Exploring the formation of spheroidal galaxies out to z~1.5 in GOODS

2.9 MB·English

by  Ignacio Ferreras

Mon.Not.R.Astron.Soc.000,1–??(2009) Printed21January2009 (MNLATEXstylefilev2.2)
∼
Exploring the formation of spheroidal galaxies out to z 1.5 in
GOODS
9
0 Ignacio Ferreras1⋆, Thorsten Lisker2, Anna Pasquali3 and Sugata Kaviraj1,4
0 1MullardSpaceScienceLaboratory,UnversityCollegeLondon,HolmburyStMary,Dorking,SurreyRH56NT
2 2AstronomischesRechen-Institut,Zentrumfu¨rAstronomie,Universita¨tHeidelberg,Mo¨nchhofstr.12-14,D-69120Heidelberg,Germany
n 3Max-Planck-Institutfu¨rAstronomie,Koenigstuhl17,D-69117Heidelberg,Germany
a 4Astrophysicssubdepartment,TheDenysWilkinsonBuilding,KebleRoad,OxfordOX13RH
J

4
1 SubmitedforpublicationinMNRAS

]
A
ABSTRACT
G The formation of massive spheroidalgalaxies is studied on a visually classified sample ex-
tracted from the ACS/HSTimages of the GOODS North and South fields, covering a total
.
h areaof360arcmin2.Thesamplesize(910galaxiesbrighterthaniF775W = 24)allowsusto
p exploreindetailtheevolutionoverawiderangeofredshifts(0.4<z <1.5;medianredshift
o- 0.68).Threekeyobservablesareconsidered:comovingnumberdensity,internalcolourdistri-
r bution;andtheKormendyrelation.Thecomovingnumberdensityofthemostmassivegalax-
t
s iesisfoundnottochangesignificantlywithredshift.Extrapolationofoursampletoz=0gives
a anincreaseofthecomovingnumberdensityofMs >1011M⊙galaxiesbyafactor2between
[ z=1andz=0,incontrastwithafactor∼50forlowermassgalaxies(1010 <Ms/M⊙ <1011).
1 Onequarterofthewholesampleofearly-typesarephotometricallyclassifiedasbluegalaxies.
v On a volume-limitedsample outto z<0.7,the averagestellar mass of the blue ellipticals is
3 5·109M⊙comparedto4·1010M⊙forredellipticals.Onavolume-limitedsubsampleoutto
2 z=1.4probingthebrightestgalaxies(MV <−21)wefindthemedianredshiftofblueandred
1 early-types:1.10and0.85,respectively.Blue early-typesonlyamountto 4%of thissample
2 (comparedto26%inthefullsample).Theintrinsiccolourdistributioncorrelatesoverallbluer
.
1 colourswithbluecores(positiveradialgradientsofcolour),suggestinganinside-outprocess
0 of formation. The redshift evolution of the observed colour gradients is incompatible with
9 a significant variatonin stellar age within each galaxy.The slope of the Kormendyrelation
0 in the subsample of massive galaxiesdoesnot changeover0.4 < z < 1.4 and is compati-
: blewith z=0 values.The “zeropoint”of theKormendyRelation(i.e.the surfacebrightness
v
i at a fixed half-lightradius) is 1 mag fainter (in the B band) for the subsample of low-mass
X (Ms <3.5·1010M⊙)early-types.
r
a Key words: galaxies: elliptical and lenticular, cD - galaxies: evolution - galaxies: stellar
content
1 INTRODUCTION 1996; Sagliaetal. 1997) and its lack of evolution with look-
back time (e.g. Forbesetal. 1998; vanDokkum&Franx 1996),
For several decades the formation of luminous early-type galax-
the homogeneity and lack of redshift evolution in their optical
ieshasoccupiedacentraldebateinobservational astrophysics.A
colours (Boweretal. 1992; Ellisetal. 1997; Stanfordetal. 1998;
comprehensive literatureontheseobjectshasconvincingly estab-
vanDokkumetal. 2000) and evidence for short (< 1 Gyr) star
lished that the bulk of the star formation in these systems takes
formation timescales in these systems, deduced from the over-
placeathighredshift(z∼> 2e.g. Boweretal.1992;Stanfordetal. abundanceofαelements(e.g. Thomasetal.1999).
1998)withpossiblynostarformationactivitythereafter–amodel
that is often referred to as the ‘monolithic collapse hypothesis’. Whilethemonolithichypothesisreproducestheopticalprop-
This is borne out primarily by the optical properties of early- ertiesofearly-typegalaxiesremarkablywell,itdoesnotsitcom-
typepopulationsandtheirstrictobedience tosimplescalingrela- fortably within the currently accepted ΛCDM galaxy formation
tions over a large range in redshift. This includes the small scat- paradigm,inwhichearly-typegalaxiesarethoughttoformthrough
ter in the early-type ‘Fundamental Plane’ (e.g. Jørgensenetal. ahierarchical buildupprocess. Semi-analyticalmodels, thatcom-
bine simulations of hierarchical structure formation with simple
recipestodescribethebaryonicphysics(starformation,chemical
⋆ E-mail:[email protected] enrichment andfeedback fromthesupernovaeandactivegalactic
2 I. Ferrerasetal.
nuclei)haveenjoyedbroadsuccessinreproducingseveralproper- BothNorthandSouthfieldsareincluded,coveringatotalareaof
tiesofthegalaxypopulationinthelowredshiftUniverse(seee.g. 320arcmin2equallysplitbetweenthetwofields.Theimagescon-
Coleetal.2000;Hattonetal.2003). sist of four deep exposures through ACS passbands F435W (B),
Whilethepredictedbulkphotometricpropertiesofearly-type F606W(V),F775W(i)andF850LP(z).Theimagesaredrizzled
galaxieshavemetwithreasonablesuccess(e.g. Kavirajetal.2005; with a final pixel size of 30 mas and include a weight map used
DeLuciaetal.2006;Kavirajetal.2007;Kaviraj2008),difficulties for the estimates of photometric uncertainties. The weight maps
have remained inreproducing theshort star formation timescales alsoallowustogeneraterealisticmockgalaxiesusedtoassessthe
implied by the high alpha-enhancement ratios. While improved accuracyofthehalf-lightradii(seeappendixA).Thevisualclas-
prescriptionsmayproducebetteragreementtotheobservedratios sification was performed on the version 1.0 images, whereas all
(Pipino et al. 2008, in preparation) this remains one of the most thesubsequentwork:photometry,sizedeterminationandintrinsic
important challenges tothecurrent incarnationof semi-analytical colour distribution was done on the recently released version 2.0
modelsintermsofreproducingthepropertiesofearly-typegalax- images (Giavalisco and the GOODS Team, in preparation). This
ies. versionhasasignificantincreaseintheS/Nofthezandtheibands.
