A Survey of Star-Forming Galaxies in the z=1.4-2.5 `Redshift Desert' Overview

A Survey of Star-Forming Galaxies in the z=1.4-2.5 `Redshift Desert' Overview
A Survey of Star-Forming Galaxies in the z=1.4-2.5 `Redshift Desert' Overview

a r X i v :a s t r o -p h /0401439v 1 21 J a n 2004

Received 2003November 6;Accepted 2003December 9

Preprint typeset using L A T E X style emulateapj v.11/26/03

A SURVEY OF STAR-FORMING GALAXIES IN THE 1.4<~Z <~

2.5‘REDSHIFT DESERT’:OVERVIEW 1

Charles C.Steidel &Alice E.Shapley 2

California Institute of Technology,MS 105–24,Pasadena,CA 91125

Max Pettini

Institute of Astronomy,Madingley Road,Cambridge CB3OHA,UK

Kurt L.Adelberger

Carnegie Observatories,813Santa Barbara Street,Pasadena,CA 91101

Dawn K.Erb,Naveen A.Reddy,&Matthew P.Hunt

California Institute of Technology,MS 105–24,Pasadena,CA 91125

Received 2003November 6;Accepted 2003December 9

ABSTRACT

The redshift interval 1.4<~z <~

2.5has been described by some as the ‘redshift desert’because of historical di?culties in spectroscopically identifying galaxies in that range.In fact,galaxies can be found in large numbers with standard broad-band color selection techniques coupled to follow-up spectroscopy with UV and blue-sensitive spectrographs.In this paper we present the ?rst results of a large-scale survey of such objects,carried out with the blue channel of the LRIS spectrograph (LRIS-B)on the Keck I telescope.We introduce two samples of star forming galaxies,‘BX’galaxies at z =2.20±0.32and ‘BM’galaxies at z =1.70±0.34.In seven survey ?elds we have spectroscopically con?rmed 749of the former and 114of the latter.Interlopers (de?ned as objects at z <1)account for less than 10%of the photometric candidates,and the fraction of faint AGN is ~3%in the combined BX/BM sample.Deep near-IR photometry of a subset of the BX sample indicates that,compared to a sample of similarly UV-selected galaxies at z ~3,the z ~2galaxies are on average signi?cantly redder in (R ?K s ),indicating longer star formation histories,increased reddening by dust,or https://www.360docs.net/doc/ad11585159.html,ing near-IR H αspectra of a subset of BX/BM galaxies to de?ne the galaxies’systemic redshifts,we show that the galactic-scale winds which are a feature of star-forming galaxies at z ~3are also common at later epochs and have similar bulk out?ow speeds of 200-300km s ?1.We illustrate by example the information which can be deduced on the stellar populations,metallicities,and kinematics of “redshift desert”galaxies from easily accessible rest-frame far-UV and rest-frame optical spectra.Far from being hostile to observations,the universe at z ~2is uniquely suited to providing information on the astrophysics of star-forming galaxies and the intergalactic medium,and the relationship between the two.

Subject headings:cosmology:observations —galaxies:evolution —galaxies:high-redshift —galaxies:

kinematics and dynamics —galaxies:starburst —stars:formation

1.INTRODUCTION

A number of di?erent observations point to the red-shift range 1<~z <~

2.5as a particularly important epoch in the history of star formation,accretion onto massive black holes,and galaxy assembly.Recent successes in identifying the luminous but heavily obscured galaxies selected at sub-mm and radio wavelengths have shown that they are mostly at redshifts z ~2.4±0.4(Chap-man et al.2003);this is also the epoch when the number density of luminous quasi-stellar objects (QSOs)peaked (e.g.,Di Matteo et al.2003and references therein).The evolution of the ultraviolet (UV)luminosity density of the universe is now mapped out with reasonable preci-1

Based,in part,on data obtained at the W.M.Keck Observa-tory,which is operated as a scienti?c partnership among the Cal-ifornia Institute of Technology,the University of California,and NASA,and was made possible by the generous ?nancial support of the W.M.Keck Foundation.

2Present address:Department of Astronomy,University of Cal-ifornia,Berkeley,CA 94720

sion over the redshift range z ~0?6(e.g.,Madau et al.1996;Connolly et al.1997;Steidel et al.1999;Gi-avalisco et al.2003),but there remains a glaring gap between z ~1.5and z ~2.5,an epoch when much of today’s stellar mass was assembled and heavy elements were produced (e.g.,Dickinson et al.2003;Fontana et al.2003;Rudnick et al.2003).The reason for this gap is that such redshifts,while only ‘modest’by current stan-dards in distant galaxy hunting,have remained challeng-ing to direct observation from the ground.As explained below,this has only to do with accidental incompatibili-ties between technology,the atmospheric windows avail-able for sensitive observations from the ground,and the spectral features that enable redshift measurement,and not with any intrinsic changes in the galaxy populations.The fact that the z =1.5?2.5universe is to a large extent still terra incognita provides exciting opportuni-ties for new observational techniques to make substantial headway.

2Steidel et al.

Spectroscopy of distant galaxies has advanced signif-icantly since the advent of8-10m class telescopes lo-cated at the best terrestrial sites and equipped with state of the art spectrographs achieving very high e?ciency throughout the optical range.Wavelengths between4000 and9000?A in particular bene?t from the combination of low night-sky background,high atmospheric transmis-sivity,and high CCD quantum e?ciency which all re-sult in greater sensitivities(in?ux density units)than those achievable at any other wavelength observable from the ground.For this reason,galaxy surveys have tradi-tionally targeted strong spectral features that fall in this range of high sensitivity,particularly nebular lines from H II regions(e.g.,[O II]λ3727;[O III]λλ4959,5007; Hβand Hα),and the region near4000?A in the spectra of early type galaxies.These features are used both in the measurement of galaxy redshifts and in the deter-mination of basic astrophysical properties,such as star-formation rates,reddening,chemical abundances,veloc-ity dispersions,and age.

As we move from the local universe to higher red-shifts,the most straightforward strategy has been to simply follow these same spectral lines to longer wave-lengths;for redshift measurements,[O II]λ3727is ac-cessible to optical spectrographs up to z?1.4.Consid-erable success in charting galaxies up to these redshifts has been achieved recently with red-optimized3spectro-graphs(e.g.,DEEP2,Coil et al.2003)that have been de-signed speci?cally to minimize the e?ects of OH emission from the night sky at wavelengths longer than~7300?A, or z>~1for the detection of[O II]λ3727.

The optical domain has also proved ideal for surveying galaxies at redshifts z>~2.5where the rest-frame far-UV spectral region,with its wealth of spectral information, crosses over the‘horizon’imposed by the earth’s atmo-sphere(Steidel et al.1999,2003;Shapley et al.2003). Between z~1.4and z~2.5,however,lies an interval of redshift for which no strong spectral lines fall in the 4300?9000?A range where most of the spectrographs on large telescopes are optimized—hence,the‘redshift desert’.There are two obvious approaches to?nding galaxies at these redshifts:one is to extend multi-object spectroscopy into the near-infrared(near-IR)and tar-get the familiar optical emission lines from H II regions; the other is to exploit the rest-frame far-UV spectral features by observing at near-UV and blue wavelengths (~3100?4500?A).

Both strategies are observationally challenging.In the ?rst case,the sky background—both in the OH emis-sion features and in the continuum—becomes progres-sively brighter at longer wavelengths,while the falling ef-?ciency of silicon detectors requires the use of a di?erent detector technology—one that lags signi?cantly behind CCDs in terms of performance,multiplexing,and areal coverage.Moreover,the increasing importance of ther-mal noise in the IR necessitates more complex cryogenic instruments with cooled focal planes and optics.Never-theless,multi-object cryogenic near-IR spectrographs are now being built,or planned,for several8-10m telescope facilities.At the other end of the scale,the ground-based 3Red optimization involves several aspects:high spectral reso-lution,low detector fringing and/or spectrograph stability,as well as high detector quantum e?ciency in the red.near-UV spectral region has its own di?culties.High near-UV transmission and/or re?ectivity requires com-promises that can exacerbate broad-band optical per-formance.Di?erent optical glasses and coatings are re-quired;it is extremely di?cult to achieve good optical performance simultaneously at3300?A and7500?A with refractive optics,for example,and until recently good UV re?ectivity has required the use of Al mirror coatings which do not perform well,compared to other readily available materials,at wavelengths longer than4500?A. In addition,in selecting CCDs one must generally choose between those that have good UV response and those with good quantum e?ciency and low fringing amplitude in the red and near-IR.All these reasons explain why the wavelength region below4000?A has so far been largely neglected in the design of optical faint-object spectro-graphs.

