow emission-line galaxies(CNELGs)at z=0.1−0.7studied by Koo et al.(1995)and Guzm´a n et al.(1996).In the[OIII]/Hβvs.M B diagram,local emission-line galaxies can be grouped into two diﬀerent classes:starburst galaxies and active galaxies(e.g.Salzer et al.1989).The ﬁrst class consists of objects where the gas is ionized by young O and B stars,and includes starburst nuclei(SBN),dwarf amorphous nuclear starbursts(DANS)and HII galaxies. The second class contains ionization sources harder than hot main sequence stars,such as Seyfert galaxies and LINERS.Local starburst galaxies deﬁne a continuous sequence in Figure3analogous to the so-called‘HII’sequence observed in the[OIII]/Hβvs.[NII]/Hαdiagram(Veilleux&Osterbrock1987).This sequence is interpreted as being a variation in the metallicity content of the ionized gas(Dopita&Evans1986;Stasinska1990).Along the HII sequence,metallicity increases with luminosity from the HII galaxies to the SBNs.Most of the compact galaxies in the intermediate sample lie in the moderate to high-excitation regime populated by local HII galaxies and moderate-z CNELGs(i.e., log[OIII]/Hβ>0.3).Direct comparison with Dopita&Evans(1986)models yields an average metallicity Z∼0.4Z⊙for compact galaxies in this excitation regime.This value is consistent with that derived from the luminosity–metallicity relation for local emission-line galaxies in the same range of luminosities(Salzer et al.1989).A few objects have low [OIII]/Hβratios consistent with more evolved star-forming systems such as local DANS and SBNs(hereafter called‘starburst disk galaxies’).The average metallicities for these objects are Z∼0.8Z⊙.From the analysis of[OIII]/Hβvs.M B,we conclude that emission-line compact objects at z<0.7are vigorously star-forming galaxies covering a broad range in metallicity.4.2.The HβLuminosity vs.Velocity Width DiagramFor star-forming galaxies with a dominant young population,theﬂux of the Balmer lines provide a reliable estimate of the age and the strength of the on-going burst(Dopita &Evans1986;Mas-Hesse&Kunth1991;Leitherer&Heckman1995).A useful diagram to study intrinsic diﬀerences in the evolutionary state of various types of star-forming galaxies is Hβluminosity vs.velocity width since the starburst properties can be compared among galaxies with similar internal kinematics,i.e.,independently of any luminosity evolution.Figure4shows the L Hβ−σdiagram for the28compact galaxies with reliable L Hβmeasurements.Only4of these objects have z>0.7(ﬁlled squares).For comparison, we also show the sample of local HII galaxies studied by Melnick,Terlevich&Moles(1988), as well as the sample of CNELGs withσ<70km s−1presented in Koo et al.(1995).Since L Hβmeasurements for both compact galaxies and CNELGs have not been corrected for internal extinction,we have decreased the corrected L Hβvalues of local HII galaxies by∼0.7dex,which corresponds to the average value of the extinction correction for luminous HII galaxies(Gallego et al.1997).We also plot a sample of local infrared-selected starburst disk galaxies studied by Lehnert&Heckman(1996).Hβluminosities were derived from extinction-corrected Hαluminosities,i.e.L Hβ=L Hα/2.86.These values were decreased by0.9dex to account for the average extinction correction applied to starburst disk galaxies(Gallego et al.1997).Velocity widths were derived from their rotational velocity measurements,assumingσ=0.426×2V rot/sin i.In the L Hβ−σdiagram,HII galaxies with EW Hβ>30˚A follow a well-deﬁned correlation:L Hβ∝σ5(Terlevich&Melnick1981).CNELGs also follow the same trend (Koo et al.1995),while starburst disk galaxies deﬁne a similar relation that is oﬀset towards lower L Hβby a factor of∼30at a givenσ.The distributions of local HII galaxies and starburst disk systems deﬁne the boundaries for the overall observed range in L Hβat any givenσof compact galaxies.The large observed spread in L Hβreﬂects variations in metallicity(Terlevich&Melnick1981),extinction(since no corrections have been applied), and strength of the current burst of star formation(since the luminosity of the Balmer lines scales directly with the SFR).Diﬀerences in the relative contribution of turbulent and virial motions to the velocity widths in our sample galaxies may also aﬀect their distribution in Figure4,although the eﬀect seems to be noticeable mainly in objects withσ>60km s−1 (Melnick,Terlevich&Moles1988).On average at a givenσ,low L Hβcompact galaxies tend to have∼50%lower excitation(i.e.,higher metallicity)and∼6times lower SFRs than their counterparts with high L Hβ.Roughly,half of the compact galaxies shown in this diagram have Hβluminosities and velocity widths similar to those of extreme star-forming HII galaxies,in agreement with the analysis of the line-ratio diagram discussed previously.This can be interpreted as the result of having average metallicity,extinction,and SFR consistent with those valuestypical for HII galaxies.The remaining have L Hβandσvalues indicative of being more evolved star-forming systems with metallicity,extinction,and SFR approaching values characteristic of local starburst disk galaxies.4.3.The[OII]Equivalent Width vs.Luminosity DiagramThe[OII]luminosity(L[OII])is a good tracer of the SFR(Gallagher et al.1989; Kennicutt1992).In the absence ofﬂux-calibrated spectra,L[OII]can be estimated using [OII]equivalent widths and blue luminosities(i.e.,L[OII]∼1029EW[OII]L B;see Appendix). The EW[OII]−M B diagram thus provides direct information on the star formation activity. Figure5shows the EW[OII]−M B diagram for the whole sample of distant compact objects, as well as for a representative sample of local emission-line galaxies(Salzer et al.1989; Gallego et al.1997).The[OII]emission lines of compact galaxies are remarkably strong (equivalent widths of∼60˚A in the integrated spectrum),given that these are luminous galaxies with absolute B magnitudes of about−20.In agreement with the results derived in the previous sections,compact galaxies in the intermediate-z sample show[OII]equivalent widths and blue luminosities consistent with those values characteristic of local HII and starburst disk galaxies.In particular,compact galaxies with high excitation and high L Hβat a givenσtend to have large EW[OII]similar to HII galaxies.High-z compacts also show [OII]equivalent widths and blue luminosities that overlap the observed distribution for local vigorously star-forming galaxies.However,although high-z compacts exhibit a similar range in EW[OII]to that of compacts at intermediate-z,they are∼1-2mag brighter.This implies that,on average at the same equivalent width,compact galaxies at z>0.7have higher[OII]luminosities by a factor∼10than those at z<0.7,which in turn translates into their having∼10times higher SFR.The increase in the[OII]luminosity with redshift suggests an enhancement of theSFR in compact galaxies at higher redshifts.To assess whether this enhancement implies a signiﬁcant evolution in the star formation activity of compact galaxies,it is necessary to take into account the selection eﬀects at play in our sample.Because of the cutoﬀs in r1/2, I814andµI814applied,the global galaxy properties of our sample are strongly correlated with redshift,and also among each other.This is clearly shown in Figure6,where we plot the distribution of L[OII](or SFR)as a function of surface brightness for the intermediate-and high-z samples.Superimposed on the data points,we plot the approximate limits of the observable window deﬁned by our selection eﬀects at z∼0.55(i.e.,1.3<R e<4kpc, M B<−18.25,and SB e<22.0),and at z∼0.85(i.e.,1.6<R e<5kpc,M B<−19.25, and SB e<21.2).These are,in fact,the same boundaries described in Paper I using the M B−SB e diagram.Figure6demonstrates that selection eﬀects can account for the lack of compact galaxies with low L[OII]in the high-z sample.However,they cannot explain why compacts with L[OII]≥1041erg s−1are so rare in the intermediate-z sample.The shaded region deﬁned by the intersection of both observable windows represents the area of the parameter space available to our sample galaxies in the redshift range0.4<z<1, approximately.If the apparent lack of high L[OII]systems in the intermediate-z sample were due to selection eﬀects,then high-and intermediate-z compacts within the shaded region would show a similar distribution.However,the segregation in L[OII]between the two samples still remains.Note that the volumes mapped at0.4<z<0.7and0.7<z<1 are comparable(i.e.,1.6×104Mpc3and2.3×104Mpc3,respectively),so a volume-richness eﬀect is not present.The sparsity of high L[OII]compacts in the intermediate-z sample,as compared to the numbers observed at z>0.7,points towards a steep evolution of the star formation activity in compact galaxies with redshift.Given the small number of objects involved,however,it is diﬃcult to demonstrate with our sample that signiﬁcant evolution has actually occurred.Previous surveys(Glazebrook et al.1995;Cowie et al.1995)have pointed to the。

太空中做的实验英语作文

As a high school student with a keen interest in science, Ive always been fascinated by the concept of space and the experiments conducted there. The idea of pushing the boundaries of our understanding beyond the confines of Earth is thrilling. I remember the first time I learned about the International Space Station ISS and the myriad of scientific research taking place there. It was a moment that sparked my curiosity and inspired me to delve deeper into the subject.One of the most intriguing aspects of space experiments is the unique environment that space provides. The microgravity conditions, for instance, offer scientists a chance to study phenomena that are impossible to replicate on Earth. I recall reading about an experiment involving the behavior of fluids in space. On Earth, fluids are influenced by gravity, which pulls them downwards. But in space, without gravitys pull, they form into perfect spheres, a sight that is both mesmerizing and scientifically enlightening.Another captivating experiment that caught my attention was the study of plant growth in space. I learned that astronauts on the ISS have successfully grown lettuce and other plants, which is a significant step towards sustainable living in space. The process is not without its challenges, as the plants must be carefully monitored and nurtured in an environment devoid of natural sunlight and soil. This experiment taught me about the resilience of life and the potential for humans to adapt and thrive in the most unlikely of places.Space experiments also play a crucial role in understanding the effects oflongterm space travel on the human body. I remember watching a documentary that discussed how astronauts bones and muscles can deteriorate in the absence of gravity. This led to experiments on the ISS involving exercise regimens and the use of specialized equipment to counteract these effects. The findings from these studies are not only vital for future space missions but also have applications in improving health and rehabilitation on Earth.The study of space also opens up new avenues for technological advancements. I was particularly struck by the story of how a simple experiment with a spinning top in space led to the development of a new type of gyroscope. This is a perfect example of how seemingly small experiments can have farreaching implications for technology and our daily lives.Moreover, space experiments have a profound impact on our understanding of the universe. The Hubble Space Telescope, for instance, has allowed us to gaze into the depths of space and time, revealing the birth and death of stars, and the expansion of the universe. These observations have not only expanded our knowledge but also challenged our understanding of the cosmos.What I find most inspiring about space experiments is the collaborative spirit they embody. Scientists from around the world work together on the ISS, sharing knowledge and resources to push the frontiers of science. This spirit of cooperation is a testament to our collective desire to learn and grow, transcending national boundaries and political differences.In conclusion, the experiments conducted in space are not just about scientific discovery they are about our collective human endeavor to explore the unknown. They inspire us to dream big, to reach for the stars, and to never stop questioning and learning. As a high school student, I am excited about the future of space exploration and the limitless possibilities it holds for our understanding of the universe and ourselves.。