Nevertheless, the discovery, using rest-frame UV data, of Themethodologypresentedinthispaperwasalreadyappliedtothe
widespread low-level recent star formation (RSF) in early-type v1.0images.Wefindanimprovementonthephotometricandsize
galaxies,bothatlowredshift(Kavirajetal.2007)andatinterme- uncertainties, but no major difference in the overall behaviour of
diate redshifts (Ferreras&Silk 2000; Kavirajetal. 2008), points thesample.
towardsamoreextendedstarformationhistorythanpreviouslyen-
visaged. While much of the work that underpins monolithic col-
lapsewasconductedinthenearbyUniverse,early-typepopulations 2.1 SizeandMagnitudeEstimation
athigherredshiftarerapidlybecomingavailablethroughdeepopti-
We started by selecting all objects with magnitude
calsurveys.Thesesurveysofferanunprecedentedwindowintothe
MAG AUTO < 24.0 from the GOODS ACS iF775W source
starformationhistoriesofearly-typegalaxiesoverthelast8billion catalogues (v1.1; detections based on the zF850LP band). 1 We
years,crucialforunderstandingtheirformationandevolution.
use hereafter elliptical apertures for each galaxy, adopting the
Surveys that combine ground-based photometry with imag-
source centre and ellipse shape (position angle and ellipticity) as
ing from the WFPC2 and ACS cameras on board the Hubble
given in the catalogues, keeping it fixed throughout the process.
SpaceTelescope(HST)additionallyprovidetheangularresolution
Several objects missed by the GOODS detection, presumably
(FWHM∼ 0.1′′)toperformspatiallyresolvedanalysesofstellar
due to proximity of a bright neighbouring source, were inserted
populations down to physical projected sizes of ∼ 1 kpc at z∼1
manually, using Source Extractor (Bertin&Arnouts 1996) to
(e.g. Ferrerasetal. 2005). In agreement with their low-redshift
determine their shape. This initial sample contains 7,462 objects
counterparts,recentstudiesthathaveexploitedsuchearly-typedata
(3,992inGOODS-Northand3,470inGOODS-South).
atintermediateredshiftshavefoundthatthebulkofthestarforma-
TheGOODSACSimagessufferfromresidual(i.e.non-zero)
tion inthese objects does indeed format high redshift. However,
backgroundflux,whichtypicallyreachesalevelof1/5ofthenoise
tell-tale signatures of the recent star formation found in the UV
RMSintheiF775W band.Thisbackground,alongwithavalueof
studiesareclearlyvisible.Asignificantfractionofearly-types(up
thenoiseRMS,wasdeterminedindividuallyforeachgalaxyasa
to20%)exhibitbluecoresthatarecharacteristicofrecentstarfor-
single value that corresponds to the median pixel value within a
mation(Menanteauetal.2001;Ferrerasetal.2005;Pasqualietal. 21′′×21′′-box,applyingfiveiterationsofclippingoutliersat2.3
2006).Furthemore,spectroscopyhasrevealed[OII]emissionlines
standard deviations. All sources weremasked inthisprocess, us-
inasimilarfractionoffieldearly-typegalaxies(Schadeetal.1999;
ing2.0timestheKRON RADIUS,takenfromtheofficialGOODS
Treuetal.2002).
catalogues.
We present in this paper an extension of the sample in
Propermaskingofneighbouringgalaxiesiscrucialforareli-
Ferrerasetal.(2005)–whichwasrestrictedtoCDFS–tothefull
abledeterminationofthePetrosianradius(Petrosian1976).Onthe
coverageoftheACS/HSTimagesoftheGOODSNorthandSouth
onehand,objectmasksshouldbelargeenoughtocoverallcontam-
fields.However,noticethatthesamplepresentedheredoesnotap-
inatinglightofcloseneighbours. Ontheotherhand,ifneighbour
ply the selection based on the Kormendy relation in contrast to
masksaretoolargetheycouldencompassabigpartofthetarget
Ferrerasetal.(2005).Theonlyconstraintappliedonthesamplese-
galaxy,renderingunreliableestimatesofthePetrosianRadius.
lectionisvisualclassification.Thedepthandsuperbangularresolu-
Aftertestingseveralwaystodefinethemasks–e.g.usingthe
tionoftheimagesenableustoaccuratelydeterminesizesandtotal
KRON RADIUS or a preliminary Petrosian radius – we adopted
magnitudes(whichtranslateintostellarmasses).Wecandetermine
anellipticalaperturethatextendsouttoasurfacebrightnesscorre-
inarobustwaytheinternalcolourdistributionandtheKormendy
spondingtoonehalfofthelocalnoiseRMS(“half-noisesemimajor
relation, out to redshifts z∼< 1.2. We improve on Ferrerasetal. axis”or“half-noiseSMA”).Inaddition,weapplyaniterativeout-
(2005)byconstrainingtheselectionmethodtovisualclassification
lierclippingprocedureinthesurfacebrightnesscalculationsused
(i.e.removingtheKormendyRelationconstraint)andbyselecting
todeterminethePetrosianradius,asdescribedbelow.Inthedeter-
volume-limitedsubsamples.
minationofthehalf-noiseSMA,initialobjectmaskswereusedthat
werebasedontheKRON RADIUSfromthecatalogues.