From the point of view of the astrophysical informa-tion they convey,both the rest-frame far-UV and opti-cal regions are important and are in fact largely com-plementary.In the rest-frame optical,nebular emission lines yield information on the chemical abundances and kinematics of the ionized gas(e.g.,Teplitz et al.2000; Pettini et al.2001;Erb et al.2003;Lemoine-Busserolle et al.2003).In the far-UV,on the other hand,large number of stellar and(especially)interstellar absorption lines are accessible.The latter have provided informa-tion on the kinematics and chemistry of out?owing gas that may have very signi?cant implications for the galaxy formation process(see,e.g.,Pettini et al.2002a;Shap-ley et al.2003,Adelberger et al.2003).Stellar features, while weaker and therefore more di?cult to detect,can be used to place constraints on the initial mass function (IMF)and metallicity of massive stars(e.g.,Pettini et al.2000,2002b;Leitherer et al.2001).

When it comes to identifying galaxies at z=1.4?2.5 in large numbers,however,there are clear di?erences be-tween the near-UV and near-IR(in the observed frame) domains.Of course,some types of objects,such as those most heavily reddened by dust or having no current star formation,may be accessible only in the near-IR.How-ever,the fact remains that,with no moonlight,the night sky background in the blue and near-UV is nearly fea-tureless,is~3(AB)magnitudes fainter(per square arc second in the continuum)than at9000?A,and is more than5magnitudes fainter than in the H(1.65μm)and K (2.2μm)bands even between the OH sky emission lines. Since the spectral energy distribution of a typical star forming galaxy is relatively?at from the rest-frame UV to the optical,the advantage of near-UV spectroscopy is obvious,provided high spectral throughput can be achieved.This is most e?ectively accomplished with double-beam spectrographs where the spectral through-put can be optimized over the whole range0.3?1.0μm. Since the commissioning of the blue channel of the Low Dispersion Imaging Spectrograph(LRIS-B)on the Keck I telescope,described in an appendix,we have been conducting a survey for star-forming galaxies at redshifts z?1.5?2.5.The development of the photometric cri-teria used to select candidates is described in detail in Adelberger et al.(2004).In this paper we present the ?rst results of the survey,highlighting the e?ciency of UV selection for bridging this important redshift gap

Star-forming galaxies in the Redshift Desert3

in our knowledge of galaxy evolution,and and overview of the science now possible for galaxies in this redshift range.In§2we brie?y describe our photometric selec-tion of candidates.The spectroscopy is described in§3 where we present the?rst results of the survey,such as the success rate of the photometric selection and the red-shift distribution of con?rmed candidates.An appendix describes the most important aspects of the LRIS-B in-strument,which has been crucial to the success of the survey.§4illustrates some of the astrophysical informa-tion conveyed by the spectra;we consider in particular the IMF and metallicity of the young stellar populations and the kinematics of the interstellar medium in these galaxies.§5deals with the optical-IR colors of the galax-ies.Finally,in§6we summarize the main?ndings from this initial stage of our survey.We assume a cosmology with?m=0.3,?Λ=0.7,and h=0.7throughout.

2.PHOTOMETRIC AND SPECTROSCOPIC TARGET

SELECTION

2.1.Color Selection

Our searches for galaxies at z=1.4?2.5are based on deep images in the U n,G,and R passbands of similar quality(in terms of depth and seeing)as those used in our published survey for Lyman Break Galaxies(LBGs)at z?3(Steidel et al.2003).The?elds observed are listed in Table1;with the exception of GOODS/HDF-N and Westphal,the?elds are distinct from those used in the z~3LBG survey and were selected primarily because they include one or more relatively bright background QSOs suitable for studying the cross-correlation of galax-ies with H I and metals in the intergalactic medium (IGM).The imaging data were obtained at four tele-scopes(Palomar5.1m,WHT4.2m,KPNO4m,and Keck I10m)mostly between2000August and2003 April.We made use of the deep U band image in the GOODS-N?eld obtained by the GOODS team(Gi-avalisco&Dickinson2003),which was calibrated onto our own photometric system using observations through the U n?lter presented in Steidel et al.(2003).The image reductions and photometry were performed following the procedures described in Steidel et al.(2003).

The rationale and method for selecting z~2galaxies using only their optical broad-band colors are described in detail by Adelberger et al.(2004).The selection crite-ria are aimed at identifying galaxies with approximately the same range of intrinsic properties,particularly UV luminosity and reddening by dust,as the well-studied z~3Lyman break galaxies.After some‘?ne-tuning’based on the initial results of early spectroscopic follow-ups,we converged on two sets of color selection criteria designed respectively to select galaxies in the redshift ranges2.0<~z<~2.5—we call these‘BX’objects,and 1.5<~z<~2.0—the‘BM’objects.The criteria for BX objects(e.g.,Q1700-BX691)are4:

G?R≥?0.2

U n?G≥G?R+0.2

G?R≤0.2(U n?G)+0.4

U n?G≤G?R+1.0(1)

4Note that the BX color cuts given in equation1are slightly di?erent from the preliminary values published in Erb et al.(2003).and for the BM objects(e.g.,Q1307-BM1163),

G?R≥?0.2

U n?G≥G?R?0.1

G?R≤0.2(U n?G)+0.4

U n?G≤G?R+0.2(2) The methods for establishing these selection criteria,and their estimated level of completeness,are discussed in Adelberger et al.(2004).Figure1shows where the two color cuts are located in the(U n?G)vs.(G?R) plane.The resulting sample of galaxies is very simi-lar to our existing sample of z~3LBGs in terms of star formation rate inferred from their UV luminosities (uncorrected for extinction):the BX+BM spectroscopic sample has SFR=3?60M⊙yr?1,with a median of 9.9M⊙yr?1,while the spectroscopic z~3LBGs have SFR=5.5?66M⊙yr?1with a median of10.3M⊙yr?1 (estimated from the apparent G and R magnitudes re-spectively,using the conversion from1500?A luminosity advocated by Kennicutt

1998.)

Fig. 1.—Two-color diagram from one of the survey?elds,to illustrate the new selection criteria discussed in this paper(see also Adelberger et al.2004).The size of the symbols scales with object brightness,and triangles are objects for which only limits were obtained on the U n?G color.The cyan and magenta-shaded regions are the BX and BM selection windows,designed to select galaxies at z?2?2.5and z?1.5?2.0,respectively.The green and yellow shaded regions are the z~3LBG color section windows used in the survey by Steidel et al.(2003).In this?eld there are 1831BX and1085BM candidates;together they account for~25% of the11547objects brighter than R=25.5.For clarity only one in?ve objects is shown in the plot.

The average surface density of photometric candidates satisfying the BX criteria is5.2arcmin?2to R=25.55, while the corresponding average surface density of BM objects is3.8arcmin?2;together,they comprise~25% 5Corrected for9%contamination by interlopers,as discussed in §4.1below.

4Steidel et al.

Table1.Survey Fields

Field Nameα(J2000)δ(J2000)Field Size Telescope/Date a

GOODS-N12:36:51+62:13:1410′.6×14′.6KPNO/Apr02,Keck I/Apr03

Q130713:07:45+29:12:5116′.2×15′.9WHT/May01

Westphal14:17:43+52:28:4915′.0×15′.0KPNO/May96,P200/May02

Q162316:25:45+26:47:2312′.5×23′.2P200/Aug00

Q170017:01:01+64:11:5815′.3×15′.3WHT/May01

Q234323:46:05+12:49:1223′.8×11′.9P200/Aug01

Q234623:48:23+00:27:1516′.4×17′.0WHT/Aug01

a Telescope and instrument used to obtain the deep imaging data:KPNO=Kitt Peak4m Mayall telescope,with PFCCD(1996)and MOSAIC(2002);WHT=William Herschel4.2m telescope with prime focus imager;P200=Palomar5.1m Hale telescope with Large Format Camera;Keck I=Keck I10m telescope LRIS in imaging mode.

of the R band counts to this apparent magnitude limit, and exceed the z~3LBG surface density by a factor of more than?ve.This is not surprising given the larger redshift interval(1.5<~z<~2.5),and the lower intrinsic luminosities reached in the BX and BM samples com-pared to the z~3LBGs.6

2.2.Spectroscopic Follow-up

At this stage in the survey we have followed-up spectro-scopically primarily the BX candidates,for the purpose of extending to lower redshifts our study of the IGM-galaxy connection begun in Adelberger et al.(2003). As discussed in§3.1,the redshift distribution of these galaxies has z =2.20±0.32.Only very recently have we begun observing signi?cant numbers of BM galaxies targeting the range1.4<~z<~2.0,considered by some to be perhaps the most challenging range of redshifts for con?rmation with optical spectroscopy.While the current statistics for such galaxies are not as extensive as those of the BX sub-sample(we have observed only 187candidates to date),the results obtained so far al-ready show that the color selection criteria work as ex-pected;the redshift distribution of con?rmed BM objects is z =1.70±0.34.