哈勃深空场

哈勃深空场
哈勃深空视场（Hubble Deep Field, HDF）是一张由哈勃空间望远镜所拍摄的小区域夜空影像。

拍摄位置在大熊座，影像的范围仅144弧秒，等于是100米外的一颗网球。

由于拍摄目标太暗淡，整张影像由342次曝光叠加而成，拍摄时间是1995年12月18日至12月28日。

此后，哈勃分别于2003-2004年和2012年9月25日再次对该区域更小视场和更深区域进行了拍摄，分别获得了哈勃超级深场（HUDF）和哈勃极端深场（XDF）两张太空影像。

“哈勃深场”揭示了数以千计的前所未见的星系，其中一些星系的光可追溯到早期宇宙。

又称哈勃深空，所包含的区域几乎没有银河系中的恒星，可见的3,000多个物体全部都是遥远的星系，其中更包含了所知最早、以及最遥远的星系。

这张前所未有的影像被引用于400多篇科学论文，对于宇宙学研究宇宙起源有很大的帮助。

哈勃深空观测三年之后，哈勃空间望远镜于南天的杜鹃座再度以同样的方式拍摄了哈勃南天深空的影像。

两张影像的雷同之处，使天文学家更加坚定地相信宇宙的星系散布并非是紊乱的，而有统一的构造。

2004年，再度拍摄哈勃超深空影像，这是人类以可见光观察宇宙得到最远的影像。

Hubble Deep Field Image - Physics & Astronomy哈勃深场影像物理学和天文学;

A second HDF
Named Hubble Deep Field South (HDF-
S); very similar to original HDF, now named Hubble Deep Field North (HDFNorth). Same criteria, only now in Southern Hemisphere, within the Tucana constellation. Taken over in 1998 over 10 days.
Hubble Deep Field
Daniel Hazard
Background
The Hubble Deep Field (HDF) is a composite
picture of 342 different images. The HDF covers an area of 144 arc seconds. Taken by the Hubble Space Telescope (HST) over a period of 10 consecutive days in December of 1995. Taken by the Wide Field and Planetary Camera 2. Total exposure time was more than 100 hours.
… HDF within the sky
Describing HDF
As a result of the field’s size, it is thought to
display less than 10 stars from our galaxy. Most of the objects seen are distant galaxies--nearly 3,000 of them. There are also a number of bluish objects and regions. They might be:

The Hubble Deep Field

2
1. Introduction
The Hubble Deep Field (Williams et al. 1996) a ords us an unprecedented view of the optical sky at small angular scales and faint ux levels. It thus allows us to study faint (and presumably high redshift) galaxies without the complicating e ects of eld crowding that a ect ground-based faint galaxy studies. Preliminary results (Giavalisco et al. 1996) show that source counts in the HDF continue to rise as a power law to the completeness limit of the data. Such an e ect may be due to ever larger numbers of galaxies at fainter ux levels. However, it may also be due to the increasingly clumpy appearance of galaxies at high redshift, which can confuse source detection algorithms into counting several faint sources for each physically distinct galaxy. We begin by considering how redshift e ects can lead to over-counting of sources in a deep eld like the HDF. Two mechanisms may be at work. First, the high redshift of most of the objects moves the ultraviolet rest-frame light into the observed visible bands, which has commonly been demonstrated to reveal a more knotty, fragmentary appearance than in the rest-frame optical (Schade 1995). Second, the classic (1 + z)4 dimming of bolometric surface brightness for di use objects is less important for compact sources, which will enhance the relative brightness of such sources. Young, actively star-forming regions are both UV bright and compact, and thus will appear prominently in a very deep eld. Several giant HII regions may occur in a single galaxy, and lead to over-counting of sources within the eld. In the later sections of the paper, we test the extent to which HDF source counts re ect the subdivision of galaxies. Several di erent tests are possible. These tests exploit the weak dependence of angular size on redshift z at z > 1 to compare the physical separations and sizes of objects in the HDF to those of galaxies in the low-redshift universe. Particularly, Peebles (1993) demonstrates that galaxies of 10kpc physical size would subtend a minimum angle of about 1:600 for = 0:2, or 2:400 for = 1 cosmologies (nearly independent of ). The minimum value occurs at a redshift between 1 and 2, after which the angular size increases only slowly (the increase is insigni cant for the redshifts we're interested in, z < 4). In section 4, we discuss the two-point angular correlation function w( ) of HDF sources. Considerable correlations may be expected for physical scales < 10kpc if many galaxies in the eld break up into multiple giant HII regions in the source catalogs. This test is complicated by the presence of spurious source detections at the edges of nearby galaxies in simple isophotal threshold source catalogs. We discuss this issue in section 3 below and describe our chosen solution, which was to create our own source catalogs using the dao nd algorithm, which we empirically nd to yield fewer spurious sources. In section 5, we discuss the magnitude-radius and number-radius relations for HDF

遨游太空英语作文

遨游太空英语作文Exploring the Wonders of SpaceThe vastness of the universe has long captured the imagination of humanity, inspiring dreams of exploring the unknown and uncovering the mysteries that lie beyond our planet. As we gaze up at the night sky, the stars and galaxies that stretch out before us serve as a constant reminder of the incredible scale and complexity of the cosmos. It is this innate curiosity and desire to understand our place in the grand scheme of things that has driven us to venture into the realm of space exploration.The journey to the stars began with the earliest human observations of the celestial bodies, as our ancestors sought to make sense of the rhythms and patterns of the heavens. From the ancient astronomers of Mesopotamia and China to the revolutionary thinkers of the Renaissance, each generation has built upon the knowledge and discoveries of those who came before, gradually piecing together a more comprehensive understanding of the universe.The 20th century marked a pivotal turning point in the history of space exploration, with the launch of the first artificial satellite,Sputnik 1, by the Soviet Union in 1957. This momentous event sparked a fierce competition between the United States and the Soviet Union, known as the Space Race, as the two superpowers vied for technological supremacy and the prestige of being the first to achieve various milestones in space exploration.The culmination of this rivalry came in 1969, when Neil Armstrong and Buzz Aldrin became the first humans to set foot on the lunar surface, as part of the Apollo 11 mission. This historic achievement not only captivated the world but also demonstrated the incredible power of human ingenuity and determination. The Apollo program, which ultimately sent 12 astronauts to the Moon, remains one of the most remarkable accomplishments in the history of human exploration.Since those pioneering days, the scope and scale of space exploration have continued to expand, with numerous robotic probes and spacecraft venturing to the furthest reaches of our solar system and beyond. The launch of the Hubble Space Telescope in 1990 has provided us with unprecedented views of the cosmos, allowing us to peer deeper into the past and gain a better understanding of the origins and evolution of the universe.The International Space Station, a collaborative effort between multiple nations, has served as a hub for scientific research andtechnological development, furthering our understanding of the effects of microgravity and the unique challenges of living and working in the vacuum of space. Astronauts who have spent extended periods aboard the ISS have contributed invaluable data and insights, paving the way for future long-duration missions to the Moon, Mars, and beyond.As we look to the future, the prospects for space exploration are truly exciting. The rise of private space companies, such as SpaceX and Blue Origin, has ushered in a new era of innovation and competition, driving down the costs of space travel and opening up new avenues for exploration. The development of reusable launch vehicles and advanced propulsion systems promises to make space more accessible, allowing for more frequent and ambitious missions.The exploration of Mars has become a primary focus for many space agencies, with plans to establish a permanent human presence on the Red Planet in the coming decades. The potential to uncover evidence of past or present life on Mars, as well as to better understand the planet's geology and climate, has captured the imagination of scientists and the public alike.Beyond our own solar system, the search for habitable exoplanets –planets orbiting other stars – has become a key priority, as we seek to understand the diversity of planetary systems and the potentialfor life to exist elsewhere in the universe. The deployment of powerful telescopes, both on Earth and in space, has already yielded numerous discoveries of exoplanets, fueling our curiosity and driving us to continue exploring the cosmos.As we venture further into the unknown, the challenges we face are both daunting and exhilarating. The harsh environments of space, the need for reliable and efficient life support systems, and the logistical complexities of long-duration missions all require innovative solutions and a deep understanding of the physical and biological processes at work.Yet, it is precisely these challenges that make space exploration so compelling and rewarding. The pursuit of knowledge and the drive to push the boundaries of human experience have been the driving forces behind some of our greatest achievements, from the first steps on the Moon to the robotic exploration of distant worlds.As we continue to explore the vastness of space, we are not only expanding our understanding of the universe but also gaining new perspectives on our own planet and our place within it. The stunning images and data gathered by spacecraft have revealed the fragility and interconnectedness of our home, inspiring a renewed sense of stewardship and a deeper appreciation for the wonders of our world.Moreover, the benefits of space exploration extend far beyond the realm of scientific discovery. The technological innovations and engineering breakthroughs that arise from space programs have had profound impacts on our daily lives, from advancements in communication and navigation to medical breakthroughs and environmental monitoring.As we look to the future, the promise of space exploration is both exciting and daunting. The potential to uncover the origins of life, to establish a permanent human presence beyond Earth, and to unlock the secrets of the universe is a tantalizing prospect that captures the imagination of people around the world. At the same time, the challenges we face, from the logistical and financial hurdles to the ethical and philosophical questions that arise, require careful consideration and a commitment to responsible and sustainable exploration.Ultimately, the pursuit of space exploration is a testament to the boundless curiosity and ingenuity of the human spirit. It is a journey of discovery that has the power to inspire generations, to broaden our understanding of the cosmos, and to forge a deeper connection between our planet and the vast, unknown reaches of the universe. As we continue to push the boundaries of what is possible, we can be certain that the wonders of space will continue to captivate andinspire us, guiding us towards a future that is both exciting and profound.。