Foreachgalaxy,wedefinea“Petrosiansemimajoraxis”(here-
2 THESAMPLE after Petrosian SMA, aPetro), i.e., in the calculation of the Pet-
rosianradius,weuseellipsesinsteadofcircles(cf.Lotzetal.2004;
We describe in this section the definition of our i-band selected Liskeretal. 2007). As mentioned above, the elliptical shape and
(F775W), visually classified sample of early-type galaxies. The
sampleisextractedfromtheACS/HSTimagesoftheGreatObser-
vatories Origins Deep Survey (GOODS; Giavaliscoetal. 2004). 1 http://archive.stsci.edu/pub/hlsp/goods/
Early-typegalaxyformationinGOODS 3
Figure1.Distributionofconcentration,asdefinedinBershadyetal.(2000) Figure 2. Comparison between our photometric redshifts and published
andM20forallgalaxieswithiF775W 624(greydots).Ourfinal–visu- spectroscopicredshifts(seetextfordetails).Solid(open)circlescorrespond
ally classified –sampleofearly-type galaxies inthe GOODSNorth and tophotometricearly(late)types.Thecomparisonisrestrictedtogalaxies
Southfieldsisshownasblackcrosses.Atthetop-rightcornerCandM20 withiF775W <23(withthefullsampledowntoiF775W <24,thestan-
correspondtoaone-componentdistributionwithahighSersicindex.To- dard deviation of the comparison is 0.13). Thelong (short) dashed lines
wardsthelowerleftcorner,thecontributionofasecondcomponent(i.e.a showthesamplemean(rms).Thermsgiveninbracketscorrespondtothe
disk)makesM20increasefasterthanC,resultinginachangeofslope. scatterwhen3σoutliersareremovedfromthesample.Inset:Redshiftdis-
tributionofthecompletesample(bothNorthandSouthfields;thicksolid
line),usingspectroscopicredshiftswhereavailable. Theotherhistograms
splitthesampleintophotometricearly-(dashedline)andlate-types(dotted
source centre of each object were taken from the GOODS offi-
line).
cial catalogues, and kept fixed during the process. The Petrosian
SMA was defined as the semimajor axis a at which the local in-
tensityfallsbelowonefifthoftheaverageintensitywithina.The lightradiusandapparentmagnitude.Thiscorrectionwasobtained
localintensityismeasuredastheaverageintensitywithinanellipti- fromsimulationsofgalaxieswithSersicsurfacebrightnessprofiles
calannulusreachingfrom0.9ato1.1a,applyingfiveiterationsof andrealisticnoiseobtained fromtheGOODS/ACSweight maps.
clipping outlying pixel values at 2.3 standard deviations. Neigh- Noticethatthiscorrectionfixestwodifferenteffects:thedominant
bouring objects were masked using the half-noise SMA, or the oneinthiscasecomesfromthefactthattheapparentsizesareclose
KRON RADIUSforsourceswithMAG AUTO>24.0.For250 totheresolutionlimitoftheobservations.Thesecondcorrectionis
objects (3.4%), aneighbouring source was so close that itsmask requiredaslightmeasuredwithinanaperturewillloseflux,andthis
encompassedthecentreofthetargetgalaxy,renderingaPetrosian lossisrelatedtothesteepnessofthesurfacebrightnessprofile(see
SMAderivationimpossible.Foranother72objects(1.0%),thede- e.g. Graham&Driver2005;Grahametal.2005).Oursimulations
rivedPetrosianSMAisclearlytoolargefromvisualinspection,in showthatthecorrectioncanbewrittenasafunctionoftwoobserv-
mostcasescausedbydiffuselightorbackgroundinhomogeneities. ables:thedominant oneistheestimatedRe andthesecond order
Toobtain thetotal magnitude, effectiveradius, and effective oneistheconcentration,measuredasR90/R50(seeappendixAfor
surface brightness of each galaxy, we measured the iF775W flux details).
withina=1.5aPetro,andthesemimajoraxiscontaining50%and The simulations allow us to assess the accuracy of the re-
90%ofthisflux(a50anda90,respectively).Forgalaxyimagesaf- trievedsize,magnitudeandapparentsurfacebrightness.Fourmock
fectedbyaneighbourmask–thereforewithoutaPetrosianSMA– samplesof910galaxieseachweregeneratedwiththesamevalues
weusedtheKRON RADIUSinsteadofa = 1.5aPetro,andper- of iF775W and Re as the real sample. These galaxies were given
formednomaskingofneighbours.ForthosewhosePetrosianSMA a random eccentricity 0 < e < 0.6, position angle, and Sersic
wasfound tobetoolarge,wealsoadopted theKRON RADIUS, (1968)index2 < n < 4.Wecouldsucessfullyretrievethehalf-
but did mask neighbouring galaxies. From a comparison of cata- lightradiuswithanaccuracyof9%,theiF775W-bandtotalapparent
logmagnitudes(MAG AUTO)withfinalmagnitudes(mi)forour magnitudewithin0.05mag,andtheaverageiF775W-bandsurface
earlytypes,wefindthatfor3-4%oftheobjects,thefinalmagni- brightnesswithinthehalf-lightradiustowithin0.16mag/arcsec2
tudeisbrighterbymorethan0.5magthanthecatalogmagnitude. (seeappendixAfordetails).
BasedonourpreselectionofgalaxieswithMAG AUTO<24.0,
weestimateourcompletenesstobe95%atmi 623.5mag. 2.2 MorphologicalClassification
The flux lost due to masked areas within the aperture was
corrected by assigning the azimuthally averaged intensity to the Galaxymorphologystudies(e.g. Abrahametal.1994;Lotzetal.
maskedpixels.Wethenappliedacorrectiontotheestimateofhalf- 2004; Menanteauetal. 2004; Scarlataetal. 2007) still lack a re-
4 I. Ferrerasetal.
liable classification method in an unsupervised way with accept-
ablelevelsofcontaminationandcompleteness.Currently,allthese
methodsalwaysrequirevisualinspection.Wedecidedtotacklethe
selectionofearly-typesonlybyvisualinspection.TheSouthfield
sample of early-types was taken from Ferrerasetal. (2005), with
atotalof377early-typegalaxies,out ofatotalof3,470galaxies
at iF775W < 24 (10.9%). FortheNorthfieldwefollow aniden-
tical procedure. All sources brighter than iF775W = 24mag are
inspected by the four authors of this paper. We useds92 to view
eachgalaxyinallfourbands.Afterafirstselection,weacceptan
early-type galaxy if classified as such by at least three of us. A
second passof visualinspectionbythesamepeoplefollows,this
time restricted to the list of galaxies already labelled during the
first pass as early-types by one or two of us. Galaxies classified
asearly-typesbythreeofusinthissecondpassareaddedtothefi-
nalsample.Outof3,992sourcesintheNorthfield,weselected533
galaxies(13.4%),withatotalinbothfieldsof910galaxies.Thedif-
ferentfractioniscompatiblewithinPoissonstatistics.Furthemore,
CosmicVariance willcertainly havean effect on the discrepancy
betweentheNorthandSouthfields.