What is clear is that the higher surface density of BX/BM photometric candidates,coupled with the high rate of spectroscopic con?rmation achieved with the UV-optimized LRIS-B spectrograph,allows a large sample of galaxies in the‘redshift desert’to be assembled relatively easily and e?ciently.To date our survey includes692 galaxies with con?rmed spectroscopic redshifts between z=1.4and2.5.We defer to a future paper the relatively complex analysis necessary to turn the photometric and spectroscopic results into a far-UV luminosity function of z~2galaxies.

We now turn to the spectroscopic results;relevant in-formation on the performance and speci?cations of the LRIS-B instrument are summarized in the Appendix.

3.SPECTROSCOPIC OBSERVATIONS AND RESULTS

3.1.Optical Spectroscopy

For the redshift range of interest here,1.4<~z<~2.5, the rest frame far-UV region between Lyαand C IVλλ1548,1550,with its rich complement of stellar 6R=25.5corresponds to galaxies that are0.6magnitudes less luminous at z=2.2,and1.1mags less luminous at z=1.7,than z~3LBGs of the same apparent magnitude.and interstellar lines,is redshifted between the atmo-spheric cut-o?near3100?A and5400?A.This wavelength interval was therefore the primary target of our spectro-scopic survey.The spectra were obtained with slightly di?erent instrumental con?gurations over the three year period of the survey to date.In the blue channel of LRIS we used the400groove mm?1grism blazed at 3400?A throughout,since this is the grism which pro-vides the highest throughput between3100and4000?A (see Appendix and associated?gures).Initially we ob-served with a dichroic that divides the incoming beam at 5600?A,and with a600groove mm?1grating blazed at 7500?A on the red side;this set-up generally gives com-plete wavelength coverage over the entire3100–8000?A range for most slits.In2002,once the blue channel de-tector was upgraded to the larger Marconi CCD mosaic, we used the blue side only with a mirror in place of the dichroic;this con?guration covers from the atmospheric cut-o?to about6500?A(the exact red limit of the spec-tra depends on the location of a given slit within the ?eld of view)with a dispersion of1.07?A per15μm pixel. More recently we have reverted to double-channel mode, but with the6800?A dichroic and the400groove mm?1 grating blazed at8500?A which gives additional spectral coverage from6800?A to9500?A for most slits.This lat-est setup is particularly useful when observing BM can-didates,because the red side allows one to check for the presence of[O II]λ3727emission at z~1?1.5;as ex-plained below,galaxies in this redshift range turn out to be the main source of contamination of the BM sample. The Keck I f/15Cassegrain focus does not yet have an atmospheric dispersion corrector,so we took special care to avoid signi?cant light losses due to atmospheric dispersion.We designed slit masks to be used at a given time of night,with a position angle that would place the slits within20degrees of the parallactic angle at any time during the observations.At the telescope,we performed the?nal mask alignment in the blue,since this is the region of most interest for our purposes.With the1.2′′slits used in all the masks,and the typical image qual-ity of~0.8′′at the detector,the spectral resolution in the blue was?5?A FWHM,sampled with~4.5pixels. Wavelength calibration was achieved by comparison with the spectra of internal Cd,Zn,Ne,and Hg lamps,as well as by reference to night sky emission lines.

The choice of BX and BM candidates assigned to each mask was based on an algorithm that gives largest weights to objects in the apparent magnitude range

Star-forming galaxies in the Redshift Desert5

R=22.5?24.5and somewhat lower weights to brighter and fainter objects.7Each slit mask covered an area on the sky of8′.0by4′.5,and contained30?35slits.With few exceptions,each mask was observed for a total of 5400s split into three1800s integrations;the telescope was stepped by1?2′′in the slit direction between expo-sures.The data were reduced and the spectra identi?ed with the procedures described by Steidel et al.(2003). The total exposure times were deliberately kept short in order to maximize the number of galaxies for which redshifts could be measured.Because of this,the qual-ity of the spectra varies considerably(since the objects range from R=21.7to R=25.5),but the best spec-tra are already suitable for more detailed analyses,as discussed in§4below.The success rate in spectroscop-ically identifying candidates varied from mask to mask, depending on the observing conditions.On masks ob-tained in the best conditions,a redshift could be mea-sured for more than90%of the objects targeted,whereas the proportion was lower for masks observed in poor see-ing(FWHM>~1arcsec)or through thick cirrus.Because of this,we believe that spectroscopic failures are very likely to have the same redshift distribution as the suc-cesses,rather than being the result of the true redshifts falling far from expectations.The overall success rate to date in securing a spectroscopic identi?cation is69%for BX candidates and65%for BM candidates(see tables2 and

3).

Fig. 2.—Redshift distributions of spectroscopically con?rmed galaxies from various color-selected samples.The green histogram is for z~3LBGs from Steidel et al.(2003)(scaled by0.7);the blue histogram shows the redshifts of our current BX sample(749 galaxies),while the red histogram is the smaller BM sample of 114galaxies(scaled up by a factor of three for clarity).The total number of new spectroscopic redshifts in the range1.4≤z≤2.5, including both BX and BM samples,is694,with244of those in the range1.4≤z≤2.0.

7Objects lying within1–2′of a background QSO were given ad-ditional weight depending on the projected distance from the QSO. Such cases are particularly useful for investigating the galaxy-IGM connection(Adelberger et al.2003).

The current redshift histograms for the BX and BM samples are shown in Fig.2,together with the histogram for the completed z~3LBG survey of Steidel et al. (2003).The total number of new spectroscopic redshifts in the range1.4≤z≤2.5,including both BX and BM samples,is692,with244of those in the range1.4≤z≤2.0.

From the results of the survey so far we have estab-lished that star forming galaxies at z =0.17±0.09 are the main source of contamination in the BX sample (adopting the somewhat arbitrary de?nition of an inter-loper as any object at z<1).These are star forming dwarf galaxies whose Balmer break mimics the Lyman αforest decrement of galaxies in the z=2?2.5range. As can be seen from Table2,they are dominant at the bright end of the distribution of apparent magnitudes but become negligible at faint magnitudes.Con?ning the spectroscopic follow-up to objects with R>23.5would reduce the contamination to5%,but at the expense of overlooking the intrinsically brightest galaxies at z>1.5 which are the most suitable for subsequent detailed spec-troscopic studies.For this reason we have included many of these bright objects in the masks observed up to now. Based on the spectroscopic results,we estimate that the overall contamination of the BX photometric sample to R=25.5by low-redshift interlopers is~9%of which 3%are stars and6%are low redshift star forming galax-ies.However,because we did not sample the photometric candidates evenly,but favored brighter objects,the in-terloper contamination of the BX spectroscopic sample to date is?17%.Of the903BX objects with measured redshifts,749are at z>1with z =2.20±0.32. Objects satisfying the BM color criteria,on the other hand,do not su?er signi?cant contamination from very low redshift galaxies or stars.While the targeted redshift range was1.5<~z<~2.0,a signi?cant number of galaxies in the1.0≤z≤1.4range are included simply because some fraction of such galaxies are found in the same re-gion of the U n G R color space as the targeted objects(see Table3);their proportion can be reduced if additional color criteria are imposed(see Adelberger et al.2004).Of the114BM objects with secure redshifts,107have z>1, with an overall redshift distribution of z =1.70±0.34; 20objects have1.0≤z≤1.4.

