Lyman-break galaxies at z~5 -I. First significant stellar mass assembly in galaxies that are not simply z~3 LBGs at higher redshift

0.97 MB·English

by  Aprajita Verma

Mon.Not.R.Astron.Soc.000,1–21(2007) Printed5February2008 (MNLATEXstylefilev2.2)
∼
Lyman-break galaxies at z 5 - I. First significant stellar
∼
mass assembly in galaxies that are not simply z 3 LBGs
at higher redshift
Aprajita Verma1,2⋆, Matthew D. Lehnert1, Natascha M. F¨orster Schreiber1,
Malcolm N. Bremer3, Laura Douglas3
1 Max Planck Institut fu¨rextraterrestrische Physik, Postfach 1612, D-85741 Garching, Germany
2 Oxford Astrophysics, Department of Physics, Universityof Oxford, DenysWilkinson Building, Keble Road, Oxford OX1 3RH, UK
7 3 H H Wills Physics Laboratory, Universityof Bristol, Tyndall Avenue, Bristol BS8 1TL, UK
0
0
2 Accepted 2006December 29.Received2006December07;inoriginalform2006October03

n
a
J ABSTRACT

5
2 We determine the ensemble properties of z∼5 Lyman break galaxies (LBGs) se-
lectedasV-banddropoutstoiAB <26.3intheChandraDeepFieldSouthusingtheir
1 rest-frame UV-to-visible spectral energy distributions. By matching the selection and
v performingthesameanalysisthathasbeenusedforz∼3samples,weshowcleardiffer-
5 encesintheensemblepropertiesoftwosamplesofLBGswhichareseparatedby1Gyr
2 inlookbacktime.Wefindthatz∼5LBGsaretypicallymuchyounger(<100Myr)and
017 h∼afveewl×ow10er10stMel⊙laranmdas∼se3s2(0∼M1y0r9Mol⊙d)).tThahnetdhieffierrzen∼ce3cinoumntaesrspiasrtssig(nwihfiiccahntareevteynpiwcahlelny
considering the presence of an older, underlying population in both samples. Such
7
0 youngand moderately massivesystems dominate the luminous z∼5LBGpopulation
/ (&70 per cent), whereas they comprise .30 per cent of LBG samples at z∼3. This
h
result, which we demonstrate is robust under all reasonable modelling assumptions,
p showsa clearchange in the properties of the luminous LBGs between z∼5and z∼3.
-
o Theseyoungandmoderatelymassivez∼5LBGsappeartobeexperiencingtheirfirst
r (few) generations of large-scale star formation and are accumulating their first sig-
st nificant stellar mass. Their dominance in luminous LBG samples suggests that z∼5
a witnesses a period of wide-spread, recent galaxy formation. As such, z∼5 LBGs are
:
v the likely progenitors of the spheroidal components of present-day massive galaxies.
i This is supported by their high stellar mass surface densities, and is consistent with
X
theircorephase-spacedensities,aswellasthe agesofstarsinthe bulgeofourGalaxy
r and other massive systems. With implied formation redshifts of z∼6 - 7, these lumi-
a
nous z∼5 LBGs could have only contributed to the UV photon budget at the end
of reionisation. However, their high star formation rates per unit area suggest these
systems host outflows or winds that enrich the intra- and inter-galactic media with
metals, as has been established for z∼3 LBGs. Their estimated young ages are con-
sistent with inefficient metal-mixing on galaxy-wide scales. Therefore these galaxies
may contain a significant fraction of metal-free stars as has been previously proposed
for z∼3 LBGs (Jimenez & Haiman 2006).
Key words: galaxies:formation-galaxies:evolution-galaxies:high-redshift-galax-
ies: starburst
1 INTRODUCTION
⋆ [email protected] (AV); [email protected]
Oneofthefundamentalopenquestionsincosmologyiswhen
Present affiliation: Laboratoire Galaxies Etoiles Physique et
Instrumentation, Observatoire de Paris, 5 Place Jules Janssen, did galaxies form their first generations of stars? Identify-
92195 Meudon, France (MDL); [email protected] (NMFS); ing and studying such galaxies are key steps towards un-
[email protected](MNB);[email protected](LD) derstandingthephysicalprocesses thatdrivegalaxy forma-
(cid:13)c 2007RAS
2 Verma, Lehnert, Fo¨rster Schreiber, Bremer and Douglas
tion. Probing the formation and early growth of systems the ages of these systems. A strong detection with IRAC
similar to the Milky Way requires observations of galaxies normally confirms the presence of a Balmer break or an
whentheUniverseisstillyoung.Specifically,theoreticalcal- intrinsically more UV luminous system. The derived prop-
culations (Mo & White 2002) predict that the most rapid erties of these high redshift LBGs are fascinating; massive
growth of galaxies with masses comparable to that of the (few-several ×1010M⊙) and strongly star forming systems
MilkyWayoccursatredshiftsofapproximatelyz∼5.Thisis with ages comparable to the age of the Universe at that
notlongaftertheendofthe(complex)reionisationprocess, epoch. The presence of such massive and evolved systems
which recent three-yearWMAPresults indicate was under- at high redshift, in place less than a billion years after the
wayatz∼11(Alvarez et al.2006)andwaslargelycomplete Big Bang, challenges the expectations from bottom-up hi-
by redshifts z∼6-6.5 (Fan et al. 2001; Beckeret al. 2001; erarchical structure formation scenarios (however, also see
Malhotra & Rhoads2004;Fan et al.2005).Estimatesofthe McLure et al. 2006).