科学探索宇宙的英语作文

科学探索宇宙的英语作文Exploring the Universe through Science。

The universe has always been a subject of fascination for humanity. For centuries, we have gazed up at the night sky, wondering about the mysteries that lie beyond our planet. It is through the lens of science that we have been able to embark on a journey of exploration, seeking to understand the vastness and complexity of the universe. In this essay, we will delve into the various scientific methods and technologies that have enabled us to explore the universe and the significant discoveries made along the way.One of the most important tools in our quest to understand the universe is the telescope. The invention of the telescope in the 17th century revolutionized our understanding of the cosmos. It allowed us to observe celestial objects in greater detail and to make groundbreaking discoveries. Galileo Galilei, a renownedastronomer, was one of the first to use a telescope to observe the night sky. He discovered the four largest moons of Jupiter, providing evidence that not everything in the universe revolves around the Earth. This observation challenged the prevailing geocentric model and paved theway for a new understanding of the universe.Since Galileo's time, telescopes have evolved significantly. The Hubble Space Telescope, launched in 1990, has provided us with breathtaking images of distantgalaxies and nebulae. It has allowed astronomers to studythe universe in unprecedented detail and has contributed to numerous scientific breakthroughs. For example,observations made by the Hubble Space Telescope led to the discovery of dark energy, a mysterious force that iscausing the expansion of the universe to accelerate. This finding has had a profound impact on our understanding ofthe universe's fate.In addition to telescopes, another important scientific tool in exploring the universe is the spacecraft. Space missions have allowed us to venture beyond Earth'satmosphere and explore other celestial bodies. The Apollo missions, which landed humans on the moon, were a landmark achievement in space exploration. They provided us with valuable insights into the moon's geology and its formation. Moreover, unmanned spacecraft, such as Voyager 1 and Voyager 2, have traveled beyond our solar system, providing us with data about the outer planets and their moons. These missions have expanded our knowledge of the solar systemand have sparked further questions about the possibility of life beyond Earth.Furthermore, the field of astrophysics has played a crucial role in our exploration of the universe. Astrophysics combines the principles of physics and astronomy to study the properties and behavior of celestial objects. Through the use of mathematical models and computer simulations, astrophysicists have been able to unravel the mysteries of the universe. For instance, the discovery of black holes, which are regions of spacetimewith extremely strong gravitational forces, has revolutionized our understanding of the cosmos. Astrophysicists have studied the behavior of matter andenergy around black holes, providing us with valuable insights into the nature of space and time.In recent years, advancements in technology have allowed us to explore the universe in even greater detail. The development of powerful telescopes, such as the James Webb Space Telescope, promises to reveal new insights into the formation of galaxies and the origins of stars. Additionally, advancements in space exploration technology, such as reusable rockets, have made space missions more accessible and cost-effective. These advancements are paving the way for future discoveries and expanding our understanding of the universe.In conclusion, science has been instrumental in our exploration of the universe. Through the use of telescopes, spacecraft, and the field of astrophysics, we have made significant discoveries and expanded our understanding of the cosmos. As technology continues to advance, we can expect even greater breakthroughs in the future. The quest to explore the universe is an ongoing journey, and sciencewill continue to be our guide as we unravel the mysteries of the cosmos.。

探索太空英语作文模板范文

探索太空英语作文模板范文英文回答：Exploring the Cosmos: An Enduring Human Endeavor。

Space exploration, an audacious undertaking that has captivated the human imagination for centuries, has profoundly expanded our understanding of the universe and our place within it. Since the dawn of humankind, we have gazed up at the night sky, pondering the mysteries that lie beyond our earthly realm.Early civilizations left behind celestial calendars, such as Stonehenge in England and the Maya Pyramids in Mexico, which attest to their keen observations of the stars and planets. In the 16th century, Nicolaus Copernicus revolutionized our understanding of the cosmos with his heliocentric theory, placing the sun, not the Earth, at the center of our solar system. This breakthrough paved the way for the era of modern astronomy.In the 20th century, the advent of rocket propulsion technology set the stage for a new chapter in space exploration. On October 4, 1957, the Soviet Union launched Sputnik, the first artificial satellite to orbit Earth. This historic event marked the beginning of the space race between the United States and the Soviet Union, which spurred rapid advancements in rocketry, satellite technology, and manned spaceflight.The United States played a pivotal role in this cosmic competition, achieving significant milestones such as the first manned moon landing in 1969, a feat accomplished by Neil Armstrong and Buzz Aldrin. The Apollo program, which put humans on the lunar surface, provided invaluable scientific data and inspired generations of young minds to pursue careers in science and engineering.In recent years, space exploration has become a truly global endeavor, with numerous countries and international organizations collaborating on ambitious missions. The Hubble Space Telescope, launched in 1990, hasrevolutionized our understanding of the universe by providing breathtaking images of distant galaxies and celestial phenomena. The International Space Station, a joint project involving the United States, Russia, Europe, Japan, and Canada, serves as a permanent human outpost in low Earth orbit, conducting groundbreaking research in microgravity and astrophysics.Today, space exploration continues to be a driving force behind technological innovation and scientific discovery. Missions such as the Cassini-Huygens probe, which explored Saturn and its moons, and the New Horizons probe, which flew by Pluto and its moon Charon, have provided unprecedented insights into the outer reaches of our solar system.Private companies are also playing an increasingly significant role in space exploration. SpaceX, founded by Elon Musk, has developed reusable rockets that have significantly reduced the cost of accessing space. Blue Origin, another private spaceflight company, is focused on developing reusable spacecraft for suborbital tourism andcommercial satellite launches.The future of space exploration holds endless possibilities. Plans are underway to send humans back to the moon and eventually to Mars, where we may establish permanent settlements. The exploration of other planets and moons, such as Jupiter's moon Europa and Saturn's moon Enceladus, which are believed to possess subsurface oceans, may yield tantalizing discoveries about the potential for life beyond Earth.Space exploration is not merely a scientific pursuit; it is a profound human endeavor that transcends national boundaries and inspires us to dream big. It represents our insatiable curiosity, our boundless ingenuity, and our eternal quest for knowledge and understanding of the vast cosmos that surrounds us.中文回答：探索太空，一项永恒的人类事业。