Example spectra of galaxies in the‘redshift desert’are shown in Figs.3and4;the properties of these exam-ple objects,including their positions,magnitudes,and colors,are summarized in Table4.In Fig.3we have reproduced the LRIS-B spectrum of Q1307-BM1163,a bright(R=21.67,G=21.87,U n=22.22)galaxy at z=1.411.We have plotted raw counts vs.observed wavelength speci?cally to show the relative count rate as a function of wavelength from3000to7100?A,the range encompassed by the blue channel of LRIS for this https://www.360docs.net/doc/ad11585159.html,eful signal is obtained all the way down to3100?A;some of the most prominent interstellar absorption and emission lines covered in our spectra of BM and BX galaxies,from C IIλ1334to Mg IIλλ2796,2803,are indicated in the?gure.In§4we use this example to illustrate the information which can be deduced on the stellar populations and the interstellar medium of galax-ies at z=1.4?2.5from the study of their rest-frame UV spectra.

Figure4is a montage of ten spectra of some of the

6Steidel et al.

Table 2.BX Spectroscopic Sample Completeness and Contamination R Mag Attempted Identi?ed %Identi?ed

Interlopers a

%Interlopers b

19.0?22.0c 525096.24284.622.0?22.51212100.01191.722.5?23.0323093.82170.023.0?23.511810689.83230.223.5?24.022317779.32514.024.0?24.536324868.312 4.824.5?25.027517563.69 5.125.0?25.523110545.72 1.9Total

1309

903

69.1

154

17.1

a Number

of objects with z <1.

b Fraction of identi?ed objects with z <1.

c 7of the 8objects with R <22an

d z >1

are QSOs.

Table 3.BM Spectroscopic Sample Completeness and Contamination R Mag Attempted

Identi?ed %Identi?ed

Interlopers a

%Interlopers b

19.0?22.0c 77100.000.022.0?22.511100.000.022.5?23.022100.000.023.0?23.5211781.01 5.823.5?24.0382976.3310.324.0?24.5674059.737.524.5?25.0301756.700.025.0?25.521838.100.0Total

187

121

64.7

7

5.8

a Number

of objects with z <1.

b Fraction of identi?ed objects with z <1.

c 6of the 7objects with R <22an

d z >1are

QSOs.

Fig. 3.—LRIS-B spectrum of Q1307-BM1163,a bright (R =21.67,G =21.87,U n =22.22)galaxy at z =1.411.The spectral resolution is ~5?A FWHM.We have deliberately shown this spec-trum before ?ux calibration to illustrate how the count rate varies over the typical wavelength range spanned by LRIS-B.Like nearly all of the spectra obtained in the survey,this is the sum of three 1800s exposures.The most prominent interstellar absorption and emission lines used to measure galaxy redshifts between z =1.4and 2.5are indicated below the spectrum.

brighter BM and BX galaxies chosen to span the range of redshifts and spectral properties seen among our sample.Broadly speaking,the far-UV spectra of color-selected z ?1.4?2.5galaxies resemble those of Lyman break galaxies at z ~3in terms of continuum slope and spec-tral lines seen in emission and absorption.There is some evidence to suggest that the Lyman αline appears less frequently in emission at these redshifts than at z ?3:

while 33%of the z ~3LBGs in the survey by Steidel et al.(2003)have no measurable Lyman αemission,the corresponding fraction in the current sample of BM and BX galaxies at z >1.7(such that Lyman αfalls long-ward of ~3300?A )is 57%.It remains to be established whether this is due to a subtle selection e?ect or to real redshift evolution in the escape fraction of Lyman αpho-tons.We intend to address this issue in the future with simulations which are beyond the scope of this paper.Excluding QSOs which were already known in our sur-vey ?elds (these QSOs are all brighter than R ?20),we have identi?ed 21QSOs and seven narrow-lined AGN among the sample of 863objects with z > 1.This AGN fraction of 3.2%is essentially the same as that deduced by Steidel et al.(2002)for LBGs at z ?3.However,we caution that we have not yet quanti?ed the AGN selection function imposed by our color criteria and spectroscopic follow-up,so that the above estimate is only indicative at this stage.On the other hand,it is encouraging that the redshift distribution of the AGN (both BX and BM)is similar to that of the galaxies: z AGN =2.31±0.48and z GAL =2.14±0.37),perhaps suggesting that di?erential selection e?ects may not be too severe.

3.2.Near-Infrared Spectroscopy

In parallel with the LRIS-B optical spectroscopy,we have been observing a subset of the BX and BM galax-ies in the near-IR,targeting in particular the H αand [N II ]λλ6548,6583emission lines in the H and K -band.

Star-forming galaxies in the Redshift Desert

7

Table 4.Example BX/BM Galaxies

Name α(J2000)δ(J2000)R G ?R U n ?G z em a z abs b Q1307-BM116313:08:18.06+29:23:19.221.660.200.35··· 1.409GWS-BX48914:17:20.41+52:33:18.322.830.010.26 1.557 1.557HDF-BX144312:36:44.87+62:18:37.923.330.310.57··· 1.683Q2343-BX36423:46:14.97+12:46:53.921.960.310.71··· 1.797Q1623-BX39216:25:46.56+26:45:52.323.230.110.54··· 1.878HDF-BMZ114812:36:46.15+62:15:51.123.380.200.29 2.053 2.046HDF-BX165012:37:24.11+62:19:04.723.240.180.74 2.100 2.093Q1623-BX47016:25:52.80+26:43:17.522.800.200.93 2.193 2.182Q2343-BX43623:46:09.07+12:47:56.023.070.120.47 2.332 2.326Q1623-BX27416:25:38.20+26:45:57.123.230.250.89 2.415 2.405HDF-BX1485

12:37:28.12

+62:14:39.9

23.29

0.35

0.96

···

2.551

a Redshift

of the Lyman αemission line,when observed.

b Average redshift de?ned by the interstellar absorption

lines.

Fig. 4.—Examples of portions of the spectra of galaxies in

the redshift range 1.5<~z <~

2.5.The spectra have been shifted into the rest frame and normalized to unity in the continuum.The strongest spectral features in the spectra of star forming galaxies at these redshifts are indicated at the top of each panel.The galaxies shown here are selected from the brightest ~10%of the spectro-scopic sample of 863BX/BM galaxies obtained to date.Typical galaxies in the sample are ~1mag fainter,and their spectra have S/N lower by a factor of 2–

3.All exposures were 5400s.

Initial results from this aspect of the survey were re-ported by Erb et al.

(2003).

Fig.4.—(Continued).

Brie?y,we use the near-infrared echelle spectrograph (NIRSPEC)on the Keck II telescope (McLean et al.1998)with a 0.76′′×42′′entrance slit and medium-resolution mode.In the K -band,this set-up records a ~0.4μm wide portion of the near-IR spectrum at a dis-persion of 4.2?A per 27μm pixel;the spectral resolution

is ~15?A FWHM (measured from the widths of emission

lines from the night sky).In the H -band,~0.29μm are recorded at a dispersion of 2.8?A per pixel and ~10?A

8Steidel et al. resolution.The42′′length of the rotatable slit is nor-

mally su?cient to include two galaxies by choosing the appropriate position angle on the sky.Individual expo-

sures are900s and we typically take between two and

four exposures per galaxy(or pair of galaxies),o?setting

the targets along the slit by a few arcseconds between ex-posures.The data are reduced and calibrated with the procedures described by Erb et al.(2003).

This near-IR spectroscopy has several goals,most of

which are complementary to those achievable with the

optical spectra described above:(1)Determine the red-

shifts of the nebular emission lines which,to our knowl-

edge,give the closest approximation to the systemic red-

shifts of the galaxies.This is particularly important for examining the connection between the galaxies and the absorption lines seen along nearby QSO sight-lines,since uncertainties in the systemic galaxy redshifts are reduced from~150?200km s?1(see Adelberger et al.2003)to ~20?30km s?1;(2)Measure the velocity dispersion of the ionized gas and look for evidence of ordered motions, such as rotation,in order to estimate the galaxies’dy-namical masses(see Erb et al.2003);(3)Determine the metallicity of the H II regions from consideration of the [N II]/Hαratio;(4)Compare the star formation rate de-duced from the Hαluminosity with that from the far-UV continuum.Initial results relevant to these topics were presented by Erb et al.(2003);in§4we brie?y touch on some of them again.More extensive results from the near-IR spectroscopic follow-up will be presented else-where.