ionising photon density in the early Universe suggests that These derived masses and ages of z&5 LBGs are sim-
the UV emission from currently known high redshift galax- ilar to the average properties of LBGs at redshift 3. Fol-
ies, whether star-formation or AGN dominated, is insuffi- lowing thepioneering work of Steidel et al., the last decade
cient to have caused reionisation. However, these galaxies has seen intensive observational and theoretical studies on
must have had an impact on the intergalactic medium at the properties of LBGs at z∼3, providing a wealth of in-
high redshift. Only through comprehensive studies of the formation about this abundant population of UV-bright
physical properties (including masses, star formation rates galaxiesatthisepoch.Thesestar-formation dominatedsys-
andhistories, andclustering strength)ofgalaxies thatwere tems are seen to host strong outflows (Pettiniet al. 2001;
in place at this epoch, can we accurately assess their con- Adelberger et al. 2003; Shapley et al. 2003), typically have
tribution to the mass growth of galaxies like our own, and sub-solar-to-solar metallicities (∼0.3-1Z⊙; Pettini et al.
their effect on the gaseous intergalactic medium (IGM) at 2001; Shapley et al. 2003) and possibly contain a sig-
theend of reionisation. nificant component of metal-free (population III) stars
As part of an ongoing study of high redshift galax- (Jimenez & Haiman 2006). LBGs are shown to be highly
ies, we have investigated the rest-frame UV-optical prop- clustered on both large and small scales, the latter being
erties of candidate LBGs at 4.6.z.6 selected as V- indicative of common halo objects (Giavalisco et al. 1998;
dropouts using the now standard Lyman-break tech- Adelberger et al. 1998; Ouchiet al. 2004b, 2005; Lee et al.
nique (Steidel & Hamilton 1993; Steidel et al. 1995, 1999). 2006). Recent follow-up of z∼3 LBGs with Spitzer has re-
Through spectroscopic confirmation, we have successfully vealed a division in the population between LBGs with
demonstrated the efficacy of this technique for unam- significant dust-attenuated star-forming regions as well as
biguously identifying z∼5 galaxies from deep imaging the UV-emitting ones (Huanget al. 2005). This division is
surveys in the rest-frame UV using 8m-class telescopes also seen for LBGs at z∼1 (Burgarella et al. 2006) with 40
(Lehnert & Bremer 2003). From these data it has been percentbeinginfraredbright.RecentlyReddyet al.(2005)
possible to determine the comoving density (Bunkeret al. have reported on the potential relation between UV- and
2004), the likely contribution to the end of reionisation IR selected populations. Huanget al. (2005) suggest IR-
(Lehnert & Bremer2003),andthefractionofsourceswhich luminous LBGs are the missing link between LBGs and
hostsuper-massiveblackholes(Bremer et al.2004)ofz∼5 sub-mm galaxies and are the progenitors of present-day gi-
LBGs. However, the rest-frame UV data alone are thus far antellipticals(seealsoAdelberger & Steidel2000;Shu et al.
insufficient to accurately constrain the ages, dust content, 2001; Ouchiet al. 2004b; Rigopoulou et al. 2006).
star formation rates and masses of the LBG population at But how do these properties compare to those samples
high redshift. of LBGs at higher redshifts? Do we see an evolution in the
Rather,accurately constrainingtheseparametersrelies properties ofsimilar LBGs at thesetwoepochs? Because of
on well measured rest-frame UV-to-optical SEDs. At z∼5, the small number of sources investigated, and the inhomo-
the rest-frame UV emission from ongoing star-formation is geneity of the selection criteria used, it has not yet been
redshifted into the observed visible, while emission in the possible to consistently compare the derived properties of
rest-framevisibletonear-infraredfromolderstars,diagnos- z&5 LBGs directly with those of Lyman-break galaxies at
tic of longer or earlier periods of star formation, shifts into z∼3. In a preliminary study, Andoet al. (2004) find that
the mid-infrared. Several fields now have excellent multi- unlike z∼3 LBGs (Shapley et al. 2003) the majority of lu-
wavelength data sets from both the Hubble and Spitzer minous z∼5-6 LBGs have weak or absent Lyman alpha
Space Telescopes (HST and Spitzer, respectively), supple- (Lyα) emission and strong low-ionisation absorption lines,
mented by ground-based data sets, forming an ideal basis already indicating key differences between the populations
forselectingandstudyingsamplesofdistantgalaxiesacross at redshift 5 and redshift 3. We complement this compari-
thefull rest-frame UV to visible wavelength range. sonbyanalysingthepropertiesofalargeandreliablesample
The most detailed multiwavelength studies to date of the most luminous LBGs (L>L*) at z∼5 which match
of 5<z<7 LBGs have been centered upon a small frac- theluminositytowhichLBGsat redshift 3havebeenspec-
tion of dropouts that benefit from amplification due troscopically confirmed. This matched selection permits a
to lensing and/or those detected with Spitzer-IRAC direct comparison between theproperties of similarly lumi-
(Egami et al. 2005; Schaerer& Pell´o 2005; Yan et al. 2005; nousLBGsinplaceina∼1.2GyroldUniverseandthoseat
Mobasher et al. 2005; Chary et al. 2005; Eyles et al. 2005; redshift 3, ∼1Gyr later. This difference is longer than the
Dow-Hygelund et al.2005;Yan et al.2006a).Since,atz>5, typical duration of the UV luminous phase of a z∼3 LBG,
the Balmer-break lies between the K and the IRAC pass- thus we are not comparing the same galaxies, but galaxies
s
bands, the IRAC data are highly effective in constraining at two epochs with similar UV-emission characteristics.