Two “non-parametric” measures of galaxy morphology are
shown in Fig. 1 for our sample: the concentration index (C
Bershadyetal. 2000), and the M20 parameter (Lotzetal. 2004). Figure3.Distributionofvariousobservables.Thehistogramsseparatebe-
C = 5logR80/R20, where RX is defined as the radius within tweentheNorth(grey)andtheSouth(black)fieldsinordertoillustratethe
whichonehasX percentofthetotalflux.M20 isthenormalized effectofCosmicVariance.
second-order moment of the brightest 20% of galaxy pixels (no
matterwheretheyare),andthusquantifieswhetherthesepixelsare
confinedtoasmallregionorwidelyspread.Thefigure,analogous fixedaperture.ThephotometrycomesfromtheGOODS/ACSim-
toFig.1inFerrerasetal.(2005),showsthatthemajorityofgalax- ages(B,V,i,z)andisdirectlyextractedfromthepubliclyavailable
iesfollowawell-definedrelationintheC-M20-plane,whichcon- v2.0 images. For HDFN we also include ground-based photome-
sistsoftwolinearpartswithdifferentslope. Thechangeinslope try from Capaketal. (2004); which includes KPNO/4m U-band
marksthetransitionbetweenbulge-dominatedanddisk-dominated andUH/2.2mHK′ data.InCDFSopticalandNIRphotometryis
orirregularobjects:fortheformer,thebrightest20%ofpixelsare takenfromWFI/NTTandISAAC/VLTasusedinthephotometric
fullylocatedinthecentre,whereasforthelatter,agrowingfraction redshiftcatalogueofMobasheretal.(2004).
islocatedoutsideofthecentralregion. Figure 2 shows a comparison of photometric and spectro-
scopicredshiftsforgalaxiesbrighter thaniF775W < 23. Theav-
erage(0.002)andscatter(0.091)ofthediscrepancy–measuredas
2.3 RedshiftandRest-frameEstimates ∆z/(1+zS)–iscomparabletothebestredshiftestimatesavail-
able (Mobasheretal. 2004, 2007). If outliers at the 3σ level are
Weretrievedthelatestpubliclyavailablespectroscopicredshiftcat-
removed fromthesample, thescatter reduces to0.061. Thefrac-
aloguesfromtheGOODSfields:FORS2(Vanzellaetal.2008)and
tionofoutliersis2.9%withnosignificantdependence ongalaxy
VVDS (Popessoetal. 2008) in CDFS and the Team Keck Red-
type.
shiftSurvey(Wirthetal.2004)inHDFN.InFerrerasetal.(2005)
Ourphotometricredshiftcodealsoidentifieseachgalaxywith
photometric redshiftsweretaken fromMobasheretal.(2004)for
thetemplatethatgivesthebestfit.Thisallowsustosplitthesample
galaxies without a spectroscopic measurement. In this paper we
into“photometric”redandbluegalaxiesdependingonwhetherthe
determine our own photometric redshifts both in the North and
bestfitcorrespondstooldoryoungerstellarpopulations.Wecom-
South fieldsfor consistency. Our code isbased on a templatefit-
paredthisnewclassification–usingthetemplatespresentedabove
tingtechniquewithoutanypriorsandusesasetofeighttemplates
– with those done in Ferrerasetal. (2005), which were based on
builtfromBruzual&Charlot(2003)populationsynthesismodels.
the photometric types from Mobasheretal. (2004). In this paper
Each template is a τ-model, i.e. a composite stellar population
wechoosetheseparationbetweenphotometricredandblueellip-
atsolarmetallicitywithanexponentially decaying starformation
ticalsatthethirdtemplate(t=2):allgalaxieswhicharebestfitby
rate(τ).WefixtheformationredshiftzF=3foralltemplates,with anexponential timescaleof1Gyrorshorterareconsidered“red”
timescales:τ ={0.05,0.5,1.0,2.0,4.0,5.0,6.0,8.0}Gyr.Wela-
throughoutthispaper.Withthischoice,thenumberof“red”early-
belthesestarformationhistorieswithanintegert={0,···,7}.The
typegalaxiesinthetotalsampleis670(∼75%).Thefractionofred
templatesaretranformedaccordingtoaFitzpatrick(1999)lawfor
early-typesintheNorthandSouthfieldsis375/533and295/377,
Galactic reddening with a colour excess E(B-V)=0.012 (HDFN)
compatiblewithinPoissonerrorbars.
and0.010(CDFS),valuestakenfromSchlegeletal.(1998)forthe
Inaddition,weusedredshiftsdeterminedforasubsampleof
centralpositionsofbothfields.Eachmodelisrunoverarangeof
our dataset, for which IF and AP have access to ACS/HSTslit-
redshiftsouttoz=3withtheavailablephotometryestimatedovera
lessgrismspectroscopy (G800L) fromthePEARSproject(Prob-
ing Evolution And Reionization Spectroscopically; PI S. Malho-
tra; HST Proposal ID # 10530) The low-resolution spectra (R∼
2 http://hea-www.harvard.edu/RD/ds9/ 50 − 80) allowed us to secure redshifts with comparable accu-
Early-typegalaxyformationinGOODS 5
Figure4.Redshiftevolutionofthecomovingnumberdensity:TheB-bandabsolutemagnitude(left)andstellarmass(right)iscomparedwithV(<z),i.e.the
comovingvolumewithinagivenredshift(theredshiftappearsinthetopaxis).Verticallinesdelimitredshiftbinsusedintheanalysisofthenumberdensityin
thenextfigure,withtheupperandlowerlimitsshownasshadedregions.Galaxiesareshownasgreydots(photometricearly-type)oropencircles(late-type).