4.THE SPECTRAL PROPERTIES OF UV-SELECTED

GALAXIES AT1.4

In this section we illustrate the astrophysical informa-tion that can be deduced from the analysis of the rest-frame far-UV spectra of galaxies in the‘redshift desert’, using Q1307-BM1163(Fig.3)as an example.This is ad-mittedly one of the brightest galaxies discovered in the survey to date(R=21.66),and the spectrum repro-duced in Fig.3is of higher signal-to-noise ratio than most.However,it must be borne in mind that this spectrum was recorded with only three1800s exposures. Data of similar quality can be secured for large numbers of galaxies in the sample with integrations of~10hours, and such observations are well underway.The character of the spectrum of Q1307-BM1163is similar to those of many other BX and BM objects;thus we expect that our conclusions from its analysis should be applicable to at least a subset of the galaxies at1.4

4.1.Integrated Stellar Spectra

In the far-UV spectra of star forming galaxies we see the integrated light of young stars of spectral types O and B;such spectra are most e?ectively analyzed with popu-lation synthesis models such as Starburst998(Leitherer et al.1999)which generally provide a good match to the observed spectral characteristics at high(e.g.Pettini et al.2000;de Mello,Leitherer,&Heckman2000),as well as low(Leitherer2002),redshifts.This is also the case for Q1307-BM1163.In Fig.5we compare our LRIS-B spec-trum of this galaxy with that calculated by Starburst99 8Available

from https://www.360docs.net/doc/ad11585159.html,/science/starburst99/for a continuous star formation episode which has been on-going for100million years,with solar metallicity and a power law IMF with Salpeter(1955)slopeα=2.35. The spectrum of Q1307-BM1163has been reduced to the rest frame at redshift z HII=1.411(see§4.4)and normal-ized to the continuum;continuum windows were selected with reference to the Starburst99model spectrum.

Fig. 5.—Comparison between the observed far-UV spectrum of Q1307-BM1163(black histogram),normalized to the contin-uum and reduced to rest-frame wavelengths,and the simplest model spectrum produced with the population synthesis code Star-burst99,one which assumes continuous star formation,solar metal-licity,and a Salpeter IMF(green or gray histogram).The model’s match to the data is remarkably good—the features which are not reproduced in the Starburst99composite stellar spectrum are in most cases interstellar absorption lines.

This simplest of models—100million years is the age beyond which the far-UV model spectrum no longer changes with time—is an excellent match to the inte-grated stellar spectrum of Q1307-BM1163.The low-contrast blends of photospheric lines are reproduced very well,and most of the features which di?er between the two spectra in Fig.5are interstellar absorption lines. Starburst99makes no attempt to reproduce interstellar features;it is often the case that these lines are stronger in the observed spectra of star forming galaxies than in the models because the latter use empirical libraries of stellar spectra of relatively nearby OB stars,whereas the observations sample much longer pathlengths through a whole galaxy.

Figure6shows the comparison on an expanded scale, centered on the C IVλλ1548,1550doublet which is a blend of P-Cygni emission/absorption from the expand-ing atmospheres of massive stars,and narrower inter-stellar absorption.The dependence of mass-loss rate on both stellar luminosity and the ionization parameter that is modulated by stellar temperature makes the C IV P-Cygni pro?le most pronounced in the most massive stars (e.g.Walborn,Nichols-Bohlin,&Panek1985).Thus, its strength relative to the underlying continuum light, produced collectively by all of the O and early B type stars,is very sensitive to the slope and upper end cut-o?of the IMF,as illustrated in Fig.6.Even relatively minor changes inαand M up alter the appearance of the P-Cygni pro?le:excluding stars with masses M>50M⊙reduces both emission and absorption components(mid-dle panel of Fig.6),while increasing the proportion of stars at the upper end of the IMF,by changingαfrom 2.35to1.85,over-produces the P-Cygni feature(bottom panel of Fig.6).The best overall agreement(from among these three models)is obtained with a standard Salpeter

Star-forming galaxies in the Redshift Desert

9

Fig. 6.—Sensitivity of the C IV P-Cygni pro?le to the upper end of the IMF.Black histogram:portion of the observed spec-trum of Q1307-BM1163;green (or gray)histogram:model spectra produced by Starburst99with,respectively,a standard Salpeter IMF (top panel),an IMF lacking stars more massive than 50M ⊙(middle panel),and an IMF ?atter than Salpeter (bottom panel).These changes were deliberately chosen to be relatively small,to illustrate the fact that it is possible to discriminate between them on the basis of even a relatively short exposure spectrum of this galaxy.Overall,there is no evidence for a departure from the stan-dard Salpeter IMF at the upper end of the mass distribution of stars in Q1307-BM1163.

IMF (top panel).

To a lesser degree,the relative strength of the C IV P-Cygni pro?le is also sensitive to other parameters,in particular to the metallicity (Leitherer et al.2001),since metal-poor stars experience lower mass-loss rates,and to age-dependent dust extinction (Leitherer,Calzetti,&Martins 2002).While one could imagine contrived scenarios in which all of these e?ects (IMF,metallicity,and dust obscuration)somehow balance each other,the most straightforward conclusion from the comparisons in Fig.6is that the metallicity of the early-type stars in Q1307-BM1163is close to solar,and that the youngest stars do not su?er,overall,signi?cantly higher extinc-tion than the whole OB population.Evidently,at a red-shift z =1.411,which corresponds to a look-back time of ~10Gyr,Q1307-BM1163had already evolved to a stage where its young stellar population closely resembled the Population I stars of the Milky Way,at least in their spectral characteristics.

4.2.Metallicity

Stellar Abundances

While we have concluded that the metallicity of Q1307-BM1163is likely to be close to solar,the wind lines are not ideal for abundance measurements because they re-spond to several other parameters,as explained above.On the other hand,the far-UV spectrum of star form-ing galaxies is so rich in stellar photospheric lines,as can be readily appreciated from Fig.5,that it is worthwhile considering whether any of these can be used as abun-dance indicators.Since all of these features are blends of di?erent lines,this question is best addressed with spectral synthesis techniques.For example,Leitherer et al.(2001)have used Starburst99to show that the blend of Si III λ1417,C III λ1427,and Fe V λ1430,which they de?ne as the ‘1425’index,becomes stable after ~50Myr (that is,its strength no longer depends on age in a continuous star formation episode)and its equivalent width decreases by a factor of ~3?4,from W 1425?1.5?A to ~0.4?A ,as the metallicity of the stars drops from Milky Way to Magellanic Cloud values,that is from ~solar to ~1/4solar.In Q1307-BM1163we measure W 1425?1.2?A [adopting the same continuum normalization as Leitherer et al.(2001)],which suggests a near-solar metallicity.

Very recently,Rix et al.(in preparation)have explored the possibility of extending this type of approach to other photospheric blends and to a wider range of metallicities.These authors have identi?ed a blend of Fe III lines be-tween 1935?A and 2020?A ,and de?ned a corresponding ‘1978’index which is potentially very useful for two rea-sons.First,it is considerably stronger than ‘1425’index of Leitherer et al.(2001)and can thus be followed to lower metallicities.Speci?cally,at metallicity Z =Z ⊙,W 1978?5.9?A ,or ~4×W 1425,and even at the low-est metallicity considered by Rix et al.,Z =1/20Z ⊙,W 1978?2?A ,which is greater than W 1425at Z =Z ⊙.Second,this index is in a ‘clean’region of the spectrum,where there are no strong interstellar,nor stellar wind,lines to complicate its measurement.In Q1307-BM1163we measure W 1978?6.2?A which again implies that the metallicity of the young stars is close to solar.

The measurement of stellar abundances from UV spec-tral indexes is a technique which is still very much under development.It would be premature,for example,to use the above values of W 1425and W 1978to draw con-clusions concerning the relative abundances of di?erent elements.Nevertheless,these initial indications are cer-tainly promising and raise the possibility that,once the indices are calibrated with independent abundance mea-sures,it may be possible to determine stellar abundances to within a factor of ~2for large numbers of high red-shift galaxies from their rest-frame UV spectra,even if of only moderate signal-to-noise ratios.

Interstellar Gas Abundances

The most obvious way to calibrate the UV spectral in-dices is with reference to nebular abundances determined from the familiar rest-frame optical emission lines from H II regions.While nebular diagnostics generally apply to di?erent elements (mostly oxygen),to a ?rst approx-imation they should give the same ‘metallicity’as the young stars which have recently formed out of the inter-stellar gas seen in emission.This is one of the scienti?c motivations to obtain near-IR spectra of a subsample of

10

Steidel et al.

BX and BM galaxies (§

4.3).

Fig.7.—Portion of the NIRSPEC H -band spectrum of Q1307-BM1163,obtained with a 1800s exposure through the 0.57arcsec slit.Note that we detect the rest-frame optical continuum from the galaxy,as well as the emission lines indicated.The feature marked ‘n.s.’is a residual from the subtraction of a strong sky line.