(cid:13)c 2007RAS,MNRAS000,1–21
Lyman-break galaxies at z∼5 3
To address these issues, we present the results from an 2.2 Input data
analysis of the properties of a sample of luminous V-band
dropouts selected from the multiwavelength datasets of the For each object we combined the ACS data with publicly
Chandra Deep Field South (CDFS, Giacconi et al. 2001). availablenear-andmid-infrareddatasetstoconstructmulti-
Thisfieldwaschosen asthesouthernfieldoftheGreatOb- wavelength SEDs covering 0.45 to 8µm (Figure 1). The
servatories Origins Deep Survey (GOODS, Dickinson et al. ground-basedJ-andKs-banddataweretakenwithISAAC
2003a), for which public imaging data are available over at the Very Large Telescope (VLT) (Olsen et al. 2006, and
ten bands covering the optical (HST-ACS), near-infrared http://www.eso.org/science/goods/imaging/products.html),
(VLT-ISAAC)andmid-infrared(Spitzer-IRAC)wavelength and the mid-infrared data with Spitzer/IRAC at 3.6,
ranges.Insection2wedescribeourmethodologyandselec- 4.5, 5.6 and 8.0µm (Dickinson et al., in prep. and
tion criteria that define this robust sample of z∼5 LBGs. http://data.spitzer.caltech.edu/popular/goods/, see also
Thestellar evolutionary modelling isdiscussed insection 3. brief descriptions in Yan et al. 2004 and Stark et al. 2006).
Theresultant physicalproperties of mass, ages, star forma-
Whileweusedthepublicly-availablereducedimagesfor
tion rate and extinction of the galaxies in this sample are
theACSand ISAACdata, we performed post-pipeline pro-
detailed in Section 41 and the comparison to the proper-
cessingontheGOODS-S/Spitzerdata(fromepochs1and2)
ties of z∼3 galaxies is presented in Section 5. Unlike sim- creating deep 1′×1′ mosaics centered on each high redshift
ilarly luminous z∼3 LBGs, and other z>5 LBG samples,
candidateusingtheSpitzer/MOPEXpackage.Theresultant
we find our sample to be dominated by young (<100Myr)
drizzled mosaics have had instrumental effects and cosmic
and moderately massive (∼ 109M⊙) galaxies. We discuss rayeventsremoved.ThemosaicswereregisteredtotheACS
theimplications of our findingsin Section 6.
astrometric reference frame using common detections to an
We adopt the following flat cosmology throughout this accuracyof∼0.1”.WeverifiedourIRACmosaicgeneration
paper: H0=70kms−1Mpc−1, Ωm=0.3, ΩΛ=0.7. All magni- and photometriccalibration byperforming thesame proce-
tudesarebasedontheABmagnitudesystem(Oke& Gunn
dure to extract photometry for A and F stars identified in
1983).
the field (Groenewegen et al. 2002). Such stars rarely show
an infrared excess orhavestrong absorption features in the
mid-infrared, therebyenabling accurate predictions of their
mid-infrared emission. The fluxes predicted by black-body
fitstothetemplatesoftheirspectraltypeswerereproduced
2 DATA AND SAMPLE SELECTION
towithin0.1,0.1,0.2and0.3magofouraperturephotome-
2.1 Selection Criteria tryat3.6,4.5,5.8and8µm,respectively,consistentwithour
adoptedaccuracies(seelaterforadescriptionofouradopted
We applied a Lyman-break colour selection to the pub- photometricuncertainties).Additionally,weconfirmedthat
licly available HST/Advanced Camera for Surveys (ACS) our measured fluxes were consistent with those measured
imaging datasets of the CDFS obtained through the fil- forourLBGcandidatesandthestarsfrom theepoch1and
ters F435W (B), F606W (V), F775W (i) and F850LP 2 GOODS Enhanced Legacy products (Dickinson et al., in
(z) (Giavalisco et al. 2004b). Specifically, we selected ob- prep.and http://data.spitzer.caltech.edu/popular/goods/).
jects with V -i > 1.7mag which were not detected
AB AB
The deep GOODS/IRAC data have the largest spatial
(signal-to-noise ratio < 3) in the F435W (B-band) image
(i.e.short-wardofthe912˚ALyman-breakatz∼5)ensuring pixels and PSFs amongst our datasets and, as a result, in
crowdedregionstheemissionprofilefromagivensourcecan
that we only selected sources with a clear Lyman discon-
overlap with those of adjacent sources. For those sources
tinuity in their emission. We then required that the ob-
selected according to the above criteria, but blended with
jects were reliably detected (signal-to-noise ratio > 5 in
neighboursin theSpitzerdata, we treated themid-infrared
the z-band) in order to accurately measure the sizes of the
IRACfluxesasupperlimitsinthesubsequentSEDanalysis.