你校将以四月十二日世界航天日英语作文

你校将以四月十二日世界航天日英语作文全文共3篇示例，供读者参考篇1The Infinite Frontier: Celebrating World Space DayAs students of the modern age, we stand at the precipice of an era defined by unprecedented technological advancements and an insatiable thirst for exploration. It is with great pride and excitement that our school commemorates World Space Day on April 12th, a celebration that transcends mere boundaries and unites us in our shared fascination with the vast expanse of the cosmos.From the moment our ancestors first gazed up at the twinkling stars, the allure of the heavens has captivated the human spirit. Today, we find ourselves living in an era where that primal curiosity has blossomed into a vibrant reality, with humanity's footprints etched upon the lunar surface and our robotic emissaries venturing ever deeper into the unknown.On this momentous occasion, we pause to reflect on the remarkable journey that has brought us to this point. The history of space exploration is a tapestry woven with tales ofunparalleled bravery, unwavering determination, and groundbreaking scientific discoveries. From the pioneering work of visionaries like Galileo Galilei and Isaac Newton, whose theories laid the foundation for our understanding of the cosmos, to the audacious dreamers who dared to reach for the stars, each chapter in this epic saga has propelled us forward, fueling our collective drive to unlock the secrets of the universe.As we gaze upon the night sky, our minds are transported to the pivotal moments that have shaped our understanding of the cosmos. We remember the awe-inspiring sight of Neil Armstrong taking that first small step onto the lunar surface, forever etching humanity's mark on another celestial body. We recall the breathtaking images beamed back from the Hubble Space Telescope, revealing the intricate beauty and complexity of distant galaxies. And we marvel at the astonishing achievements of robotic explorers like the Curiosity rover, which has unveiled invaluable insights into the enigmatic landscapes of Mars.Yet, as we bask in the glory of these accomplishments, we must also acknowledge that space exploration is not merely a pursuit of scientific knowledge but a testament to the indomitable spirit of human endeavor. It serves as a powerfulreminder of what we can achieve when we set our sights on the seemingly impossible and harness the collective genius of our species.The exploration of space has transcended borders, uniting nations in a shared quest to unravel the mysteries of the cosmos. It has fostered international cooperation and inspired generations of young minds to pursue careers in science, technology, engineering, and mathematics (STEM). The profound impact of space exploration extends far beyond the confines of the Earth's atmosphere, influencing virtually every aspect of our lives, from advanced communication systems to cutting-edge medical technologies.As students, we are the torchbearers of this incredible legacy, inheriting the mantle of a generation that dared to dream and challenged the boundaries of human potential. It is our responsibility to carry forth this spirit of exploration, to nurture the flames of curiosity that burn within us, and to push the frontiers of knowledge ever further.On World Space Day, we celebrate not only the achievements of the past but also the boundless possibilities that lie ahead. As we stand in awe of the celestial wonders that adorn the night sky, we are reminded of the vast expanse thatawaits our exploration. Perhaps one day, our footsteps will grace the rusty dunes of Mars, or our robotic ambassadors will venture beyond the confines of our solar system, unlocking the secrets of exoplanets and unveiling the enigmas that have captivated humanity for eons.This is a call to action, a rallying cry for the dreamers, the visionaries, and the pioneers of tomorrow. Let us embrace the spirit of exploration that has guided humanity thus far, and let us pledge to push the boundaries of what is possible. For in the vast expanse of the cosmos, there lie infinite frontiers waiting to be conquered, mysteries yearning to be unraveled, and wonders beyond our wildest imaginations.Today, as we honor World Space Day, let us raise our gaze to the heavens and allow our minds to soar among the stars. Let us revel in the accomplishments of those who came before us while casting our sights towards the boundless horizons that beckon. For it is in the pursuit of knowledge, in the relentless quest to unlock the secrets of the universe, that we truly embody the essence of what it means to be human.The cosmos is our canvas, and we are the artists, painting the narrative of humanity's journey through the vast expanse of space. Let us embrace this opportunity to leave an indelible mark,to inspire generations to come, and to forge a legacy that will echo throughout the ages.篇2The Infinite Frontier: Celebrating World Space DayAs students of the 21st century, we stand at the precipice of a new era of exploration and discovery. The vast expanse of space, once an unyielding mystery, has become our canvas upon which we paint the boldest dreams of humanity. On April 12th, we celebrate World Space Day, a date that carries profound significance for all those who dare to gaze upwards and ponder the infinite wonders that lie beyond our terrestrial confines.From the moment our ancestors first glimpsed the twinkling lights in the night sky, an insatiable curiosity has burned within the human spirit. What secrets do those celestial bodies hold? What lies beyond the veil of our atmosphere? These questions have ignited the imaginations of generations, fueling a relentless pursuit of knowledge that has propelled our species to remarkable heights.The journey into the cosmos began with baby steps, as our primitive understanding of the universe gradually gave way to scientific breakthroughs. From Galileo's pioneering use of thetelescope to Newton's laws of motion, each revelation ushered in a paradigm shift, redefining our perception of the cosmos. Yet, it was the dawn of the Space Age that truly shattered the boundaries of what we believed possible.On that fateful day in 1961, when Yuri Gagarin became the first human to venture into space, the world watched in awe as he orbited our planet, defying the shackles of gravity. This momentous achievement not only symbolized the triumph of human ingenuity but also ignited a fervent race to conquer the final frontier. Nations rallied their resources, pouring their collective determination into reaching for the stars, and the ensuing decades witnessed a flurry of milestones etched into the annals of space exploration.From the awe-inspiring footprints left on the lunar surface by Neil Armstrong and Buzz Aldrin to the breathtaking images captured by the Hubble Space Telescope, each achievement has broadened our horizons and deepened our reverence for the cosmos. We have witnessed the birth of stars and the death throes of ancient galaxies, unraveling the intricate tapestry of our universe thread by thread.Yet, our journey into the unknown is far from over. As we celebrate World Space Day, we are reminded of the boundlesspotential that lies ahead. The exploration of Mars, once a distant dream, is now within our grasp, with robotic emissaries paving the way for future human expeditions. The uncharted mysteries of exoplanets beckon, tantalizing us with the possibility of discovering life beyond our solar system.Moreover, the realm of space has transcended mere scientific curiosity, becoming an integral part of our daily lives. Satellite technology has revolutionized communication, navigation, and our understanding of our planet's intricate systems. Space-based observations have unveiled the intricate dynamics of climate change, prompting us to take action and safeguard our fragile world.As we stand in awe of these achievements, we must also acknowledge the collaborative spirit that has propelled us forward. Space exploration has served as a unifying force, transcending borders and ideologies, bringing nations together in pursuit of a common goal. It has ignited the imaginations of countless young minds, inspiring them to pursue careers in science, technology, engineering, and mathematics, shaping the innovators and explorers of tomorrow.On this World Space Day, let us pause and reflect on the incredible journey we have undertaken thus far. Let us honor thepioneers who dared to dream and the visionaries who turned those dreams into reality. But let us also look ahead, for the infinite expanse of space holds secrets yet to be uncovered, challenges yet to be surmounted, and wonders yet to be revealed.To the young minds of today, the baton is passed to you. Embrace the boundless curiosity that resides within you, for it is the fuel that will propel humanity towards new frontiers. Dare to dream of distant worlds, to unravel the mysteries of black holes, and to push the boundaries of what we thought possible. The cosmos beckons, and the limitless potential of space exploration awaits your inquisitive minds and unwavering determination.As we celebrate this momentous occasion, let us raise our eyes to the heavens and revel in the magnificence of the universe that surrounds us. For in the vast tapestry of stars and galaxies, we find not only the answers to our deepest questions but also a humbling reminder of our place in the grand scheme of existence.The infinite frontier lies before us, and with each step we take, we etch our names into the cosmic tapestry, forever intertwined with the boundless wonders of space. On World Space Day, let us embrace the spirit of exploration, for it isthrough this unwavering pursuit that we unlock the secrets of the universe and fulfill our destiny as custodians of knowledge, pioneers of discovery, and ambassadors of the infinite.篇3A Voyage Among the Stars: Celebrating World Space DayAs I gaze up at the twinkling celestial tapestry that adorns the night sky, my mind is consumed by a profound sense of wonder and curiosity. The vast expanse of the cosmos, once an impenetrable enigma, has gradually unveiled its secrets to the relentless pursuit of human ingenuity and exploration. It is with great anticipation that our school eagerly awaits the arrival of World Space Day on April 12th, a momentous occasion that commemorates the triumphs and aspirations of space exploration.From the moment our ancestors first cast their gaze upon the heavens, a primal yearning to unravel the mysteries of the universe has been etched into the human psyche. The ancients, while lacking the technological advancements of today, wove intricate mythologies and cosmologies in an attempt to make sense of the celestial bodies that danced across the night sky. Little did they know that their celestial musings would one daypave the way for the extraordinary feats of space exploration that have reshaped our understanding of the cosmos.It was not until the 20th century that humanity's dreams of venturing into the great unknown began to crystallize. The pioneering work of visionaries like Konstantin Tsiolkovsky, Robert Goddard, and Wernher von Braun laid the foundations for the development of rocket technology, propelling us into the realms of space travel. The launch of Sputnik 1 in 1957 marked a watershed moment, igniting the Space Race between the United States and the Soviet Union, and forever altering the course of human history.Yet, it was the Apollo program that truly captured the world's imagination and solidified our place among the stars. On July 20, 1969, the world held its collective breath as Neil Armstrong became the first human to set foot on the lunar surface, uttering those immortal words: "That's one small step for man, one giant leap for mankind." This momentous achievement not only demonstrated the boundless potential of human ingenuity but also served as a poignant reminder that even the most audacious dreams can be realized through unwavering perseverance and determination.Since those pioneering days, space exploration has undergone a remarkable transformation, transcending the boundaries of nations and evolving into a collaborative endeavor that unites humanity in its quest for knowledge and discovery. The International Space Station stands as a shining example of this global cooperation, a celestial outpost where astronauts from diverse backgrounds work in harmony to advance our understanding of the universe and the intricacies of life in space.As we celebrate World Space Day, we are reminded of the countless contributions that space exploration has made to our daily lives. From weather forecasting and global communication networks to advancements in medical technology and materials science, the ripple effects of our cosmic endeavors have profoundly enriched our terrestrial existence. Furthermore, the study of celestial bodies and phenomena has shed light on the origins of our universe and our place within it, prompting us to ponder existential questions that have captivated philosophers and stargazers alike throughout the ages.Yet, our journey among the stars is far from over. As we stand on the precipice of new frontiers, our insatiable curiosity and thirst for knowledge propel us ever forward. The exploration of Mars, once a distant dream, is now a tangible reality, withrobotic rovers and future human missions poised to unravel the secrets of the Red Planet. Beyond our celestial neighborhood, the quest to detect and study exoplanets – worlds beyond our solar system – has opened up a universe of possibilities, igniting speculation about the existence of extraterrestrial life and the potential for future interstellar travel.Moreover, the advent of commercial space enterprises and the democratization of space exploration have ushered in a new era of accessibility and innovation. Private companies are now developing reusable launch vehicles, making space travel more affordable and sustainable, while also pushing the boundaries of what was once thought impossible. This burgeoning industry holds the promise of ushering in a new age of space tourism, where ordinary citizens can experience the wonders of the cosmos firsthand, forever altering their perspective on our fragile yet awe-inspiring planet.As we gather to celebrate World Space Day, let us embrace the spirit of exploration that has propelled humanity to the stars. Let us honor the courageous men and women who have risked their lives in the pursuit of knowledge, and let us inspire the next generation of explorers, scientists, and dreamers to carry the torch of discovery into the vast unknown.For it is through our insatiable curiosity and unwavering determination that we will continue to push the boundaries of what is possible, unlocking the secrets of the universe one celestial body at a time. As we gaze upon the stars, let us remember that our journey among them has only just begun, and that the limitless expanse of the cosmos beckons us to embrace the unknown, to dare greatly, and to never cease in our quest to understand the wonders that lie beyond our terrestrial abode.。

50个星系小知识

50个星系小知识1. 银河系（Milky Way）：我们所在的星系，包含数百亿颗星星。

2. 仙女座星系（Andromeda Galaxy）：是离我们最近的大型螺旋星系，将在未来几十亿年内与银河系发生碰撞。

3. 大麦哲伦星云（Large Magellanic Cloud）：是银河系的伴随星云，可在南半球看到。

4. 小麦哲伦星云（Small Magellanic Cloud）：也是银河系的伴随星云，位于大麦哲伦星云附近。

5. 三角座大星系团（Triangulum Galaxy）：是本地群中的第三大星系，与银河系和仙女座星系一同组成本地群。

6. M87星系：包含了世界首张黑洞照片的星系，黑洞称为M87*。

7. 半人马座α星系（Alpha Centauri）：是距离地球最近的三颗恒星，包括Proxima Centauri。

8. 螺旋星云（Whirlpool Galaxy）：与伴随星系NGC 5195一同构成一对交互作用的星系。

9. 和平座流星团（Pleiades）：一群年轻的恒星，以七姐妹而闻名。

10. 狮子座不规则星系（Leo I）：是银河系的卫星星系之一。

11. NGC 1300：一座螺旋星系，以其引人注目的臂旋结构而著称。

12. 椭圆星系（Elliptical Galaxy）：具有椭圆形状的星系，通常由老年恒星组成。

13. 蝎子座X-1：包含一颗质量极大的恒星和一个紧密伴星，是X射线双星系统。

14. NGC 2244：位于猎户座大星云中的一个年轻星团。

15. 哈勃深空场（Hubble Deep Field）：是哈勃太空望远镜拍摄的一个小区域，显示了许多远离地球的星系。

16. 蓝色大理石星系（Blue Marble Galaxy）：以其蓝色的颜色而闻名，是由气体和尘埃组成的星系。

17. NGC 6822：也称为巧克力盒星系，是一个不规则星系。

18. 奇迹星系（Antennae Galaxies）：两个星系之间发生碰撞，形成引人注目的星系相互作用。

有趣的实验做法作文英语

有趣的实验做法作文英语Title: Engaging Experimental Approaches: Unveiling the Fascinating Realm of Scientific Inquiry。