Figure 7shows a portion of the H -band spec-trum of Q1307-BM1163encompassing the H α,[N II ]λλ6548,6583,and [S II ]λλ6716,6731emis-sion lines.After subtracting the faint continuum,and ?tting H αand [N II ]λ6583with Gaussian pro?les,we measure a ?ux ratio [N II ]/H α=0.22±0.06(1σrandom error).Denicol′o ,Terlevich &Terlevich (2002)have shown that,in a statistical sense,this ratio scales with the oxygen abundance.Adopting the recent calibration by Pettini &Pagel (2004):

12+log(O /H)=8.90+0.57×N 2

(3)

where N 2≡log([N II]λ6583/H α),we deduce 12+log(O /H)=8.53±0.25[the error includes both the random error in our measurement of the N 2in-dex and the ±0.2dex accuracy of the N 2calibrator (at the 68%con?dence level)].This value of (O/H)is consistent,within the errors,with the the most re-cent estimates of the abundance of oxygen in the Sun,12+log(O /H)=8.66±0.05(Allende-Prieto,Lambert,&Asplund 2001;Asplund et al.2003),and in the Orion nebula,12+log(O /H)=8.64±0.06(Esteban et al.1998;2002).It would also be of interest to mea-sure element abundances in the cool interstellar medium which produces the numerous absorption lines indicated in Fig.3.This,however,will require data of higher spec-tral resolution,giving access to weaker absorption lines which are not saturated.In any case,such an analysis would only yield relative,rather than absolute,abun-dances because the Ly αabsorption line falls at 2931?A ,below the atmospheric cut-o?.

Summarizing the results of §5.2,we have estimated the metallicity of Q1307-BM1163from stellar wind lines,stellar photospheric absorption lines,and emission lines from ionized gas,and all these di?erent indicators con-cur in pointing to an approximately solar abundance.Preliminary results from our on-going near-IR spectro-scopic survey indicate that galaxies with Z ?Z ⊙are not unusual at redshifts z =1.4?2.5,at least among the brighter BX and BM objects.In Q1307-BM1163we seem to have an example of a galaxy which had already reached solar metallicity ~10Gyr ago while continuing to support an extremely vigorous star formation,at a rate of more than ~30M ⊙yr ?1(see §5.3).This mode

of star formation is very di?erent from that undergone by the Milky Way disk at any time in its past (Freeman &Bland-Hawthorn 2002)and it is likely that the de-scendents of objects like Q1307-BM1163are to be found among today’s elliptical galaxies and bulges of massive spirals.

4.3.Star Formation Rate

From the spectrum reproduced in Fig.7,we measure an H α?ux F H α=(2.9±0.1)×10?16erg s ?1cm ?2.In the adopted cosmology,this implies an H αluminosity

L H α=(3.5±0.1)×1042h ?270erg s ?1

.With Kennicutt’s (1998)calibration

SFR (M ⊙yr ?1)=7.9×10?42L H α(erg s ?1)

(4)

we then deduce a star formation rate SFR =(28±1)M ⊙yr ?1.

An independent estimate of the star formation rate is provided by the UV continuum at 1500?A SFR (M ⊙yr ?1)=1.4×10?28L 1500(erg s ?1Hz ?1)(5)(Kennicutt 1998);both eq.(4)and (5)assume contin-uous star formation with a Salpeter slope for the IMF from 0.1to 100M ⊙.At z =1.411,1500?A corresponds to an observed wavelength of 3617?A ,close to the center of the bandpass of our U n ?lter (Steidel et al.2003).Then,the measured U n =22.22(AB)of Q1397-BM1163corresponds to SFR =30M ⊙yr ?1.The good agreement between the values of SFR deduced from the H αand the far-UV continuum luminosities is not unusual for bright UV-selected galaxies (Erb et al.2003).To a ?rst approx-imation,it presumably indicates that the UV continuum does not su?er a large amount of extinction by dust (see the discussion of this point by Erb et al.2003).

4.4.Kinematics of the Interstellar Medium

From the Gaussian ?t of the H αline in Q1307-BM1163we deduce a one-dimensional velocity dispersion of the ionized gas σ=126km s ?1and a redshift z HII =1.4105.The former is close to the mean σ =110km s ?1of the sample of 16(mostly BX)galaxies at z HII =2.28an-alyzed by Erb et al.(2003).It is however signi?cantly higher than the mean σ =78km s ?1of the 16Lyman break galaxies at z ?3studied by Pettini et al.(2001),and in fact exceeds the highest value found in that sam-ple,σ=116±8km s ?1.Erb et al.(2003)commented on the apparent increase in velocity dispersion of star form-ing galaxies between z ~3and ~2;it will be interesting to explore such kinematic evolution in more detail and to lower redshifts once our near-IR survey of BX and BM galaxies is more advanced.

Another important aspect of the internal kinematics of LBGs are the large velocity di?erences which are nearly always measured between interstellar absorption lines,nebular emission lines,and Ly αemission.If we take the nebular lines to be at the systemic redshifts of the galax-ies,the interstellar absorption lines and Ly αare respec-tively blue-and red-shifted by several hundred km s ?1(Pettini et al.2001;Shapley et al 2003).In this respect also Q1307-BM1163is no exception–the numerous in-terstellar lines in the spectrum have centroids that are blue-shifted by 300km s ?1with respect to the redshift de?ned by the H αemission line,and have velocity widths

Star-forming galaxies in the Redshift Desert

11

of ~650km s ?1;both of these values are quite typi-cal [cf.Pettini et al.2001,2002,Shapley et al.2003].)This kinematic pattern is most simply explained as being due to large-scale out?ows from the galaxies,presumably powered by the energy deposited into the ISM by the star formation activity.The resulting ‘superwinds’are likely to have a far-reaching impact on the surrounding intergalactic medium and are probably at the root of the strong correlation between LBGs and IGM metals found by Adelberger et al.(2003).

Indeed,a major motivation for pursuing galaxies in the ‘redshift desert’is to investigate how the galaxy-IGM connection evolves from z ~3to lower redshifts.We can already address one aspect of this question by ex-amining the velocity di?erences between interstellar ab-sorption,nebular emission,and Ly αin 27BX and BM galaxies which we have observed at H αwith NIRSPEC and which also have LRIS-B spectra of su?ciently high quality to measure absorption and (when present)Ly αredshifts with

con?dence.

Fig.8.—Velocity o?sets of the interstellar absorption lines (blue-hatched histogram)and,when present,Ly αemission (red-hatched histogram)relative to the systemic redshifts de?ned by the nebular emission lines (vertical long-dash line).The top panel shows the results for z ~3LBGs presented by Pettini et al.(2001);the bottom panel shows results for a sub-sample of 27BX and BM galaxies which have been observed with NIRSPEC at H αand also have high quality LRIS-B spectra.The velocity o?sets seen in BX and BM galaxies are similar,in both magnitude and distribution,to those typical of LBGs at z ~3.

The results of this exercise are shown in Fig.8.For these 27galaxies,the mean velocity o?set of the inter-stellar lines is ?v IS ,abs =?175±25km s ?1.For the ten galaxies among them which exhibit detectable Ly αemission, ?v Ly α =+470±40km s ?1.These values,and their observed distributions,are very similar to those found at z ~3by Pettini et al.(2001)and Shapley et al.(2003)—a comparison with the data presented in Pettini

et al.(2001)is included in Fig.8.Thus,the superwinds generated in active sites of star formation appear to have similar kinematic characteristics from z ~3down to at least z ~1.5,even though their parent galaxies may be-come more massive over this redshift interval,if the hints provided by the nebular line widths have been correctly interpreted (Erb et al.2003).These tentative conclu-sions make it all the more interesting to investigate how the galaxy-IGM connection may evolve to lower redshifts.

5.NEAR-IR PHOTOMETRIC PROPERTIES

We end with a brief comment on the K s -band mag-nitudes and R ?K s colors of UV-selected star-forming galaxies in the ‘redshift desert’.In 2003June we initiated a program of deep K s -band photometry of these galaxies using the Wide Field Infrared Camera (WIRC)on the Palomar 5.1m Hale telescope.The camera employs a 2048×2048Rockwell HgCdTe array,and has a ?eld of view of 8′.7×8′.7with a spatial sampling of 0′′.25per pixel.With ~12hour integrations,the images reach 5σphotometric limits (in 2arcsec diameter apertures)of K s ?21.7,su?ciently deep to detect ~85%of the galaxies with spectroscopic redshifts.In Fig.9we show initial results from the ?rst three WIRC pointings (in the Q1623+27,Q1700+64,and Q2343+12?elds,where only BX candidates have so far been observed spectro-scopically);while preliminary,these data already allow a coarse comparison with other faint galaxy samples.