UV emitting regions. Furthermore, as mentioned above, we
matched oursampleto themagnitudelimit towhich LBGs For each source that satisfied our Lyman break crite-
atz∼3havebeenspectroscopicallyconfirmed.Thislimitof ria, we built a 10-band SED with fixed diameter aperture
RAB∼25.5 probes z∼3 LBGs brighter than ∼ 0.4L*z=3 photometry performed on each image (see Figure 1). Aper-
(Steidel et al. 2003). At redshift z∼5 RAB∼25.5 corre- tures,appropriateforthecharacteristicsofeachimage,were
sponds toiAB∼26.3mag, themagnitude to which we select determined based on enclosing as large a fraction of the
our sample of z∼5 LBGs. This colour selection is similar source flux as possible while minimising the impact of the
to those previously used to select V-band dropouts (or the skybackgroundnoiseandfluxcontributionsfromneighbour-
analogous R-banddropoutsinLehnert & Bremer2003,but ing sources. We used apertures with diameters of 1′′, 2′′,
see also Vanzella et al. 2006; Stark et al. 2006 for a discus- and 4.5′′ for the measurements on the ACS, ISAAC, and
sionofalternativecolourcuts),whichhavebeensuccessfully IRAC images, respectively. Small aperture correction fac-
spectroscopically confirmed to lie at z∼5 (Giavalisco et al. tors were applied to account for the fraction of the total
2004a; Yan et al. 2005;Vanzella et al. 2006). flux not enclosed by our fixed-diameter apertures: 1.07 for
thefourACSbands,1.10forthetwoISAACbands,1.18for
the IRAC 3.6 and 4.5 µm bands, and 1.35 and 1.47 for the
5.8 and 8 µm bands. We determined the aperture correc-
1 Weexplorehowdifferentassumptionsinthemodellingprocess tionsbasedon thepointspread function(PSF)of eachmo-
affectstheresultsofouranalysisinAppendixAandthepotential saic,constructedfromisolated,brightbutunsaturatedstars
contributionfromanunderlyingoldpopulationinAppendix B. throughouttheCDF-Sandtakingreference total apertures
(cid:13)c 2007RAS,MNRAS000,1–21
4 Verma, Lehnert, Fo¨rster Schreiber, Bremer and Douglas
Figure1.(i)Thisfigureshows5”×5”imagesofatypicalgalaxyfromoursampleinthe10filter-bandsweconsider.Sixspectralenergy
distributions (SEDs) are shown in panels (ii-vii). The best-fit model is plotted over the data points. Panels (ii-vi) show SEDs for five
galaxiesfromourrobustsampleofz∼5LBGs.TheSEDsarearranged(fromlefttorightthendown)inapproximatebest-fitageorder
showing the emergence of the Balmer-break between the Ks and 3.6µm data points. The SED of a low redshift interloper is shown in
panelviiforcomparison.Panelsiii,ivandviarespectroscopicallyconfirmedz∼5LBGs.
of diameters 6′′ for the optical and near-infrared data, and withverysmallerrors.Wenotethatadoptingalessconser-
12.2′′ for themid-infrared data. vativevalueforthisuncertaintywouldmainlyacttoreduce
The formal uncertainty assigned to each photometric therangeofacceptableolderages, whichismostinfluenced
data point was derived from measuring the background bytheIRACphotometry,andthereforewouldnotalterour
fluctuations in the parent image using ∼103-104 synthetic findingsof young ages for themajority of our sources.
apertures with fixed diameters laid down at random on
regions that are empty of flux from discrete sources (e.g.
2.3 Defining a robust sample of z∼5
F¨orster Schreiber et al. 2006a), thereby accurately charac-
Lyman-break galaxies
terisingthephotometricuncertaintydirectlyfromthedata,
ratherthancalculatingitfromthepixel-to-pixelrmsassum- Our initial rest-frame UV selection yielded a sample of 109
ing uncorrelated Gaussian noise statistics. The simulations z∼5LBGcandidates.Thissamplewillincludeafractionof
wereperformedforarangeofaperturediameters.Asafunc- spurious sources that are not LBGs (low redshift galaxies,
tion of linear aperture size N, the empirically derived rela- quasars and stars) and also some sources with insufficient
tionship shows that thephotometric uncertaintieslie above photometric constraints (regardless of their nature) to re-
and grow faster with size than the σ ∝ N dependence for liably determine their properties. We have therefore culled
pureGaussiannoise.However,foragivenaperturesize,the theinital sample as follows.
histogram of background fluxes is very well described by AssistedbySEDsextendingtotheobserved-framemid-
a Gaussian distribution (of which the dispersion is taken infrared, we have reliably excluded 22 candidates from our
as the 1σ uncertainty for photometric measurements in the samplewhicharelowredshiftgalaxies(orinterlopers).Syn-
givenaperturesize).Aconservativeabsolutecalibrationun- thesis modelling of the colour evolution of stellar popu-
certainty of ten per cent was additionally included in the lations predicts that galaxies with z<4 can also satisfy
finaladopteduncertaintytocomfortablyaccountfortherel- our UV selection criteria. These galaxies have intrinsically
ativeuncertaintiesinphotometriccalibrationacrosstheten redder near and mid-infrared SEDs than true high red-
bands. In our SED modelling, this additional uncertainty shift Lyman break galaxies. The SED of an example in-
prevents the fits being driven by a few photometric points terloper is shown in panel (vii) of Fig. 1 and is easily dis-
(cid:13)c 2007RAS,MNRAS000,1–21
Lyman-break galaxies at z∼5 5
tinguished from the SEDs of high redshift LBGs (panel
ii-vi). Furthermore, Figure 1 demonstrates how the addi-
tion of the IRAC data has greatly enhanced our ability to
screen for such low redshift galaxies which are generally
the brightest sources in the IRAC wavelengths that sat-
isfy our optical selection criteria. These interlopers mostly
satisfy the criterion for being extremely red objects (ERO,
i −K >2.48, Roche et al. 2003) and IRAC-selected
AB s,AB
EROs(iERO,m3.6µm,AB−zAB >20,Yan et al.2004),con-
sistent with being galaxies at z∼1-2. The importance of
thisscreeningisclearwhenoneconsidersthatmostsamples
of systems satisfying the high-redshift LBG selection crite-
ria distinctly lack confirmatory spectroscopy and several of
the recently reported IRAC-detected modelled z>5 LBGs
donot haveconfirmed spectroscopic redshifts (Egami et al.
2005; Mobasher et al. 2005; Yan et al. 2005). In the case of
thez∼6LBGinMobasher et al.(2005),aninterlopersolu-
tion is more plausible Dunlop et al. (2006). Figure 2. Histogram showing the z-band magnitude distribu-
Our initial optical selection potentially includes low tions of the full set of 109 objects satisfying the initial selection
massstarswithintheGalaxyandQSOs.Thesearespatially criteria,withhistogramsofthecomprisingsub-samplesoverlaid.