Thecurved,thinsolidlinesintheleftpanelarethelimitingiF775W = 24foraphotometricearly-orlate-type.Thethicklineanddashedlinesaboveand
belowillustratethesampleseparationinto“bright”and“faint”subsets(orhighandlowstellarmassintherightpanel;seetextfordetails).Greyareasinthe
leftpanelareL∗valuesfromIlbertetal.(2006).ThedashedlineinthemassplotfollowsM∗fromFontanaetal.(2006).
racy to the spectroscopic ones because of the prominent 4000A˚
Table1.Thesampleofearly-typegalaxies(iF775W <24)
breakfoundinmostofthesegalaxies.Fordetailswerefertheinter-
estedreadertothepaperontheanalysisofthegrismdatafromthe
GOODSearly-types(Ferrerasetal.,inpreparation),ortoasimilar Sample Number fBa zb Mbs(M⊙) MVb Rbe(kpc)
studyontheHUDF(Pasqualietal.2006).Outofthetotalsample
of910early-typegalaxiesinthispaper,wehavespectroscopicor HDFN 533 0.30 0.78 4.2·1010 −20.9 1.10
CDFS 377 0.22 0.64 3.3·1010 −20.6 1.26
grism-basedredshiftsfor 541galaxies.Hence, 59.5%of thetotal
All 910 0.26 0.68 3.7·1010 −20.8 1.15
samplehavespectroscopicallyaccurateredshifts. S1c 411 0.04 0.85 1.2·1011 −21.9 2.11
The redshifts are used to determine rest-frame properties of S2c 280 0.11 0.56 3.4·1010 −20.6 1.55
thegalaxies.Thephysicalprojectedhalf-lightradiusisdetermined
byusingtheangulardiameterdistanceforaconcordancecosmol- aBlueearly-typefraction:Red/Bluetypesselectedfromphotometricred-
ogy,namelyΩm = 0.3,ΩΛ = 0.7andH0 = 72km/s/Mpc.The shiftbesttype(Red:t62,i.e.starformationtimescaleτ 61Gyr,seetext
bestfittemplateiscomparedtotheapparenttotalmagnitudetoes- fordetails).bMedianvalue.c S1andS2arevolume-limitedsubsamples,
timatetheabsolutemagnitudeandstellarmass–aChabrier(2003) definedin§4.
InitialMassFunctionisusedthroughoutthispaper.FromtheACS
B (where available), V,i,z photometry we choose the passband ical size, absolute magnitude and redshift. The sample is shown
with the lowest K-correction for the transformation into absolute separatelyfortheNorth(grey)andSouth(black)fields.Bothfields
magnitude. As an estimate of the amount of correction applied, have a similar range in mass or size, but the redshift distribu-
theRMSoftheK-correctionsforthefullsampleis0.42magfor tion illustrates the effect of cosmic variance over the individual
MB and0.27magforMV.Regardingstellarmassestimates,even 160arcmin2 areas.Thevariationinthetotalnumberofearly-type
though the age and metallicity distribution of a composite stellar galaxies(533intheNorthvs.377intheSouth)isalsocausedby
populationcannotbeconstrainedwithbroadband photometry,the such effect (the selection method being identical in both fields).
stellar mass can be reliably constrained to within 0.2−0.3 dex Some of the basic properties of this sample is shown in table 1.
providedtheadoptedIMFgivesanaccuraterepresentationofthe Thetablealsoshowsthepropertiesofacoupleofvolume-limited
trueinitialmassfunction(seee.g.Ferreras,Saha&Burles2008). subsamples(S1andS2)definedin§4.
Inorder toassesstheeffectofthepassband onthedetermination
ofstellarmasswecompareourstellarmasses–basedoniF775W –
withtheKsphotometryoftheCDFSfieldfromISAAC/VLT(Ret-
3 NUMBERDENSITYEVOLUTION:LUMINOSITYVS.
zlaffetal.,inpreparation).Overtheredshiftrangeprobedbyour
MASS
sample,i-bandandNIRmassestimatesdifferbylessthan0.15dex
(seeappendixB). Asimplewaytoassesstheredshiftevolutioninthenumber den-
Figure3showsthedistributionofstellarmass,projectedphys- sityof early-type galaxies isillustratedin figure4, where the B-
6 I. Ferrerasetal.
Figure 5 shows the evolution in comoving number density
withrespect toabsolute magnitude (top) or stellarmass (bottom)
obtainedfromthepreviousdiagram. Theerrorbarsareestimated
fromasuminquadratureofthePoissonnoiseineachbinandthe
uncertaintyintheabsolutemagnitudeormass.Thisuncertaintyis
computedusingaMonteCarloalgorithminwhichnoiseisadded
totheabsolutemagnitudeorstellarmassofeachgalaxy.5,000real-
izationsaregeneratedandeachisanalysedinthesamewayasour
originalsample.Theerrorbarsgivethestandarddeviationwithin
the ensemble. The evolution of the number density can be mod-
elled by a simple power law n ∝ (1+z)α. In luminosity there
isno significant difference between the “bright” and “faint” sub-
samples. However, the most massive galaxies show a remarkable
weaktrendwithredshift,withaslopesignificantlydifferenttoless
massivegalaxies.Withtheadoptedfitstothedata,anextrapolation
tolowerredshiftyieldsadecreaseofafactor∼ 50inthenumber
density of early-type galaxies between z=0 and z=1 for the mass
range1010−1011M⊙;whereasthedecreaseisjustafactor2for
galaxieswithstellarmassesbetween1011and1012M⊙.Thisresult
ispresented inmoredetailinaletter(Ferrerasetal.2008) where
weshowthatthisresultisinagreementwithrecentworkinother
surveys (e.g.Conseliceetal. 2007)and withrecent semi-analytic
modelsofgalaxyformation(e.g.Khochfar&Silk2006).