Introduction:In the realm of scientific inquiry, experimentation stands as a cornerstone for unraveling the mysteries of the universe. The allure of conducting captivating experiments lies not only in the pursuit of knowledge but also in the sheer joy of discovery. In this discourse, we delve into intriguing experimental methodologies that not only captivate the imagination but also yield profound insights into the workings of the natural world.1. The Magic of Microscopy:Microscopy, the art of exploring the minute, unveils a mesmerizing world invisible to the naked eye. One captivating experiment involves the observation ofmicroorganisms thriving in diverse environments. By collecting samples from seemingly mundane sources like pond water or soil, scientists can peer into a bustling ecosystem teeming with life. Through the lens of a microscope, amoebas gracefully glide, bacteria engage in intricate dances, and algae form intricate patterns, showcasing the beauty and complexity of the microbial world.2. Quantum Quirks:Venturing into the realm of quantum mechanics unveils a host of peculiar phenomena that defy classical intuition. The double-slit experiment stands as a quintessential example of this enigmatic realm. By passing particles, such as electrons or photons, through a barrier with two narrow slits, researchers observe a wave-like interference pattern on the detection screen, indicative of the particles behaving as waves. However, when the particles are observed or measured, they exhibit particle-like behavior, manifesting as distinct impacts on the screen. This experiment not only challenges our understanding ofparticle-wave duality but also underscores the fundamentalrole of observation in shaping reality at the quantum level.3. Psychological Illusions:Exploring the intricacies of the human mind unveils a treasure trove of psychological illusions that challengeour perception of reality. One captivating experiment involves the Rubber Hand Illusion, wherein participants experience a profound sense of ownership over a rubber hand when it is synchronously stroked alongside their hiddenreal hand. This phenomenon highlights the brain'sremarkable ability to integrate multisensory information, blurring the boundaries between self and non-self. Bydelving into such illusions, researchers gain invaluable insights into the mechanisms underlying body representation and embodiment.4. Galactic Glances:Peering into the depths of the cosmos unveils a breathtaking tapestry of celestial wonders. In the field of astronomy, the Hubble Space Telescope stands as a beacon ofexploration, capturing awe-inspiring images that redefine our understanding of the universe. Through pioneering experiments like the Hubble Deep Field observation, astronomers have unveiled a myriad of distant galaxies, each a testament to the vastness and beauty of the cosmos. By gazing into the depths of space, we not only witness the wonders of distant realms but also confront profound questions about our place in the cosmos.Conclusion:In the grand tapestry of scientific exploration, experiments serve as the threads that weave together our understanding of the natural world. From the microscopic realm to the cosmic expanse, each experiment offers a glimpse into the intricate workings of the universe. By embracing curiosity and embarking on captivating experimental journeys, we unravel the mysteries that shroud our reality, inching ever closer to the elusive truths that lie beyond. So, let us continue to explore, to question, and to marvel at the wonders that await us in the vast expanse of scientific inquiry.。

我们在太空探索中做了哪些事情英语作文

我们在太空探索中做了哪些事情英语作文Exploring the Universe: Our Accomplishments in SpaceHumans have always been curious about the vastness of the universe and our place in it. Over the years, we have embarked on numerous space missions to explore outer space and gain a deeper understanding of our cosmic surroundings. In this essay, we will delve into some of the key accomplishments we have achieved in our quest to explore the unknown, uncovering remarkable advancements along the way.One landmark achievement in space exploration is undoubtedly the successful landing of Apollo 11 on the moon in 1969. This historic mission, led by Neil Armstrong and Buzz Aldrin, marked the first time humans set foot on another celestial body. The images and videos captured during their lunar exploration not only captivated audiences worldwide but also significantly expanded our knowledge about the moon's surface composition and geological history.Beyond our closest neighbor, Mars has served as an intriguing target for exploration. Various robotic missions such as Pathfinder, Spirit, Opportunity, Curiosity, and most recently Perseverance have been launched to this red planet looking for signs of past or present life. These missions have provided us with invaluable information about Martian geology, weather patterns, and potential habitats that may sustain life forms.In addition to rovers exploring Mars' surface, scientists have sent spacecraft to study other planets and their respective moons. For instance, Voyagers 1 and 2 explored Jupiter's moons Io and Europa as well as Saturn's moon Titan. These missions allowed us to observe these celestial bodies up close while dishing out new insights into their unique compositions and geologic features.Furthermore, humans have taken an interest in studying other galaxies beyond our own Milky Way. The Hubble Space Telescope deployed into Earth's orbit has granted us breathtaking visualizations of distant stars, nebulas, andgalaxies. Thanks to this innovative telescope's observations and data collection capabilities, astronomers have made groundbreaking discoveries concerning universal expansion rates—a crucial piece in understanding the origins of our universe.Space exploration is not limited to distant planets and galaxies; it also encompasses the search for habitable zones beyond Earth. The discovery of exoplanets, planets orbiting stars outside our solar system, has become a focus of scientific research. Missions like Kepler and TESS have identified thousands of exoplanets, some of which exhibit conditions that could potentially support life as we know it. These findings have sparked excitement and further investigations into the possibility of extraterrestrial life.While space exploration is undeniably awe-inspiring, it is also essential for practical reasons. Satellite technology, developed through extensive research and experimentation, provides improved communication systems, weather forecasting capabilities, and GPS navigation services thatbenefit society as a whole.In conclusion, our endeavors in space exploration have yielded remarkable achievements that expand the frontiersof human knowledge. From landing on the moon to exploring Mars' surface and observing distant galaxies, we continueto push the boundaries of what we thought was possible. By embarking on these quests to unravel the mysteries of space, we open doors to new opportunities and uncharteddiscoveries that shape not only our understanding of the universe but also our own existence as inhabitants ofplanet Earth.。

TheHubble一只望远镜的革命英语美文

TheHubble一只望远镜的革命英语美文The Hubble一只望远镜的革命英语美文The Hubble 一只望远镜的革命When most people think of space, what come to mind are names like John Glenn and Neil Armstrong. When scientists think about space, the name that comes to mind is Hubble, a space telescope we sent on a twenty-year Journey to explore the origins of the universe. It"s already being called the most scientifically significant space project we ever embarked on. Taking pictures of the universe that literally let you and me and everyone else look back in time and see what the universe looked like13 billion years ago.很多人想到太空时,首先在脑海中出现的名字不是约翰·格林就是尼尔·阿姆斯特朗。

而当科学家想到太空时,他们脑海中出现的是哈勃太空望远镜。

二十年来哈勃望远镜一直在为我们探索宇宙的起源之谜,它被称为是人类所进行的最具科学意义的太空项目。

从它拍摄的宇审照片上,我们每人都能回顾到宇宙在130亿年前的模样。

The images are like nothing ever seen before, as much art as science, visions of a universe more violent and fantastic than anyone had dared to imagine. Everything from razor-sharp views of the planets in our own solar system, to the vast stellar nurseries where stars and planets are born. Some show us the explosive outbursts of dying suns, others the swirling masses of stars that make up the galaxies. But Hubble isn"t just giving us extraordinary pictures, it"s helping astronomers unlock the secrets of the universe.照片所展示的图像是我们前所未见的：科学犹如艺术，宇宙图像比任何人敢想象的.还要热烈，奇异。

哈勃望远镜英文阅读理解

哈勃望远镜英文阅读理解The Hubble Space Telescope: A Window to the CosmosThe Hubble Space Telescope has been a groundbreaking achievement in the field of astronomy since its launch in 1990. This remarkable instrument has revolutionized our understanding of the universe, providing us with unprecedented insights into the celestial bodies and phenomena that lie beyond our planet. Through its powerful lens, the Hubble has captured breathtaking images and gathered invaluable data, shedding light on the mysteries of the cosmos.One of the Hubble's most significant contributions has been its ability to observe distant galaxies with unparalleled clarity. By peering deep into the universe, the telescope has allowed us to witness the evolution of these galactic structures over billions of years. This has enabled astronomers to study the formation and development of galaxies, as well as the role of dark matter and dark energy in shaping the large-scale structure of the universe.The Hubble has also been instrumental in the study of exoplanets, or planets orbiting stars other than our own Sun. Through its preciseobservations, the telescope has helped identify and characterize numerous exoplanets, providing valuable insights into their size, composition, and potential for supporting life. This knowledge has been crucial in the search for habitable worlds beyond our solar system, fueling our curiosity and the hope of one day discovering extraterrestrial life.In addition to its groundbreaking discoveries, the Hubble has also captivated the public's imagination with its breathtaking images of celestial objects. From colorful nebulae to distant galaxies, the Hubble's stunning visuals have not only advanced our scientific understanding but have also inspired awe and wonder in people around the world. These images have become iconic representations of the beauty and complexity of the universe, sparking the curiosity of both scientists and the general public.The Hubble's impact, however, extends beyond its scientific and visual achievements. The telescope has also played a significant role in the education and outreach of astronomy, inspiring and engaging people of all ages to explore the wonders of the cosmos. Through its educational programs and collaborations with schools and universities, the Hubble has brought the excitement of space exploration to classrooms and communities worldwide, fostering a greater appreciation for the scientific endeavor and the pursuit of knowledge.Moreover, the Hubble's success has paved the way for the development of even more advanced astronomical instruments, such as the James Webb Space Telescope, which was launched in 2021. These new telescopes, building upon the Hubble's legacy, will continue to push the boundaries of our understanding of the universe, unlocking even more secrets and mysteries.In conclusion, the Hubble Space Telescope has been a true marvel of human ingenuity and scientific exploration. Through its groundbreaking observations, the Hubble has revolutionized our understanding of the cosmos, from the formation of galaxies to the search for habitable exoplanets. Its stunning visuals have captivated the public's imagination, while its educational initiatives have inspired generations of scientists and space enthusiasts. As we look to the future, the Hubble's legacy will undoubtedly continue to shape the course of astronomy and our exploration of the universe.。

高中英语作文探索宇宙的奥秘

高中英语作文探索宇宙的奥秘Exploring the Mysteries of the UniverseThe universe, with its vast expanse of space, has always captivated the human imagination.From ancient times to the present day, people have gazed up at the stars and asked questions about our place in the cosmos.The desire to explore the mysteries of the universe is a driving force behind many scientific endeavors.One of the most significant achievements in the exploration of the universe is the discovery of the expanding universe.In the early 20th century, Edwin Hubble observed that galaxies are moving away from us, and the farther away they are, the faster they are moving.This observation led to the realization that the universe is expanding, and it is still expanding today.This discovery revolutionized our understanding of the universe and laid the foundation for modern cosmology.Another fascinating aspect of the universe is the existence of dark matter and dark energy.These components make up the majority of the universe's mass and energy, but they cannot be directly observed.Dark matter does not emit, absorb, or reflect light, making it invisible.Dark energy, on the other hand, is a mysterious force that is causing the universe to expand at an accelerating rate.The existence of dark matter and dark energy is inferred through their gravitational effects on visible matter and the cosmic microwave background radiation.The study ofdark matter and dark energy is one of the most active areas of research in cosmology today.The universe is also home to a wide variety of celestial objects, such as stars, planets, black holes, and neutron stars.These objects have unique properties and behaviors that can be studied using various instruments and observatories.For example, the Hubble Space Telescope has provided stunning images of distant galaxies and helped scientists understand the evolution of the universe.The Chandra X-ray Observatory has revealed the presence of black holes and neutron stars through the detection of X-ray emissions.These observations have deepened our understanding of the universe and its most extreme phenomena.In addition to studying the universe through observations, scientists have also sent spacecraft to explore our solar system and beyond.Missions such as the Mars rovers Curiosity and Perseverance have provided valuable insights into the geology and potential for life on Mars.The Cassini mission to Saturn has yielded fascinating information about the planet's rings, moons, and atmosphere.These missions have not only expanded our knowledge of the solar system but have also inspired future generations to pursue careers in science and engineering.As we continue to explore the mysteries of the universe, we may never fully understand its intricacies.However, the pursuit of knowledge in this field has led to significant advancements in technology,mathematics, and physics.It has also fostered a sense of wonder and awe about our place in the cosmos.The exploration of the universe is a testament to human curiosity and our desire to push the boundaries of knowledge.As we look to the future, we can expect that our exploration of the universe will continue to yield new discoveries and deepen our understanding of this vast and complex entity.。