Theblack/shaded histogramrepresents thefinal’robust’sample
unresolved in the ACS data are therefore straightforwardly
ofreliableLBGcandidates.
excluded.Weidentified19suchobjects.Theseobjectsgener-
allyhavethebrightestvisibleemissionamongstthesystems
that satisfy ourselection criteria (zAB ∼23). Vanzella et al. the bright, z∼5 galaxy population selected by the Lyman-
(2006) obtained spectra of four of these objects with suffi- break technique, with the possibility that the lower mass
cient quality for them to be classified as stars, supporting and younger systems may be underrepresented. The use of
the exclusion of unresolved objects from our robust sam- thelongerwavelength near-andmid-infrared datahaveen-
ple. Had we included them and subjected them to synthe- abled us to construct a uniformly selected rest-frame UV
sismodelling,theirbest-fitmodelswouldimplyimplausibly sample that is reliable. Figure 2 shows the magnitude dis-
high star formation rates and young ages. Thus, without tribution of the robust sample in comparison to all objects
screeningfor theseobjects, ourderivedensembleproperties satisfying ourselection criteria and theculled members. As
would be heavily biased towards young ages and high star expected, we find that the Star/QSO candidates and low-
formation rates. redshift galaxies are in the most part the brightest sources
Finally, as accurate estimates of the ages, masses and that satisfy our initial selection criteria. The histograms of
star-formation histories of our galaxies are dependent upon therobustsampleandtheLBGcandidatesflaggedashaving
well-sampled SEDs, it is imperative to analyse the objects insufficient-photometry suggest that the former comprises
withthemostrobustphotometry.Therefore,weadditionally the brighter members of the z∼5 population and the lat-
excluded the results from 47 sources with secure detections ter the fainter members. Indeed 6 LBGs from the robust
in only two or three bands (including i and z), with the sample,and9candidateswithinsufficientphotometry,have
remaining constraints being upper limits. There are insuffi- beenspectroscopically confirmedtobez∼5LBGs(seeSec-
cient data to constrain the models to accurately constrain tion 4.1, Vanzella et al. 2005).
their physical properties, and these systems are flagged as
having ’insufficient photometry’. A significant fraction of
thesesystemsareblendedwithanearbysourceintheIRAC
3 EVOLUTIONARY STELLAR SYNTHESIS
bands. Because blending is purely a random alignment of
MODELLING
foreground sources with our high redshift candidates, there
is no reason these should be different to the isolated candi- Using a library of synthetic spectra (Bruzual & Charlot
dates. Accordingly, we do not find any evidence for a sys- 2003),wemodelledtheSEDofeachLBGcandidatesimulta-
tematic difference in the derived properties of the blended neously deriving photometric redshifts and the key proper-
and isolated sub-samples. We note that this step may pref- tiesof age, extinction,star formation rate andstellar mass.
erentially excludeLBGswithhighstarformation rates,low We explored a range of star formation histories (constant
massesandyoungagesasthesearelesslikelytobedetected star formation rate, instantaneous burst, and exponentially
intheIRACbands.Massive,oldLBGsselectedatthesame decayingstarformationrateswithe-foldingtimescalesrang-
i−band magnitude as younger, lower mass systems are in- ingfrom10Myrto1Gyr).TheUVemissionfromgalaxiesis
trinsically brighter in the observed IR and so require more dominatedbythepopulationofshort-livedmassiveOBand
of the photometric bands to be compromised in order to Astars.Modelsofcontinuousstarformationenablethepro-
excludethem in this way. ductionof thesemassive starsevenat old ages. Incontrast,
Thisculling processrendersafinalrobustsample of21 massive stars die away rapidly in an instantaneous burst
galaxies which is the focus of our study. Since each step in modelandconspicuousUVemissionisonlyproducedatvery
thecullingprocess(apartfromthelast)isunbiasedwithre- young ages. For a given initial mass function (IMF), model
specttothephysicalpropertiesofourhighredshiftgalaxies, fits with these two star formation histories will bracket the
the final robust sample is expected to be representative of possibleagerangeofthegalaxies.Ouremphasisonthecon-
(cid:13)c 2007RAS,MNRAS000,1–21
6 Verma, Lehnert, Fo¨rster Schreiber, Bremer and Douglas
stant star formation model therefore provides upper limits
tothebest-fitstellarages.Forthisreasonweconcentrateour
analysis on the results obtained for a constant star forma-
tion rate.Theeffectsof adoptingalternativestarformation
histories on ourresults are discussed in AppendixA.
For all of our models, we used a Salpeter stellar IMF
between 0.1 and 100M⊙. The actual IMF is unconstrained
for our objects. A steep rise down to the lower mass cutoff
likely is unrealistic in view of the turnover below 1M⊙ in-
ferred for thelocal IMF(e.g. Kroupa2001;Chabrier 2003),
and recent comparisons of dynamical masses and photo-
metric stellar masses suggest that Kroupa/Chabrier-type
IMFs would be more appropriate at higher redshift as well
(e.g. F¨orster Schreiber et al. 2006b). The main impact of
changing the IMF on our SED modelling is on the derived
masses and star formation rates which, for a given rest-
frame V-band luminosity, would be about a factor of 1.4-
2 lower with the Kroupa (2001) or Chabrier (2003) IMFs
(see Bruzual & Charlot 2003; see also the discussion by
Papovich et al. 2001, in the context of SED modelling of
z∼3LBGs).Atz&2,andmoresoatz∼5,top-heavyIMFs
with a larger proportion of massive (> 10M⊙) stars, or
evenextremelymassive(>100M⊙)metal-freestarsformed
fromprimordialgas(“populationIII”stars)maybeanissue
(e.g.Schneideret al.2006;Yoshidaet al.2004;Baugh et al.