Figure5. Redshift evolution ofthetop endofthe massandluminosity
distribtutionasillustratedinthepreviousfigure.Theblackdotscorrespond
tothebrighter(toppanel)ormoremassive(bottom)subsample.Thefainter
subsampleisshownasgreycrosses.ErrorbarsincludePoissonnoiseand
4 INTRINSICCOLOURDISTRIBUTION
theuncertaintyintheK-correction.Thelinesgivefitstoapowerlaw,with
indexα,whichislabelledforallsubsamples. Theinternalcolourdistributionofagalaxycanrevealimportantin-
formationregardingitsstarformationandassemblyhistory.Early-
typegalaxieshavesmoothsurfacebrightnessdistributions,which
bandabsolutemagnitude(left)orstellarmass(right)ispresented makethemidealtargetsforthissortofstudyinGOODS.Foreach
withrespecttothecomovingvolumeenclosedwithintheredshift colourmapthreepiecesofinformationcanbegatheredinarobust
ofeachgalaxy.Hence,countinggalaxieswithincellsinthisfigure way:theaveragecolourwithinanaperture(theprojectedhalf-light
gives the comoving number density at a given redshift. The thin radius in our case); the radial gradient of the colour distribution,
solidlinestrackthefluxlimitimposedbyoursearch(iAB < 24). and the scatter about the best fit (assumed to be linear between
ThislimitdependsontheK-correctionadopted,therebyweshow colour and logR).Wefollow thesametechnique asdescribed in
inthefiguretheabsolutemagnitudelimittakingtheextremaofthe Ferrerasetal.(2005),brieflyoutlinedhere.
templatesusedinthephotometric redshiftanalysis(i.e.typest=0 Thecolour map isgenerated byconvolving theimage taken
andt=7).Galaxiesareshownaccordingtotheconventionadopted inonefilterwiththePointSpreadFunction(PSF)oftheotherfil-
throughoutthispaper,namelyredgalaxiesasgreysoliddots(pho- terusedtodefinethecolour.Thisisthemostrobustwaytoreduce
tometrictypet62)andbluegalaxiesasblackopencircles(t>2). thecolourtrendscausedbyadifferentresolutionbetweenimages
Inbothcases–absolutemagnitudeandstellarmass–wesplitthe indifferentbandpasses.WeusedaPSFgeneratedfromstellarim-
sample in two bins. Inabsolute magnitude we define 1 mag bins agesinthesamedataset.Ferrerasetal.(2005)includesanappendix
(dashed lines) above and below a typical L⋆ galaxy represented thatshowsthe–negliglible–differenceinthecolourgradientsand
bythethicksolidline,whichcorresponds toM0 ∼ −21. Inor- scatterwhen usingeither astellarimagefromthesamedataor a
B
der to track its evolution with redshift, we assume an exponen- syntheticPSFgeneratedbyTinyTimKrist(1993).
tialstarformationhistorywithformationredshiftzF=5,timescale The images span a widerange of signal-to-noise ratioson a
τSF = 0.5Gyrandsolarmetallicity.Forcomparison, theshaded pixel-by-pixelbasis.Hence,itisbesttobinthepixelsalthoughpre-
boxes delimitthevalue oftheM⋆ Schechter fitsof theVIMOS- servingspatialinformationasmuchaspossible.AVoronoitessella-
B
VLT Deep Survey for early-type galaxies (Ilbertetal. 2006). We tionmethodwasadopted,inwhichatargetS/Nisspecifiedsothat
decidedtochoosethisbinningcriterioninsteadofafixedabsolute thealgorithmmergesnearbypixelsintotiles,keepingthesetilesas
magnitude in order to overcome the passive fading of the stellar roundaspossible,followingthealgorithmbyCappellari&Copin
populations. The shaded areas show that our low-luminosity bin (2003).TheprocessisfinishedwhenatargetS/Npertileisreached.
roughlymapsL∗galaxies. InFerrerasetal.(2005)weshowedthatestimatesofradialcolour
Regarding stellar mass, we choose a fixed reference mass profilesaremuchimprovedastheouterpartsofthegalaxyhavea
(1011M⊙;thicksolidline).Thesampleissplitin1dexbinsabove significantly lower noise after a Voronoi tessellation. We impose
and below thisreference (dashed lines). Forreference, thedotted a target S/N=10 per tile for each colour. The radial distribution
line tracks the characteristic stellar mass scale of Fontanaetal. of colour vs logR/Re isfittoastraight line. Wealsodetermine
(2006). A typical error bar is shown in both panels. The vertical thecolour scatter about thisbest fit by using abiweight estimate
linesinbothpanelsmarktheredshiftbins,withthefullrangefor (Beersetal.1990).
a complete volume-limited sample given by the hatched vertical The left panel of figure 6 shows the observerframeV − i
regions. colour distribution as a function of redshift, half-light radius and
Early-typegalaxyformationinGOODS 7
Figure6.Left:Theobserver-framecolourgradient(top),scatter(middle),andintegratedcolourwithinthehalf-lightradius(bottom)areshownwithrespect
tothreeobservables:redshift(left);apparenthalf-lightradius(middle)andiF775Wapparentmagnitude(right).Thefullsampleisshowningrey.Blackpoints
correspondtoaveragevalues ofsubsamples(aftera5σclipping)binnedatconstantnumberofgalaxiesperbin,andseparatedwithrespecttothebestfit
populations:old(filleddots;photometrictypet62)andyoung(hollowdots;t>2).Right:Redshiftevolutionoftheintrinsicdistributionofthethreeavailable
colours,aslabelled.Inbothfiguresthelinesinthebottompanelscorrespondtotheevolutionofasetofexponentiallydecayingstarformationhistorieswith
solarmetallicity,startedatzF=5withadecaytimescaleof0.5(solidline)1(dashedline)and8Gyr(dottedline).