关于地球和宇宙英语作文带中文

关于地球和宇宙英语作文带中文【中英文版】**English Version:**The Earth and the UniverseOur planet, Earth, is a fascinating and unique place in the vast expanse of the universe.It is the only known planet to support life, with its rich biodiversity and a delicate balance of ecosystems.The Earth is a complex and interconnected system, where life has thrived for billions of years.The universe, on the other hand, is a vast and mysterious expanse that extends beyond our comprehension.It encompasses everything we know, from the tiniest particles to the largest galaxies.The universe is governed by fundamental laws of physics, which dictate the behavior of celestial bodies and the forces that shape them.The Earth is a part of the Milky Way galaxy, which is just one of billions of galaxies in the observable universe.Our solar system is located in a relatively quiet corner of the galaxy, with the Sun at its center and eight planets orbiting around it.Earth is the third planet from the Sun and is often referred to as the "blue planet" due to the abundant presence of water.The study of the Earth and the universe is a never-ending journey of discovery.Scientists have learned much about our planet and the cosmosthrough observations, experiments, and space exploration.The Hubble Space Telescope, for example, has provided us with stunning images of distant galaxies and has deepened our understanding of the universe"s origins and evolution.However, there is still much to learn.The existence of dark matter and dark energy, which make up the majority of the universe, remains a mystery.Additionally, the search for extraterrestrial life continues, as scientists explore the possibility of habitable planets beyond our solar system.In conclusion, the Earth and the universe are subjects of immense curiosity and wonder.Our planet is a precious gem in the cosmic ocean, and it is our responsibility to protect it.As we continue to explore the vastness of the universe, we must never forget the importance of preserving the only known home to life.**中文版：**地球与宇宙在浩瀚无垠的宇宙中，我们的星球——地球，是一个引人入胜且独一无二的地方。