2005).Ifso,usingsuchanIMFinthemodelling wouldlead
to a reduction of thederived stellar masses and SFRs.
We adopted Bruzual & Charlot (2003) models with a
metallicity of one-fifth solar and, for consistency, a Small
Magellanic Cloud-type (SMC-type) dust extinction law
(Pr´evot et al.1984;Bouchet et al.1985).Observationalcon-
straints on the metallicity of z∼5 galaxies are scarce; how-
ever, the metallicities derived from absorption lines in the
spectra of eigth z∼5 LBGs in the Subaru Deep Field are
Z ∼ 0.2 Z⊙ (Andoet al. 2004), suggesting that our choice
of 0.2 Z⊙ is reasonably representative. For each individ-
ual galaxy, best-fit parameters were derived from minimi-
sation ofthereducedχ2 statistic.Allmodelfittingfollowed
standard procedures applied in similar studies of high red-
shiftgalaxies(e.g.,Papovich et al.2001,2006;Shapley et al.
2001,2005;F¨orster Schreiberet al.2004;Yan et al.2006a).
Weassumedaconstantmarginalisationforeachofthemod-
els. While our results are marginalised over the extinction
law, metallicity, star formation history and IMF, we have
actuallyexploredarangeofalloftheseparameters(seeAp-
pendix A) but the results presented correspond to our pre-
ferred single values of these properties as described above.
Detailed aspects of the modelling procedure will be de-
scribed in a forthcoming paper (N.M. F¨orster Schreiber et
al., in preparation, Paper II).
We note that it is difficult to differentiate between the
Figure 3. Composite probability distributions of age and (a)
exact metallicity, extinction by dust and star formation
stellar mass, (b) star formation rate and (c) extinction for the
histories for each individual galaxy based solely upon the
robustsampleofz∼5LBGsasdeterminedforeachgalaxyfrom
goodness-of-fit because of degeneracies among these model
our 500 Monte Carlo simulations. The points overlaid show the
parameters.Therefore,todeterminetheconfidenceintervals best-fit properties for each object in the sample and the large
for the modelled properties, we ran 500 Monte Carlo sim- crossindicatestheformalbest-fitmedianpropertiesoftheentire
ulations for each object. We applied the same best-fitting ensemble.Theoverlaidcontoursindicatethe68%(solidline)and
procedure after perturbing the input broadband photome- 95%(dashedline)integratedprobabilitiesontheensembleprop-
tryassumingthephotometricuncertaintiesareGaussian as erties measured from the centroid of the distribution. In other
indicatedbyourbackgroundnoisefluctuationsanalysis(see words,68%(and95%)ofallsourcesshouldhaveagesandstellar
masses that fallwithinthese regions. Thebulkof the composite
Section2).Theresultsofthesesimulationsprovidetheprob-
probabilitydistribution liesat ages less than 100Myr, as do the
ability distribution in parameter space for each source. By
best-fitages forthemajority(two-thirds)ofthesources.
(cid:13)c 2007RAS,MNRAS000,1–21
Lyman-break galaxies at z∼5 7
combiningtheseindividualprobabilitydistributions,wede- expected for young stellar populations with no more than
rivedthoseforthepropertiesoftheensembleofsources(see, moderate dust obscuration. The results of our stellar evo-
e.g., Papovich et al. 2001, for an analogous approach). Ex- lutionary synthesismodelling with theparameters specified
amplecombinedprobabilitydistributionsfortheage,stellar in Section 3 yields the following typical properties for the
mass,starformationrateandextinctionforallofthegalax- galaxies. These are typically strongly star forming galaxies
ies in the robust sample are shown in Figure 3. These dia- with a median star formation rate of 40M⊙yr−1. The me-
grams show the probability that any galaxy satisfying the dian photometric stellar mass is 2×109M⊙, about a factor
Lyman-break criteria has of having those properties. From tenlowerthanLyman-breakgalaxiesatz∼3(Shapley et al.
these we can therefore determine the characteristic proper- 2001; Papovich et al. 2001). The stellar mass estimates are
ties of the ensemble of galaxies without relying solely upon themostrobustofallderivedproperties,varyingbyafactor
themean of the (degenerate) best-fit solutions. of 2-5 depending on the model assumptions (see Appendix
A).Whilea large spread in thebest-fitage isseen amongst
individualsourcesinthesamplethemedianageisrelatively
4 ENSEMBLE PROPERTIES OF THE ROBUST young, ∼25Myr. The best-fit models imply only moderate
SAMPLE OF z∼5 LBGS extinction with a median value in the V-band of AV = 0.3
mag. Models including extinction at this level fit the SEDs
4.1 Redshifts: Photometric and spectroscopic of thez∼5 LBGs better than models without extinction.
TheLBGsintherobustsamplehavebest-fittingphotomet- As discussed in Section 3, we use the results of our
ric redshifts in the range 4.60 to 5.54 (<z> = 4.8) Monte-Carlo simulations to characterise the properties of
median
as expected given our initial photometric selection2. Our the ensemble of z∼5 LBGs. Figure 3 shows the combined
photometric redshifts agree exceptionally well (within 2σ, probability distributions for the parameters of mass, star
orbetter),forboththehighredshiftLBGsandlow-redshift formation rate and extinction as a function of age of the
interlopers,withspectroscopicredshiftswhereavailable(see stellarpopulation.Whiletheensembleprobability distribu-
Figure 4 and Table 1). tion in age extends over 1Myr to 1Gyr, there is a clear
As part of the public spectroscopic concentration at young ages. As indicated by the contours
surveys of the CDFS conducted by ESO shown in Figure 3, more than 68 per cent of the age dis-
(Vanzella et al. 2005, 2006, but also see v1.2 release tribution lies at ages less than 100Myr, consistent with the
http://www.eso.org/science/goods/spectroscopy/CDFS Mastercamt/a)j,ority (two-thirds) of the galaxies having formal best-fit
spectroscopy has been performed for 7 LBGs from our ages younger than this. In addition, Figure 3(a) shows that
robust sample, 6of which areconfirmed tobeat z∼5.The theseyoungsources havestellar masses <1010M⊙ and typ-
low signal-to-noise ratio of the spectrum of the remaining ically∼109M⊙,andFigure3(b)suggeststheyarethemost
robust LBG prevented assignment of a redshift. This high stronglystar-formingsystemswithinoursample.Thisisun-
confirmationratereinforcesthatourselectionofz∼5LBGs surprising as they must have the lowest UV mass-to-light
is robust. Spectroscopic redshifts were also secured for 17 ratios in our sample given their low mass.