As can be seen from the left-hand panel of Fig.9,~10%of UV-selected galaxies at z ~1.9?2.5are brighter than K =20;thus we expect a relatively small overlap between the BX population and the high redshift tail of published K -selected samples (Cohen et al.1999ab;Cimatti et al.2002;Daddi et al.2003).However,going only one magnitude deeper to K ~21should pick up an appreciable fraction of the spectroscopic BX sample.It is intriguing to ?nd a clear di?erence in the R ?K s colors of BX galaxies at z ~2.2and LBGs at z ~3(Shapley et al.2001),in the sense that the former are signi?cantly redder than the latter,on average (see right-hand panel of Fig.9).For a given star formation history and extinction,galaxies at z ~2.2and z ~3.0would have identical R ?K s color (the k-corrections are iden-tical in the two bands for model SEDs that ?t the ob-served colors),meaning that whatever the cause,there is a signi?cant intrinsic color di?erence in the UV-selected galaxies in the two redshift intervals.Since the K s and R ?lters straddle the age sensitive Balmer break at these redshifts,one interpretation of the redder colors would be that,on average,star formation has been proceeding for longer periods of time in the z ~2galaxies as compared to similarly-selected galaxies at z ~3.If the z ~3LBGs continued to form stars during the ~800Myr interval between the two epochs,such reddening of the R ?K s color would be expected.However,we caution that there may be other reasons for the o?set evident in Fig.9,possibly related to higher dust extinction and/or larger contamination of the broad-band colors with line emis-sion (most of the z ~2galaxies would have H αin the K band,whereas most of the z ~3galaxies had [OIII]and H βin the K band–although these tend to have roughly the same equivalent widths for the sub-samples that have been spectroscopically observed in the near-IR),as well

12Steidel et

al.

Fig.9.—Left Panel:Color-magnitude diagram for the283BX galaxies with K s band measurements to date from the Q1623,Q1700, and Q2343?elds(blue points);the5σdetection limit is K s?22.The red points are the sample of108galaxies at z~3from Shapley et al.2001.The dotted line indicates galaxies with R=25.5,the limit for both spectroscopic samples.Approximately15%of the BX galaxies with spectroscopic redshifts were not signi?cantly detected in the images;most of these have optical magnitudes R>25.0.Right Panel:The distributions of R?K s colors for the same BX sample at z =2.2(blue)as compared to that of the z~3LBGs studied by Shapley et al.(2001).The BX galaxies are signi?cantly redder in their optical/IR colors despite having been selected to span the same range of UV color as the LBGs(see text for discussion).

as to the way the di?erent samples were selected9At

present,both the observed increase in one-dimensional

line widths and the reddening of the optical/IR colors are

qualitatively consistent with a signi?cant overall increase

in stellar mass among at least a substantial fraction of

the UV-selected populations between z~2and z~310.

Ongoing near-IR spectroscopy targeting BX(and even-

tually BM)galaxies with red R?K s colors,and full

modeling of the observed optical/IR SEDs using popula-

tion synthesis,will allow us to reach?rmer conclusions

on the cause of the observed evolution.

6.SUMMARY

The main thrust of this paper has been to show that

the redshift interval1.4<~z<~2.5,which has so far been

considered hostile to observations,is in fact ripe for scien-

ti?c exploration.Galaxies in what used to be called the

‘redshift desert’can in reality be easily identi?ed from

their broad-band U n G R colors and can be studied very

e?ectively with a combination of ground-based optical

and near-IR spectroscopy,provided the optical instru-

mentation has high e?ciency in the near-UV.It is then

possible to cover most of the rest-frame UV and optical

spectra of these galaxies,from Lyαto Hα,and gain ac-

cess to a wider range of important spectral diagnostics

than is usually available for the so-far better studied Ly-

man break galaxies at z~3(or even galaxies at z<~1).

In addition,because the luminosity distances are lower,

the galaxy luminosity function can be probed~0.6?1.1

magnitudes deeper than is the case at z~3,and there

are more galaxies brighter than R~23.5whose spectra

can be recorded at high spectral resolution and S/N for

9The z~3sample of Shapley et al.2001over-sampled the

optically brightest galaxies and galaxies having the reddest UV

colors relative to a random R-selected spectroscopic sample,so that

the two samples may not be exactly analogous.Possible di?erential

selection e?ects will be better quanti?ed in future work.

10Very recently,an increase in the stellar mass of UV-selected

galaxies has been inferred between z~4and z~3in the GOODS-

S?eld,using similar arguments;Papovich et al.2003

further,detailed,investigation.

We have presented the?rst results from our survey for

galaxies at1.4<~z<~2.5,in seven?elds totaling~0.5

square degrees;?ve of the?elds were chosen because they

include one or more bright background QSOs.Over this

area,we have identi?ed thousands of candidates which

satisfy newly de?ned BX and BM color selection criteria

and have spectroscopically con?rmed863of them to be

at z>1;692are in the targeted z=1.4?2.5range.

The rest-frame UV spectra of BX and BM galaxies are

very similar to those of LBGs,with a rich complement

of stellar and interstellar lines.There seem to be propor-

tionally fewer galaxies with detectable Lyαemission,but

we have not established yet whether this is related to the

color selection cuts we have adopted or is a real e?ect.

The fraction of faint AGN within this sample is3.2%,

essentially the same as in the LBG sample at z~3.

We have illustrated the range of physical properties

which can be investigated with the combination of rest-

frame UV and optical spectroscopy using,as an exam-

ple,one of the brightest objects in the sample,Q1307-

BM1163.This z=1.411galaxy is forming stars at a

rate~30M⊙yr?1and with a Salpeter slope at the up-

per end of the IMF.Various abundance indicators,based

on stellar wind and photospheric lines,show that the

metallicity of the youngest stars is close to solar;this

is in good agreement,as expected,with the solar abun-

dance of oxygen in its H II regions implied by the high

[N II]/Hαratio.We draw attention to the potential for

abundance determinations of newly developed UV spec-

tral indices which measure the strengths of blends of pho-

tospheric lines;once properly calibrated,these indices

may allow the metallicities of large numbers of galax-

ies to be approximately assessed from spectra of only

moderate signal-to-noise ratios.Viewed at a look-back

time of~10Gyr,Q1307-BM1163is clearly turning gas

into stars,and enriching its ISM with their products,at

a much faster rate than that experienced by the Milky

Way disk at any time in its past.We speculate that by

Star-forming galaxies in the Redshift Desert13

z=0it will have become an elliptical galaxy or perhaps the bulge of a massive spiral.

The galactic-scale winds which are commonly seen in LBGs at z~3are still present in star-forming galaxies at later epochs,generating velocity di?erences of sev-eral hundred km s?1between absorption lines produced by the out?owing ISM and the emission lines from the star-forming regions.The typical velocity di?erence of ~200?300km s?1between emission and absorption does not seem to change between z~3and~2,even though there is evidence that the velocity dispersion of the ionized gas increases by~40%between these two epochs,possibly re?ecting a growth in the typical galaxy mass.

Initial results from deep K s-band imaging of spectro-scopically con?rmed BX galaxies show that a larger pro-portion of the z~2galaxies have relatively red R?K s colors as would be expected if the z~2galaxies have been forming stars at close to their observed rate for a longer period of time than their z~3counterparts(and hence would have correspondingly larger stellar masses). Approximately10%of the BX spectroscopic sample are very bright in the near-IR(K s<20)and thus would be expected to comprise part of the high-redshift tail of current K s-selected spectroscopic surveys.

There are several other issues of interest which can be addressed with a large sample of galaxies at these intermediate redshifts,such as their luminosity function and integrated star formation rate density,the impact of the‘superwinds’on the galaxies’environment and the intergalactic medium at large,and the relationship between galaxy morphology and kinematics.We intend to consider these topics in the future,as our survey progresses beyond the initial stages which have been the subject of this paper.