apparent i band magnitude of the whole sample (grey dots). The tracksaiF775W = 24galaxyasafunctionofredshift,assuminga
largerdotscorrespondtobinningthesampleatafixednumberof typicalstarformationhistoryforanearly-typegalaxy(photometric
galaxiesperbin,separatingtheminphotometrictypebetweenred typet=1).Acompromisehastobemadewhengeneratingvolume-
(t6 2; solid circles) and blue (t>2; hollow dots). Error bars rep- limitedsamples:eitherweprobeasdeepaspossibleinredshift–
resentthestandarddeviationwithineachbin.Forcomparison,the cuttinginabsolutemagnitude,andtherebysacrificingfaintgalax-
linesatthebottom-leftcornertrackexponentiallydecayingstarfor- ies,orweprobeasfaintaspossibletheluminosityfunction–cut-
mation historiesat solar metallicitywithformation redshift zF=5 tinginredshift.Inordertoexplorethesetwochoicesweselecttwo
and timescales τ = 0.5 Gyr (solid), 1 Gyr (dashed) and 8 Gyr subsamples,shownasblackdotsineachpanel:S1(412galaxies;
(dotted). left)is asample targeted toexplore the redshift evolution, and is
Inthetoppanels,theradialcolourgradientisshown.Galax- restrictedtogalaxiesbrighter than MV < −21. Wealsoapply a
ieswithred(blue)coreshavenegative(positive)gradients.Weob- cutinredshift(z<1.4).SubsampleS2(280galaxies;right)targets
tainaconsistentresultwithrespecttoFerrerasetal.(2005).Galax- awiderangeofluminositiesouttoz<0.7.Inthiscasethevolume-
ieswithbluecoresarealmostalwaysphotometricallyclassifiedas limitedsampleextendstofaintergalaxiesMV <−19.Thebottom
“blue”andgalaxieswithredcoresareclassifiedas“red”. Thisis panels of figure 7 show the histograms in stellar mass (left) and
not a trivial statement, as the overall colour of the galaxy need projectedphysicalsizeofeachsubsample.
not correlate withthe radial gradient. The scatter does not corre- Figure8focusesontheredshiftevolutionofthecolourdistri-
late with redshift, but there isa clear trend with size or apparent butionandshowstheobserver-frameV−icolourgradient(top)and
magnitude. Thiscan becaused by acombination of physical and scatterofsubsampleS1.Forcomparison,thecolourgradientsob-
passband-shiftingeffectsandwedeferthediscussiontoalaterfig- servedinasampleofmoderateredshiftclustersbyTamura&Ohta
ure. (2000)isshownasstars.Oursampleisshownwiththeusualcoding
Ontherightsideoffigure6,asimilarplotispresentedforthe withrespecttophotometrictype:red(greydots)andblue(opencir-
threeavailablecoloursfromtheACSimages.TheB−i,V −iand cles).IntheappendixofFerrerasetal.(2005)simulationsofgalax-
i−z colour distributionsareshownwithrespecttoredshift.The ieswiththesamecharacteristicsastheGOODSgalaxiesweredone
linesinthebottompanelscorrespondtothesamemodelsasinthe to show that from photometric errors one could expect a typical
leftsideofthefigure.Thetrendisconsistentwithallthreecolours, colourscatterσ(V −i)∼< 0.03mag,withamaximumof0.05mag
withbluegalaxieshavingpositivecolourgradients. forthefaintestgalaxies.Figure15intheappendixofFerrerasetal.
In order to extract meaningful trends of the intrinsic colour (2005)showsthatboththeobservedscatterandslopeareintrinsic.
distributionswehavetodefinevolume-limitedsubsamplesthatwill Weincludeinthefigureasimplemodelpredictionwhichuses
removebiasesinherenttoflux-limitedsamples.Figure7illustrates the local B −R colour gradients of elliptical galaxies in the lo-
our selection process. The complete sample isshown in absolute cal Universe as constraint (Peletier 1990). The set of two nearly
magnitudevsredshiftspace.Thedistributionshowsthecharacter- horizontallines–justbelowthe∆(V −i)/∆log(R) = 0axis–
istic boundary caused by the iF775W = 24 limit. The solid line assumes the z=0 intrinsic colour gradient to be caused by a pure
8 I. Ferrerasetal.
Figure8.IntrinsicV −icolourdistributionofsubsampleS1:Theslope
(top)andscatteroftheradialcolourgradientisshownwithrespecttored-
shift.Thesampleisseparatedintophotometricearly-(greydots)andlate-
Figure 7. Volume limited subsamples of early-type galaxies in GOODS types(opencircles).Starsaregradientmeasurementsinasampleofclus-
N+Sfields.S1focusesonmassivesystemsouttoredshiftz<1.4,whereas terearly-typesfromTamura&Ohta(2000).Twoalternativemodelsforthe
S2 limits the redshift out to z<0.7 in order to maximise the range of evolutionofthecolourgradientareshown,dependingonwhetherthegra-
masses/luminosities. ThetoppanelsshowV-bandabsolutemagnitudevs dientiscausedbymetallicity (thin,nearlyhorizontallines)orage(thick,
redshift,withtheiF775W <24fluxlimitshownasasolidline.Thebottom curvedlines).Twoformationredshiftsareconsideredforeachcase:zF=5
panelsshowthehistogramsofeachsubsampleinstellarmassandprojected (solidlines)or2(dashedlines).Thedottedlinecorrespondstoametallicity
size. sequencewithformationredshiftzF=5,addingtothecore10%(inmass)
ofastellarpopulationformedatz=0.8.
metallicitysequence,i.e.wefixanagethroughoutthegalaxyand
The relation between colour distribution and galaxy proper-
compute the difference in metallicity that will give the observed
ties is explored in figure 9; where the colour gradient (top) and
colour gradient in local early-type galaxies. Then we evolve this
scatterisshownwithrespecttostellarmass;projectedhalf-lightra-
modelbackwardsintimetoassesstheevolutioninthecolourgra-
diusandV-bandabsolutemagnitudeforsubsampleS2.Bluecores
dient.Standardmodelswithexponentiallydecayingstarformation typically have stellar masses below 1010M⊙ and half-light radii
historiesareusedforthefit.Thetimescaleisfixedatτ =1Gyrand Re ∼< 1kpc,asignatureofdownsizing. Inthissubsampletheav-
theformationepochiseitherzF=2(solidline)or5(dashedline). eragestellarmassoftheblueellipticalsis5·109M⊙comparedto
Thesecondsetsofmodels–whichcurvedownatz<0.6–assume 4·1010M⊙ forredellipticals.Thisstrongmass-dependence sug-
apureagesequenceforthecolourgradientatfixed(solar)metallic-
gests that AGN activity alone is not enough to explain the blue
ity,withasimilarsetofmodels(inthiscasethebluerouterregion
cores(Menanteauetal.2005).