additionalsources:4arestars,4arelow-redshift interlopers Theremainingthirdofoursamplehavebest-fitmasses
(all agreeing with our photometric/morphological classifi- oforder 1010M⊙ andages older thana few hundredmillion
cation) and 9 are confirmed to lie at z∼5 and are part of years.Theseoldergalaxiesareanalogoustoseveralgalaxies
our ’insufficient photometry’ sub-sample which we expect recentlyreported(Egami et al.2005;Schaerer & Pell´o2005;
includes genuine z∼5 LBGs. The right panel of Figure Yan et al. 2005; Mobasher et al. 2005; Chary et al. 2005;
4 shows the z-band magnitude and redshift distributions Eyles et al. 2005; Dow-Hygelund et al. 2005; Yan et al.
of the spectroscopically confirmed redshift 5 LBGs from 2006a) that lie at similar or slightly higher redshifts (z∼6-
the robust sample and those flagged as having insufficient 7)thanours, wherethepresenceof adiscontinuitybetween
photometry. The latter are slightly fainter than the robust the near and mid-infrared bands in the SED is identified
sample while the redshift distribution is similar indicating with the rest-frame Balmer/4000˚A break (Figure 1). This
bona fide fainter LBGs are among the sources flagged as breakisindicativeofmoreevolvedstellarpopulations,where
havinginsufficient photometry. theemissionfromstarswithagesofseveralhundredmillion
yearsdominatesoverthatfromtheshort-livedmassivestars
thatproducetheUVemission.Theabsenceorweaknessofa
4.2 Stellar Masses, ages, star formation rates and
discontinuity between the K -band and 3.6µm photometry
s
extinction
points in the SEDs of the majority of our sample of galax-
The rest-frame SEDs of the majority of our prime candi- ies with younger best-fit ages strongly limits the possible
dates are very blue from the far-UV to the visible, as is contributiontotheintegratedstellarlight bysuchsimilarly
evolvedstellarpopulations.Indeed,thelackofthisdisconti-
nuityconstrainstheirbest-fitagestolessthan∼100million
2 Usingstandardprocedures(Madauetal.1996)weestimatethe years. While old and massive systems are present in the
redshiftrangeourselectioncriteriaaresensitiveto.Thisrangeis
sample, our results clearly indicate that a substantial frac-
determined following the implementation in Lehnert&Bremer tion(>two-thirds)ofgalaxies atz∼5satisfyingtheLyman
(2003). Standard local galaxy templates were modified to ac-
breakselection criteriaaredominatedbyayoung,intensely
count forIGM opacity along the lineof sight short-wardof Lyα
star-forming component. Similarly young ages and moder-
(Madauetal. 1996). They were then further modified using the
redshiftdependentGunn-PetersonabsorptionofLyαasdescribed ate masses (ages .45Myr and stellar masses ∼109M⊙) are
inFanetal.(2005).Thisresultsinstar-forminggalaxieslyingbe- found for IRAC undetected z∼6 i-band-dropouts (z∼6)
tweenz∼4.6andz∼6beingabletosatisfyourselectioncriteria. LBG candidates recently reported by Yan et al. (2006a).
(cid:13)c 2007RAS,MNRAS000,1–21
8 Verma, Lehnert, Fo¨rster Schreiber, Bremer and Douglas
Figure 4. Left: This figure shows the agreement between spectroscopic and photometric redshifts of all objects which satisfy our
selectioncriteriawhichareconfirmedtobegalaxies.Allfilledsymbolsareforz∼5LBGsfromourrobustsample.Twoσ errorbarson
thephotometricredshiftareoverplotted.Thefigureexcludesalowredshiftinterloperwhichhasanx-raycounterpart(XCDFS265)and
islikelyto beAGN-hosting, therefore the results of the stellarevolutionary synthesis modellingdoes not makesense inthis case. As a
guide, the line zsp=zph is overplotted. Right: z-band magnitude distribution of the spectroscopically confirmed LBGs. As in Figure 2
theblack/shaded histogramcorrespondstogalaxiesinthe’robust’sample.
Table 1.Breakdown ofphotometric andspectroscopic redshiftsforalloftheLBGcandidates. Thezphot columnreferstothenominal
best-fitredshiftdeterminedfromourmodellingandthezspec tospectroscopicredshiftsfromtheliterature.
zphot zspec
Total z<4 z>4 Attempted z<4 z>4 Unassigned
(1a) (1b) (1c) (2a) (2b) (2c) (2d)
UVSelection 109 23 86 34 8 15 11
Robustsample 21 0 21 7 0 6 1
InsufficientPhotometry 47 1 46 14 0 9 5
Star/QSO 19 0 19† 5 4 0 1
Interlopers 22 22 0 8 4 0 4
Columns1(a-c)refertotheresultsoftheSEDmodellingforallcandidates (109) thatsatisfyouroriginalUV-selection,andcolumns
2(a-d)forthoseofthe109candidates whichhavebeenspectroscopicallyfollow-up.