We would like to thank the rest of the team responsi-ble for the design,construction,and commissioning of the LRIS-B instrument and the upgraded CCD cam-era,particularly Jim McCarthy,John Cromer,Ernest Croner,Bill Douglas,Rich Goeden,Hal Petrie,Bob We-ber,John White,Greg Wirth,Roger Smith,Keith Tay-lor,and Paola Amico.We have bene?ted signi?cantly from software written by Drew Phillips,Judy Cohen, Patrick Shopbell,and Todd https://www.360docs.net/doc/ad11585159.html,S,AES,MPH, and DKE have been supported by grants AST00-70773 and AST03-07263from the US National Science Foun-dation and by the David and Lucile Packard Foundation. NAR has been supported by an NSF Graduate Fellow-ship.KLA acknowledges support from the Harvard So-ciety of Fellows.

APPENDIX

THE LRIS-B SPECTROGRAPH

The blue channel of the LRIS spectrograph(LRIS-B)was anticipated as an upgrade to the LRIS instrument(Oke et al.1995)from the initial planning stages of the?rst-light Keck Observatory instrumentation suite,and was begun as a project in1995.LRIS-B was installed on the LRIS instrument during the summer of2000,and saw?rst-light in 2000September.After the installation of LRIS-B,the spectrograph now provides two independent,optimized imaging spectrograph channels which simultaneously observe the same5′.5by8′.0?eld of view in two di?erent wavelength ranges through the use of a dichroic beamsplitter.Figure10illustrates a section view of the LRIS instrument that is color-coded to indicate the light paths through the red and blue channels.The red side of the instrument maintains identical performance to the original LRIS spectrograph,and(as always)employs re?ection gratings to disperse red light that is passed by the beamsplitter.The blue channel uses UV/blue optimized grisms as dispersers.An early technical description of LRIS-B is given by McCarthy et al.(1998);more recent information is available at https://www.360docs.net/doc/ad11585159.html,/realpublic/inst/lris/lrisb.html.

In brief,the LRIS-B upgrade involved the installation of a UV/blue-optimized spectrograph camera,replacement of the camera bulkheads for both the existing spectrograph(LRIS-R)and LRIS-B,and installation of independent carousels and transport mechanisms to store and deploy blue-side dichroics,grisms,and?lters into the beam.In addition,all of the LRIS electronics were enclosed inside a glycol-cooled compartment for better control of the thermal environment at the Cassegrain focus of Keck I,and the overall instrument software was re-con?gured to accomodate the operations of both blue and red side mechanisms and the simultaneous operation of two independent detector trains.Each of the optical elements(moving from left to right on?gure10,the dichroic,grism,and?lter)can be changed in less than60seconds,and in particular a grism or?lter can be deployed or retracted from the beam in ~30seconds,enabling rapid switching from imaging to spectroscopic mode which greatly improved the e?ciency of slitmask alignment with LRIS.

The instrument currently allows the user to select one of5possible dichroic beam-splitting optics:a?at aluminized mirror,which sends all of the light into the blue camera,and dichroics that divide the beam at4600?A(d460),5000?A(d500),5600?A(d560),or6800?A(d680).The re?ectivity of the dichroics for wavelengths shortward of the design cuto?are generally better than96%(i.e.,superior to the re?ectivity of aluminum),with transmittance better than 90%longward of the cuto?.

The LRIS-B?lters,which include publicly available u’,B,G,and V,are generally only used for imaging programs or for slitmask alignment https://www.360docs.net/doc/ad11585159.html,er-supplied?lters(which must be larger than~7.5by8inches in order not to vignette the parallel beam)may be installed in the?lter carousel.With suitable choice of dichroic and red-side?lter, it is possible to image in two passbands simultaneously.

There are currently4grisms available with LRIS-B:a1200line/mm grism blazed at3400?A which can cover the wavelength range3000-4300?A with a resolution of~3600(with a0.7′′slit),a600line/mm grism blazed at4000?A (R?1600),a400line/mm grism blazed at3400?A optimized for the highest throughput at wavlengths<~4000?A (R~1000),and a300line/mm grism blazed at5000?A(R~900).With suitable choice of dichroic beamsplitter and

14Steidel et al.

Fig.A10.—Section view of the LRIS instrument after addition of the LRIS-B components.The light path is illustrated with incoming rays from the left.The light is collimated by the mirror(lower right),and then split into blue and red beams at the dichroic.The LRIS-B optical elements are shaded in blue;from left to right,they are the dichroic beam splitter,the grism,the?lter,the camera,and the CCD dewar.

LRIS-red side grating,it is possible to cover all wavelengths from the atmospheric cuto?near3000?A to1μm with high e?ciency.

Because both the red and blue channels of the spectrograph share the same paraboloidal re?ecting collimator(see ?gure10),the original protected silver coating on the0.53m diameter collimator mirror was replaced by the hybrid coatings developed by the group at Lawrence Livermore National Laboratory(Thomas&Wolfe2000)which provide re?ectivity of>~95%at all wavelengths from3100?A to1μm.

The light path for LRIS-B goes as follows:a slitmask(or long slit)is deployed in the telescope focal plane,at a position centered6′o?axis.After passing into the intrument and through a?eld lens,all light is collimated by the re?ecting collimator.The dichroic beam splitter(placed just in front of the red channel re?ection grating)receives the collimated beam and re?ects wavelengths shortward of the dichroic cuto?into the blue channel,passing light longward of the cuto?directly onto the red side grating(or?at mirror in the case of imaging).The blue light is then dispersed by a selectable grism,after which it passes through a selectable?lter(tilted by about6degrees with respect to the collimated beam to avoid internal re?ections)and into the LRIS-B camera.The camera is all refractive,constructed from12CaFl and fused silica elements with optimized coatings that achieve<0.5%re?ection losses at each of the8 air/glass surfaces.The LRIS-B camera has an aspheric?rst element that is placed slightly o?-axis to correct much of the coma introduced by the parabolic collimator.Tests carried out during the commissioning period con?rmed that the camera achieves images with RMS image diameters better than22μm(0.20′′)over the full8.0′by5.5′?eld of view and from3100to6000?A without the need to re-focus.The camera performs well to~6800?A,beyond which the image quality and throughput of the system deteriorate somewhat;in general,this is not a problem since red light is directed into the red channel of the spectrograph.

Initially,the LRIS-B detector was an engineering-grade SITe2048×2048pixel CCD.This was replaced in2002 June by a science grade mosaic of two EEV(Marconi)2048×4096devices selected to have particularly high near-UV and blue quantum e?ciency.With the new detector,LRIS-B records images and spectra over the full?eld of view of the instrument with the exception of a13′′inter-chip gap that runs parallel to the dispersion direction,and which coincides with the bar that is used to support the slit masks in the slit mask frames.The plate scale at the detector is0.135′′per15μm pixel.

To our knowledge LRIS-B is the only UV/blue optimized faint object spectrograph on an8-10m class telescope. It is particularly worthwhile implementing this type of instrument at a site such as Mauna Kea which,because of its high altitude,has atmospheric opacity in the3100?4000?A range that is20–30%lower at zenith than that at many other observatory sites.Because every optical element has been optimized for the blue and UV,the instrument achieves remarkably high e?ciency.The total spectroscopic system throughput of Keck I+LRIS-B measured during the commissioning run with the science-grade CCD mosaic averages~40%in the3800?6000?A range and25%at 3500?A;the corresponding values for the instrument alone(that is,neglecting telescope losses)are higher by about 30%.The measured instrumental throughput for a number of grism and dichroic combinations is shown in?g11. LRIS-B represents an increase in spectroscopic e?ciency over the original LRIS instrument(now LRIS-R)of a factor

Star-forming galaxies in the Redshift Desert

15

Fig.A11.—The total spectroscopic system throughput of the LRIS-B instrument (excluding slit losses and the telescope)as a function of wavelength for several con?gurations.The 400/3400grism was used in all the observations presented in this paper because it o?ers the highest throughput between 3100and 4000?A —the most important wavelength range for spectroscopic con?rmation in the z =1.4?2.5‘redshift desert’.Note that slightly higher UV throughput using this grism is achieved when a dichroic beam splitter rather than the ?at mirror is used.

of ~2.2at 4000?A and even at 5000?6000?A the throughput gain is ~40%relative to LRIS-R;the gain at wavelengths λ<4000?A is more than a factor of 10.Coupled with the very dark night sky background in the UV-visual range,LRIS-B allows for unprecedented spectral throughput and is optimized for very faint objects at intermediate to low spectral resolution.

At present,there is no atmospheric dispersion corrector (ADC)for the Keck I Cassegrain,so that the full broad-band capabilities of the instrument using multislit masks are not yet realized.However,at the time of this writing an ADC is being designed and contructed at the University of California Observatories,for deployment in late 2004.

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