The Hubble Ultra Deep Field

a r X i v :a s t r o -p h /0607632v 1 27 J u l 2006The Hubble Ultra Deep FieldSteven V.W.Beckwith 1,2,Massimo Stiavelli 1,Anton M.Koekemoer 1,John A.R.Caldwell 1,3,Henry C.Ferguson 1,Richard Hook 5,6,Ray A.Lucas 1,Louis E.Bergeron 1,Michael Corbin 7,Shardha Jogee 1,8,Nino Panagia 4,Massimo Robberto 4,Patricia Royle 1,Rachel S.Somerville 1,9,and Megan Sosey 1Space Telescope Science Institute,3700San Martin Drive,Baltimore,MD 21218,USAsvwb@ABSTRACTThis paper presents the Hubble Ultra Deep Field (HUDF),a one million second exposure of an 11square minute-of-arc region in the southern sky with the Advanced Camera for Surveys on the Hubble Space Telescope using Director’s Discretionary Time.The exposure time was divided among four ﬁlters,F435W (B 435),F606W (V 606),F775W (i 775),and F850LP (z 850),to give approximately uniform limiting magnitudes m AB ∼29for point sources.The image contains at least 10,000objects presented here as a catalog,the vast majority of which are galaxies.Visual inspection of the images shows few if any galaxies at redshifts greater than ∼4that resemble present day spiral or elliptical galaxies.The image reinforces the conclusion from the original Hubble Deep Field that galaxies evolved strongly during the ﬁrst few billion years in the infancy of the ing the Lyman break dropout method to derive samples of galaxies at redshifts between 4and 7,it is possible to study the apparent evolution of the galaxy luminosity function and number density.Examination of the catalog for dropout sources yields 504B 435-dropouts,204V 606-dropouts,and 54i 775-dropouts.The i 775-dropouts are most likely galaxies at redshifts between 6and ing these samples that are at diﬀerent redshifts but derived from the same data,we ﬁnd no evidence for a change in the characteristic luminosity of galaxies but some evi-dence for a decrease in their number densities between redshifts of 4and 7.Assessing the factors needed to derive the luminosity function from the data suggests there is considerable uncertainty in parameters from samples discovered with diﬀerent instruments and derived using independent assumptions about the source populations.This assessment calls into question some of the strong conclusions of recently published work on distant galaxies.The ultraviolet luminosity density of these samples is dominated by galaxies fainter than the characteristic luminosity,and the HUDF reveals considerably more luminosity than shallower surveys.The apparent ultraviolet luminos-ity density of galaxies appears to decrease from redshifts of a few to redshifts greater than 6,although this decrease may be the result of faint-end incompleteness in the most distant samples.The highest redshift samples show that star formation was already vigorous at the earliest epochs that galaxies have been observed,less than one billion years after the Big Bang.Subject headings:astronomical data bases:miscellaneous —cosmology:early universe —galaxies:evolution —galaxies:high-redshift6SpaceTelescope European Coordinating Facility7US Naval Observatory,FlagstaﬀStation 8University of Texas 9Max-Planck-Institut f¨u r Astronomie1.INTRODUCTIONA primary motivation for deep exposures of the sky has been to detect the most distant objects al-lowed by the observing technology.Over the last ten years,the use of ground-based telescopes com-bined with the Hubble Space Telescope produced large samples of galaxies at redshifts as high as5 to study early structure formation and the assem-bly of stars into present-day galaxies(Steidel et al. 1996a,b,1999;Ellis1998;Giavalisco2002).These programs successfully revealed the distant popula-tions recognized for several decades as important for understanding how the present-day universe came to be(Eggen et al.1962;Partridge and Pee-bles1967a,b;Tinsley1972a,b).Because of the complications arising from star formation,gas dy-namics,and feedback into the early intergalactic medium,theoretical predictions about the earliest galaxies are challenging,and the subject has been driven mainly by observations.Even though it has been possible to detect galaxies at redshifts above one,it has been diﬃ-cult to determine the redshifts and thus distances to objects from images only,where large samples may be rapidly assembled.Early workers rec-ognized that Lyαradiation should be especially prominent around theﬁrst generation of galax-ies,despite some uncertainty about the amount of scattering and absorption,and there should be a strong edge or break in the rest frame UV spec-tra at912˚A owing to absorption by hydrogen internal to the galaxies and in the intergalactic medium(e.g.Partridge1974;Davis&Wilkinson 1974;Koo&Kron1980.)Subsequently,Steidel and Hamilton(Steidel&Hamilton1992;Steidel 1996a,b)developed search techniques to exploit the Lyman edge using broad band colors toﬁnd galaxies with a paucity of short-wavelengthﬂux, the so-called“dropout”galaxies.This technique has proven most productive in discovering large samples of high redshift galaxies in multi-band im-ages.There are now samples of several thousand galaxies at redshifts between about2and5(Stei-del et al.1999,hereafter SAGDP99;Steidel et al. 2003;Giavalisco et al.2004.)When it became evident that the most dis-tant galaxies were characterized by compact high-surface brightness features(Driver et al.1995), the Hubble Space Telescope took a prominent role in the study of young galaxies.An important ad-vance came from the Hubble Deep Field(HDF; Williams et al.1996),a four-band,0.5million second exposure with the Wide Field Planetary Camera2.This seminal program using150orbits of Director’s Discretionary time on Hubble uncov-ered a large number of sources at redshifts above1 that would have been diﬃcult to discover from the ground.The HDF revealed a population of small, irregular galaxies that often appeared in pairs or small groups.Much of the light from these objects was high surface brightness—owing to high rates of star formation—but concentrated,requiring the resolution of Hubble to identify them as distant galaxies as opposed to red stars,say.Extension of the deepﬁeld approach to the southern hemi-sphere(Williams et al.2000)conﬁrmed the main conclusions of the HDF but also showed the limi-tations of a pencil beam survey in drawing broad conclusions about distant populations;cosmic er-ror within smallﬁelds can be substantial.Several advances since the HDF suggested that even deeper observations could reveal important aspects of the way that galaxies were created. The standard cosmology holds that the atoms in the universe were neutral following recombination at a redshift,z∼1100,until somewhere around z∼6−10,at which time they were reionized by stars and black holes.Theﬁrst observation of this epoch came with detection of the Gunn-Peterson hydrogen edge in the spectra of distant quasars discovered in the Sloan Digital Sky Survey(Becker et al.2001,Fan et al.2002),putting the redshift of complete reionization around6.The WMAP experiment made an indirect determination of a reionization era that started as early as redshift 11(Kogut et al.2003;Spergel et al.2003;Spergel et al.2006).Cold Dark Matter(CDM)models with a cosmological constant had some constraints that were not in accord with such an early epoch of reionization(Frenk et al.1985),although most of these models have suﬃcient freedom to accom-modate even the most discrepant data.Precise determination of the reionization history of the universe remains one of the important goals of ob-servational astronomy.The luminosity function inferred from the HDF suggested that searching a wider area to less depth than the HDF would be eﬃcient at picking up large populations of high redshift galaxies.Animportant advance since the HDF was the Great Observatories Origins Deep Survey:GOODS(Gi-avalisco et al.2004.)GOODS used the Advanced Camera for Surveys(ACS;Ford et al.2003) on Hubble to image an area thirty times larger but1magnitude shallower than the HDF.The GOODS sample contains more than60,000galax-ies with photometric magnitudes in four bands, B435(F445W),V606(F606W),i775(F775W),and z850(F850LP),and suﬃcient resolution to study structures as small as1kpc at redshifts approach-ing6.This sample is excellent for statistical stud-ies of bright galaxies at high redshifts.Deepﬁelds have an advantage over shallow ﬁelds to study the faint end of the luminosity func-tion and for increasing sample sizes when the slope of the luminosity function is large near the limit-ing magnitude of the survey.For a steep slope,the sample size will increase faster by investing addi-tional observing time in more exposure on a single ﬁeld rather than covering more area.The luminos-ity function at high redshifts is imprecise,but the current evidence indicates that it is consistent with a Schechter function(Schechter1976)with a char-acteristic luminosity,L∗,somewhat brighter than the local value and a faint end slope steep enough to warrant investment in a deepﬁeld(SAGDP99, Gabasch2004a,b.)The redshift at which the limiting magnitude of GOODS makes a deepﬁeld preferable to a wide ﬁeld can be estimated using a standard Schechter function.A deepﬁeld becomes preferable at a redshift greater than5,where the GOODS limit-ing magnitude is∼L∗,depending on the exact assumptions about how L∗evolves with redshift. The upper limit to the redshifts of the objects in a deep optical survey is when the Lyman edge goes beyond the longest wavelengthﬁlter.A practical limit for the ACS is when intergalactic absorp-tion shortward of Lyαshifts through the z850ﬁl-ter,z∼7.4.A deepﬁeld should produce samples of objects in the range5<z<7that are fainter than those found in GOODS and other wide sur-veys and allow a good characterization of the lu-minosity function in the early universe.It is most important to characterize the lu-minosity function below L∗to see the transition from exponential to power-law form,to measure the slope,and to assess the total luminosity of faint galaxies.The GOODS survey was limited to studying galaxies at the bright end of the lu-minosity function for redshifts greater than about 5.Even for lower redshifts,a deepﬁeld is use-ful to observe galaxies fainter than the character-istic brightness,providing important information about samples in the intermediate redshift ranges 2<z<5where much of the early star forma-tion in the universe took place.There is a strong degeneracy between derivations of object density and characteristic luminosity unless the luminos-ity function is well characterized below L∗.Such a degeneracy hampers the interpretation of shallow surveys even with large samples.As shown in the next section,it is possible to reach well below L∗out to redshifts near7with ACS on Hubble.This capability motivated a deep ACSﬁeld.The appearance of high redshift galaxies in the HDF and shallower surveys indicates substantial evolution in size and structure between early times and today.This evolution was already known at the time of the HDF,and subsequent observations tend to conﬁrm the conclusion that the galaxy populations look markedly diﬀerent at high red-shift compared to the present time.But the ap-parent morphology of high redshift galaxies is af-fected strongly by the loss of low-surface bright-ness features owing to cosmological dimming.An important way to test whether the loss of these features signiﬁcantly distorts our perception of galaxies at high redshift is to make deeper ob-servations of the sample at intermediate redshifts. Thus,an ultra deepﬁeld can provide an impor-tant complement to the pioneering observations of the HDF,GOODS,and ground-based surveys by searching for low-surface brightness components of faint galaxies.It was evident that a deepﬁeld with the new capabilities of Hubble following the installation of the ACS could address several important issues in early galaxy formation.In addition to augment-ing the samples of galaxies at redshifts greater than2,there was also the tantalizing possibility of pushing back the observational boundaries to redshifts greater than6to reach the reionization epoch.With these motivations in mind and fol-lowing the same philosophy pioneered by Robert Williams for the HDF,we held a series of meet-ings asking for advice on the scientiﬁc importance of another deepﬁeld and then assembled a Scien-tiﬁc Advisory Committee with a wide range of ex-pertise to recommend speciﬁc parameters for the survey:choice ofﬁeld,choice ofﬁlters,and depth needed for a meaningful advance.As with the original HDF,our purpose was to provide a public database using Director’s Discre-tionary Time on the Hubble Space Telescope for community use.This paper emphasizes the pa-rameters of the database rather than the subse-quent analysis,but it provides aﬁrst-order anal-ysis of the data to assess changes in the galaxy populations from the highest observable redshifts until the present.Thus,we assembled a team at the Space Tele-scope Science Institute to create the deepest visual-band image of the universe to date and put the observations in the public domain for commu-nity analysis.Like the HDF,this is a multi-color, pencil beam survey in a single ACSﬁeld.We call the resulting multi-color image the Hubble Ultra Deep Field(HUDF).2.Observations2.1.Field selectionTheﬁeld choice derived from a desire to min-imize the celestial foreground radiation,maxi-mize the accessibility to other astronomical ob-servatories,maximize the overlap with extant or planned deep observations at x-ray,infrared,and radio wavelengths,and maximize the observing eﬃciency of Hubble.The original HDF was lo-cated in Hubble’s continuous viewing zone(CVZ) to allow uninterrupted observations over a long period.The background light in CVZ orbits is often bright when observing in the part of the or-bit grazing the bright earth limb.The HDF over-came this limitation by taking images in the ul-travioletﬁlter,F300W,during the bright periods, because WFPC2images in thisﬁlter are detec-tor noise limited and relatively unaﬀected by in-creased background.The Wide Field Camera of ACS is not sensitive at ultraviolet wavelengths, and the enhanced background of the bright CVZ orbits would seriously limit their usefulness.We, therefore,decided not to require that the target ﬁeld be located in the CVZ,since it would not enhance the eﬃciency of the observations. Zodiacal dust within approximately30◦of the ecliptic plane is bright for Hubble;it was desirable to locate theﬁeld as far from the ecliptic as pos-sible.Declinations north of35◦are inaccessible from all major southern hemisphere observatories, particularly the planned Atacama Large Millime-ter Array(ALMA),designed to be an important tool for observations of distant galaxies.Declina-tions south of-40◦are inaccessible from Hawaii and all observatories northward.At the outset, we concentrated onﬁelds between-40◦and+35◦declination and more than35◦from the ecliptic plane.Within this declination range,there are a few places with very low Galactic dust and substan-tial investments of observing time from other pro-grams.The most prominent was the Chandra Deep Field South(CDF-S),a large(15′×15′)ﬁeld located in the direction3h30m-28◦.Thisﬁeld has very low Galactic cirrus emission and atomic hydrogen column density(Schlegel,Finkbeiner, &Davis1998),it passes through the zenith at the major observatories in Chile(the VLT,CTIO, Gemini South,Magellan,and ALMA),and it is ac-cessible from as far north as the VLA site in New Mexico.Furthermore,the CDF-S already has a substantial investment in deep x-ray observations with Chandra and XMM,and there are existing ACS observations through the Great Observato-ries Origins Deep Survey(GOODS,Giavalisco et al.2004)allowing some useful comparisons for the HUDF.There are also deep infrared observations with the Spitzer Space Telescope(Dickinson et al. 2004)CDF-S is larger than a single ACSﬁeld;sev-eral additional criteria guided the exact choice of pointing within it.The x-ray sensitivity with Chandra varies across theﬁeld,and it was de-sirable for the HUDF to coincide with a region of good x-ray sensitivity.There are several in-teresting objects identiﬁed through GOODS that deep observations would be most useful for,specif-ically a galaxy at redshift5.8and an old super-nova.We centered theﬁeld such that the high red-shift galaxy and old supernova were both covered, and the x-ray sensitivity was also very good.This choice produced aﬁeld centered on:RA(J2000) =3h32m39s,Dec(J2000)=-27◦47′29′′.1.Table1 lists the major characteristics of thisﬁeld.2.2.FiltersTheﬁlter choice was identical to that chosen by the GOODS team.This choice provides enough color information for rudimentary classiﬁcation of objects and enough wavelength coverage to search for the highest redshift galaxies.It also makes pos-sible an easy comparison of samples derived from both surveys.To detect objects at the highest possible redshifts,the observations needed to in-clude the longest wavelengthﬁlter,F850LP(z850), a band that was suﬃciently insensitive in WFPC2 (F814W)to limit its use for the HDF.The adja-cent F775W(i775)ﬁlter gives minimal overlap but contiguous wavelength coverage.Together,these two bands provide excellent sensitivity to the high-est redshift objects detectable with ACS,the i775-dropout sources,and are mandatory to search for objects at redshifts approaching7.Four bands are desirable to provide crude spec-troscopic analysis of the objects.Since the long-wavelength observations are background-limited, additional sensitivity could be gained by adding images at shorter wavelengths without loss of signal-to-noise ratio.The V606ﬁlter,F606W,is immediately adjacent to i775,broad,and provides excellent sensitivity to all objects at redshifts less than about4.The HDF incorporated an ultravioletﬁlter use-ful for identifying dropout sources at redshifts near 3.The ACS wideﬁeld camera is optimized for red wavelengths,making ultraviolet observations relatively insensitive.Since a primary goal was to identify samples at higher redshifts than the HDF, we chose the bluestﬁlter to be the B435band, F435W,immediately adjacent to V606band.Fol-lowing the conventions of the GOODS team,the bands are hereafter called B435,V606,i775,and z850.There are two other advantages to using the sameﬁlter set as GOODS.The overlap between the GOODS CDF-Sﬁeld and the HUDF makes it possible to compare objects directly in both sur-veys to calibrate completeness estimates for the shallower survey.Furthermore,science analyses can be carried out on the two data sets using iden-tical methods and minimizing systematic diﬀer-ences.Figure1plots the total detection eﬃciency for the four bands used for the observations.Theﬁg-ure includes the spectrum of a model galaxy at a redshift of5.8for comparison.Figure1shows that this very high redshift galaxy produces a sharp drop inﬂux between z850and i775with noﬂux at all in the shorter wavelength bands.The rela-tive detection eﬃciency also indicates the need for longer exposures in the longest wavelength bands to detect objects whose spectra are eitherﬂat or blue,typical of star forming galaxies at high red-shift.4000500060007000800090001000011000Wavelength Å0.20.40.60.81Transmission,FΝB435V606i775z850100Myr CSFz 5.8Fig. 1.—Theﬁlter transmission curves for the four bands used in this survey(colored lines)along with the spectrum of a model star forming galaxy at z=5.8(black).The spectrum is from the model of Bruzual&Charlot(2003)for a0.4so-lar metallicity galaxy undergoing continuous star formation for100Myr using the intergalactic ab-sorption curves of Madau(1995).2.3.DepthThere are several ways to estimate the depth needed to address the diﬀerent problems described in the introduction.However,we caution at the outset that so little is known about objects at the highest redshifts of interest that a conserva-tive approach would require depths well beyond those possible even with Hubble.We recognized at the outset that our goal was to obtain as deep an observation as possible with the amount of discretionary time available to the Director,and the depth would be constrained by the available pool.The resulting sensitivity is,nevertheless, well-suited to make progress on each issue.We adopt the concordance cosmology,ΩΛ=0.73,ΩM=0.27,h=0.71,throughout this paper for all analyses.The primary goal was to detect a statistically signiﬁcant sample of galaxies at redshifts between5and7.We set this goal at about100objects.Two additional goals are to study the luminos-ity function of high redshift galaxies at the faintend down to∼0.1L∗,and to observe low surface brightness features in galaxies that are missed inshallower surveys such as GOODS and the HDF,both of which will be aided by the faintest lim-iting magnitude that can be achieved by Hubble. The following paragraphs estimate what might be achieved with a limiting AB magnitude of∼29m, say.Assuming galaxy luminosity distributions are described by a Schechter function,it is possible to estimate the number of galaxies accessible to observation in any volume element of the uni-verse.SAGDP99determined a characteristic ap-parent magnitude at redshift3,m∗(3),of24.5(B-band)in the rest frame ultraviolet,correspond-ing to M∗=−20.8corrected to the assumed cos-mology;the local value is−20.2(Schechter1976.) Assuming M∗=−20.2,φ∗=0.016Mpc−3,and α=−1.6,the number of galaxies detected as a function of redshift per unit redshift in a single ﬁeld of∆Ωsr(11arcmin2for ACS)is:dNdz ∞x(z)φ∗Lαe−L dL,(1) where dV(1+z)10(M∗+k b(z)−m lim)/2.5,(2)with the luminosity distance,D L,in units of10pc and k b(z)the k-correction at redshift,z.These equations may be evaluated numerically;the re-sults are given in Table2for an assumed magni-tude limit between28and29.We also calculated the expected number of objects appearing in the V606,i775,and z850bands assuming all objects had the spectrum of a source undergoing continuous star formation for100Myr(Bruzual and Charlot 2003)and accounting for intergalactic hydrogen absorption(Madau1995)to illustrate where the objects drop out of eachﬁlter.The numbers in Table2are certainly much larger than a real survey would see.We have notattempted to correct for many observational ef-fects that would preclude galaxy detection in asurvey or for the expected mixture of source sizes,types,colors,etc.Furthermore,the density,φ∗, of distant populations appears to be several timessmaller than the local value used in these calcu-lations(SAGDP99.)The point of this estimate is to show that the expected number of sources is so large that even a few percent detection probability would yield statistically useful samples of galaxies.It is evident from Table2that even a detectionlimit of28m in z850would easily satisfy the goal of detecting∼100objects above redshift5.It would also reach below L∗at redshifts approach-ing7.There are several large uncertainties that could change these numbers in either direction:if L∗continues to increase beyond redshift3(e.g. Gabasch et al.2004a,b),there will be more faint objects to increase the counts and move further into the faint end of the luminosity function;how-ever,ifφ∗decreases owing either to a smaller num-ber density of galaxies in the early universe or cos-mic error,the number of detected galaxies will decrease.The two eﬀects oﬀset one another for number counts and introduce a diﬃculty of inter-pretation without a well-characterized luminosity function.A z850band limiting magnitude of28requires oforder one hundred orbits of dedicated Hubble ob-servations.The maximum available was400,and those needed to be divided among the four bands to give adequate spectral information.We chose to allocate144orbits each to the z850and i775 bands with estimated limiting magnitudes of28.7 and29.2,respectively.The remaining112orbits were split equally between B435and V606,whose estimated limiting magnitudes were then29.1and 29.3.With these sensitivities,the HUDF would, therefore,moveﬁrmly into the range needed to as-semble samples of high redshift galaxies and might even see substantial evolution in the properties of galaxies when compared with later epochs.2.4.Schedule andﬁeld orientationThe observations were scheduled for two peri-ods during which the roll angles could be con-trolled to produce a nearly square image.Schedul-ing400orbits at the same pointing and with con-strained orientations required the use of four roll angles in all:40◦,44◦,310◦,and314◦for the po-sition angle of the+U3axis on the sky to increase the target visibility and facilitate scheduling.Ta-ble3lists the schedule of observations in orbits for eachﬁlter.Each observation or visit consisted of two orbits with two exposures per orbit.The exposure time was typically1200seconds but in a few cases the exposures had to be shortened to850seconds.The total exposure time is just under1million seconds.2.5.Small Scale Pointing:DithersIn addition to the large rotations between dif-ferent phases of the observations that were re-quired for scheduling,smaller shifts in telescope pointing were applied to diﬀerent observations at the same position angle.These small changes in pointing between exposures,referred to as dithers, were introduced on two scales.First,small-scale dithers were applied to each of the four exposures within a two orbit visit. This dither pattern improved the sampling of theﬁnal image by introducing half-pixel oﬀ-sets.The ACS/WFC detector critically samples the point spread function(PSF)in the reddest bands but signiﬁcantly undersamples the PSF at shorter wavelengths.Such undersampling leads to loss of spatial information and aliasing artifacts. The introduction of sub-pixel dithering improves the sampling and allows the reconstruction of a higher-resolutionﬁnal image and a reduction of artifacts.In the case of ACS/WFC,half-pixel dithers or small integer numbers of pixels plus a half-pixel in both X and Y directions provide adequate sam-pling.The integer pixel components of the dither-ing were chosen to create the most compact dither pattern that ensured that a bad row or column could not overlap in the combined image because the pointings were always at least1.5pixels away from others in both X and Y.Thisﬁnal four-point dither,suggested by Stefano Casertano,is given in Table4.The most compact pattern was chosen because the exact sub-pixel shifts will be diﬀerent far from the center of the detectors owing to the very large non-linear component of the ACS/WFC optical distortion.The dither pattern minimizes this eﬀect and ensures good sampling across the ﬁeld.Additional dithers of approximately3and6 arcseconds in length in the direction perpendic-ular to the gap between the two ACS/WFC chips were introduced between visits.These oﬀsets en-sure that the regions of sky falling in the gap be-tween the two ACS/WFC chips had at least two thirds of the exposure of the rest of theﬁeld and hence minimized the lack of uniformity of theﬁnal exposure map.3.Data analysis3.1.Basic data reductionEach of the ACS/WFC exposures was pro-cessed through the standard ACS data pipeline, CALACS.Theﬁrst step removed the bias level, subtracted the dark current,corrected theﬂatﬁeld and gain variations,eliminated known bad pixels, and calculated the photometric zeropoint.To achieve optimal calibration,several refer-enceﬁles were created speciﬁcally for these ob-servations:improved dark current correctionﬁles (hyperdarks),improvedﬂatﬁelds,and bad pixel ﬁles.The hyperdarks were created using all the dark-current frames from the6-month period encom-passing the HUDF observations.Theseﬁles pro-vide higher signal-to-noise ratios than the typi-cal dark referenceﬁles that are subtracted during standard calibration and provide a more accurate representation of the overall dark current struc-ture appropriate to the HUDF exposures.Newﬂatﬁeld images for eachﬁlter were pro-duced by applying aﬂatﬁeld technique that cor-rects only low spatial-frequency variations based on stellar photometry of47Tucanae.These new ﬂatﬁeld images(L-ﬂats)produced a more uni-form sky level across the images than the stan-dard pipeline products.After re-calibrating the data with these L-ﬂats,the images had residual ﬂux of order2%of the sky level that we ascribe to scattered light from bright sources outside the ﬁeld of view.We produced images of these residu-als from the re-calibrated images and subsequently removed the scattered light by the following pro-cedure:1.All exposures from the pipeline were com-bined to create an image of theﬁeld.。