(1a)Totalnumberofsourcessatisfyingourinitialselectioncriterion.
(1b)&(1c)ThenumbersofLBGcandidates withphotometricredshiftsaboveandbelowz=4.
(2a)TotalnumberofLBGcandidates fromourinitialsamplethathavehadspectroscopicmeasurements.
(2b)Numberofcandidates forwhichthespectrawereofinsufficientqualitytodefinitivelyassignaredshift.
(2c)&(2d)Thenumbersofsourceswithconfirmedspectroscopicredshiftsaboveandbelowz=4.
† Best-fitredshiftsofzphot>4areproducedforallobjectsclassedasstars/QSOsduetothethestrongbreakintheirSEDsbeing
identifiedwiththeLyman-breakandbecause theirSEDsarefitwithinappropriatestar-forminggalaxytemplates.
Inthissectionwehavereportedonthetypicalphysical 5 COMPARISON TO LBGS AT REDSHIFT 3
properties for the sample of luminous z∼5 LBGs for our
adopted modelling assumptions. Wehaveextensivelyinves-
tigatedtheeffectsofvaryingtheinputmodelparameterson 5.1 Stellar Masses, Ages, Star-formation Rates
the derived properties and find the key properties of young and Extinction
ages and moderate masses are robust under a wide range
In this section we compare the properties of our z∼5 sam-
of model assumptions. Wediscuss theeffects of varyingthe
ple with those published for the z∼3 LBG sample from
input parameters in the Appendix (see also N.M. F¨orster
Shapley et al.(2001)whichismatchedinrest-frameUVlu-
Schreiberet al. in preparation).
minosity. The SEDs for both samples span the same rest-
framewavelengthrangeandmodellingshowssignificantdif-
ferences in the stellar masses and ages derived even for
identical modelling assumptions. Because of this and the
matched selection, we can only ascribe these differences to
(cid:13)c 2007RAS,MNRAS000,1–21
Lyman-break galaxies at z∼5 9
Figure 5. Histograms (peak normalised) showing the distribution of best-fit properties of 74 R<25.5 z∼3 LBGs from Shapleyetal.
(2001)(shaded grey) together withthe best-fit properties of ourrobust sampleof z∼5LBGs (linefilledhistograms).These properties
correspondtoidentical modelassumptions:constant starformation,0.1-100M⊙ SalpeterIMF,Calzettiextinction lawandsolarmetal-
licity templates of Bruzual&Charlot (2003). The vertical lines indicate the median values for the z∼3 sample (dashed line) and the
z∼5sample(solidline).LBGsatz∼5aretypicallyyounger(<100Myr)andarelessmassive(few×109M⊙)thanz∼3counterparts of
similarluminosity(seetextforadetaileddiscussion).
intrinsic differences in the properties of LBGs at these two 3 andaCalzettiattenuationlaw(Calzetti et al.2000).Note
epochs. that this is different a extinction law and metallicity than
thoseusedin previoussections, butallows adirectcompar-
isonbetweenthetwosamples.Anydifferencesintheresults
The i-bandselection limit of i <26.3 chosen forour
AB indicateintrinsicdifferencesinthepropertiesofthetwosam-
z∼5 sample is matched to the typical magnitude limit of
ples, either in properties like mass and star formation rate,
spectroscopically confirmed samples of LBGs at z∼3. This
orthatdifferentextinctionlawsormetallicitiesarerequired
limit (RAB <25.5), is sensitive to z∼3 LBGs that are ∼1 at thetwo epochs.
mag fainter than m∗z∼3 =24.48 (Steidel et al. 1999).
1700˚A,AB Figure 5 shows the result of this comparison for the
TheR−bandmapsto1700˚Aintherest-frameatz∼3.This best-fit stellar masses, ages, extinctions and star formation
enablesustocomparetheensemblepropertiesofourrobust rates obtained for a constant star formation history from
sample of z∼5 LBGs to those obtained for 74 z∼3 LBGs the z∼3 LBG sample of Shapley et al. (2001) (shaded his-
withapparentmagnitudesgreaterthan0.1m* fromthesam- tograms) and the results for our robust z∼5 sample (line
ple analysed by Shapley et al. (2001). As well as matching filled histograms). While the derived extinctions are well
our samples by magnitude, in order to makea fair compar-
ison we have modelled the SEDs of our robust sample of
z∼5 SEDs using the same assumptions as Shapley et al.. 3 They assume a slightly higher upper mass cut-off of 125M⊙
Specifically, we used solar metallicity model spectra gener- than our assumed 100M⊙, however this slight increase has an
atedwiththeBruzual & Charlot (2003)population synthe- insignificant effect on the generated models and resultant prop-
sis code, a constant star formation history, a Salpeter IMF erties.
(cid:13)c 2007RAS,MNRAS000,1–21
10 Verma, Lehnert, Fo¨rster Schreiber, Bremer and Douglas
Figure 6. The shading and lines are as in Figure 5 but now showing the distributions of properties derived from our Monte-Carlo
simulationsof theproperties ofz∼5LBGsfromourrobustsample(linefilledhistograms)obtained with0.2Z⊙ templates and aSMC
extinctionlaw(seetextfordetails).
Figure 7.Thisfigureshowsthat thedistributionofproperties derivedfromourMonte Carlosimulations(linefilledhistograms)trace
the distributions of nominal best-fit (shaded histogram) properties well. Stellar masses and star formation rates are shown, ages and
extinctions showsimilaragreement.Thesolidverticallineindicatesthemedianvalue.
(cid:13)c 2007RAS,MNRAS000,1–21