2022年江苏天一中学高二下学期期中考试英语试题

江苏省天一中学2021-2022学年第二学期高二期中考试高二英语学科（时间：120分钟满分150分）第二部分阅读（共两节，满分50分）第一节（共15小题；每小题2.5分，满分37.5分）阅读下列短文，从每题所给的A、B、C、D四个选项中选出最佳选项。

ABest Compact SUVs for 2022If you are looking for more interior and cargo space than sedans offer, compact SUVs can be a great choice. We highlight the best compact SUVs below to help get you started.2022 Hyundai Tucson#1 in Compact SUVsFeatures & SpecsSEA TING:5DRIVETRAIN: FWD, AWDHORSEPOWER: 187The redesigned 2022 Hyundai Tucson ranks at the top of the compact SUV class, thanks in part to its composed ride and handling, spacious and upscale interior, user—friendly infotainment system, and long list of driver assistance features. The redesigned Tucson impresses with its spacious five-seat interior and large cargo area. And it's powered by a zippy yet economical four-cylinder engine. It's also backed by a 10-year/100,000-mile powertrain warranty.2021 Mazda CX—5#2 in Compact SUVsFeatures & SpecsSEA TING: 5DRIVETRAIN: FWD, AWDHORSEPOWER:187—227The 2021 Mazda CX-5 challenges for a top spot in the competitive compact SU'V class thanks to its engaging handling and refined interior. The CX-5 doesn't have the most spacious cargo hold, hut it offers a sophisticated and elegant interior, comfortable seating, and a revamped infotainment system. The CX-5's solid combination of quality and value led us to name it as a finalist for our 2021 Best Compact SUV for the Money award. It was also named a finalist for our 2021 Best Compact SUV for Families award.2022 Honda CR—V#3 in Compact SUVsFeatures & SpecsSEA TING: 5DRIVETRAIN: FWD. AWDHORSEPOWER: 190The 2022 Honda CR—V finishes near the top of our compact SUV rankings. It's among the class leaders in several areas, including passenger and cargo space and fuel economy. The Honda CR—V is a good SUV. It delivers energetic acceleration and a comfortable ride, and it gets good gas mileage. Because of its solid combination of quality and value, we named the CR—V a finalist for our 2022 Best Compact SUV for the Money.1. If you pay much attention to a SUV's powertrain guarantee, is a good choice.A. 2021 Hyundai TucsonB. 2021 Mazda CX—5C. 2022 Honda CR—VD. 2022 Hyundai Tucson2. What can we learn about the three SUVs from the passage?A. The three SUVs share the same seating and drivetrain.B. The 2021 Mazda CX —5 is powered by a zippy four—cylinder engine.C. The 2022 Hyundai Tucson doesn't have large spacious cargo hold.D. The CR—V was named 2021 Best Compact SUV for Families award.3. What's the main purpose of this passage?A. To detail some best compact SUVs for 2022.B. To promote the best compact SUVs for readers.C. To advertise for some car designing companies.D. To guide readers to choose a SUV from rankings.【答案】1. D 2. A 3. B【解析】【导语】本文是一篇应用文。