ismodelledbyaslightlylongerstarformationtimescale).Asex-
pected,thismodelgetsverysteepwithincreasingredshiftbecause
ofthestrongdependence ofcolour withage.Athirdmodel (dot-
5 THEKORMENDYRELATION
tedline)overlaysontopofametallicitysequenceformedatzF=5
asecond, younger stellarpopulationinthecore,formedatz=0.8. TheKormendyrelation(hereafterKR;Kormendy1977)isatight
Thisyounger component contributes only 10% in mass, illustrat- linear correlation between average surface brightness – typically
ingtheeffectthatasmallepisodecausedbyaminormergerwould measured within the half-light radius (Re) – and the logarithm
haveonthecolourgradient. of Re. It is one of the projections of the Fundamental Plane
This result illustrates that blue cores have a radial range of (Djorgovski&Davis1987),whichrelatessurfacebrightness,size
ages,beingyoungestattheircentres.Thelargebluecolourgradi- andvelocitydispersion.TheFundamentalPlaneanditsprojections
entsobservedinmanyofthebluecorescannotbeexplainedbyan areaconsequenceofthedynamicsofearly-typegalaxiesandtheir
“inverted”gradientinmetallicity,i.e.moremetal-richpopulations underlyingstellarpopulations.KRisthereforethe“nextbestthing”
outwards.Ifweweretoexplainthelargerangeincolourbetween totheFundamentalPlanewhenmeasurementsofvelocitydisper-
theinnerandtheouterpartsofthegalaxybymetallicityalone,one sion arenot available. The observed dispersion of the KR at z=0
wouldhavetoinvokeunrealisticalylowmetallicitiesatthegalaxy is∼0.4magarcsec−2(LaBarberaetal.2003).Theredshiftevolu-
centre.Furthermore,thisscenariowouldpredictnoredshiftdepen- tionoftheKormendyrelationcanbeusedtoconstraintheassembly
denceonthefractionalcontributionofbluecores,incontrasttothe andformationhistory.Asignificantchange inslopewithredshift
observations. In this subsample (S1) of luminous (MV < −21) canreveal thepresenceof starformation ortherearrangement of
early-typegalaxies,themedianredshiftofblueandredearly-types thestarsbecauseof“dynamicalactivity”(i.e.mergers).
is1.10and0.85,respectively,i.e.bluerearly-typegalaxies(atleast Figure 10 shows the Kormendy relation of both volume-
atthemassiveend)contributemoresignificantlyatz∼> 1. limited samples S1 and S2 (grey dots). The surface brightness is
Early-typegalaxyformationinGOODS 9
Figure10.KormendyrelationofsamplesS1(left)andS2(right)intherest-
frameB-band(greydots).Opencirclesdotsaretheupperquartileofthe
redshift(S1)orthestellarmassdistribution(S2),aslabelled.Atypicalerror
Figure9.Intrinsic(V −i)colourdistributionofsubsampleS2:Theslope barisshown.Thedatahavebeencorrectedforthe(1+z)4cosmological
(top)andscatteroftheobservedV −icolourdistribution–roughlyrest- dimming.ThelinerepresentsthelocalrelationobservedintheComacluster
frame B −V over the redshift range of S2 – is shown with respect to (Jørgensenetal.1995).The17galaxiesfromtheK20sampleareshownas
stellarmass(left);projectedhalf-lightradius(middle)andV-bandabsolute crosses(diSeregoAlighierietal.2005).
magnitude(right).Theblackpointswitherrorbarscorrespondtoaverage
valuesofsubsamplesseparatedwithrespecttophotometricredshifttype:
old(filled dots;type t62)and young(hollow dots; t>2). 1σ error bars inFerrerasetal.(2005)fordifferentreasons.Figure4showsthat
areshown(aftera5σclipping).Thebinningisdoneatconstantnumberof mostofthefaint,low-redshiftgalaxiesinoursampleareremoved
galaxiesperbin. from S1 or S2 to enforce completeness. Those are mainly blue
early-typeswithsmallsizes.TheadditionofthosegalaxiestoS1or
S2wouldresultinsignificant scattertowardsthebottom-leftcor-
computed as the average withinthe half-light radius. Open black neroftheKormendyrelationinfigure10,inasimilarfashiontothe
dotscorrespond totheupper quartileinredshift (S1,left)orstel- presenceofthe“dwarfgalaxybranch”presentedintheKormendy
lar mass (S2, right), as labelled. The solid line is the local re- RelationplotofCapacciolietal.(1992).Henceweconcludethata
lation observed in the Coma cluster (Jørgensenetal. 1995) and significantfractionofthegalaxiesrejectedinFerrerasetal.(2005)
the crosses are a sample of 17 galaxies from the K20 survey becauseoftheKRconstraintmustbedwarfellipticalsatz∼< 0.4.
(diSeregoAlighierietal. 2005). Sample S1 – which extends to Figure11showstheredshiftevolutionoftheKRofsubsam-
higher redshift than S2 – has a significantly brighter zero point, plesS1andS2intermsofslope(top);averagesurfacebrightness
caused by the effect of lookback time on the stellar populations. (middle)andscatter(bottom;usingabiweightestimator).Sample
For comparison, a simple stellar population formed at zF=5 and S2issplitatthemedianvalueofstellarmass(3.5·1010M⊙).The
solarmetallicityundergoesafadingof∼ 1magintherest-frame fitisdonewithastandardleastsquaresalgorithm,binningthesam-
B band between redshifts z=1 and 0, similar to the offset found plesatconstantnumberofgalaxiesperbin(hencetheunequalsep-
in the left panel of figure 10. Sample S2 (right) has a KR com- arationbetweendotsinthefigure).Thearrowinthetoppanelrep-
patiblewithlocalearly-typegalaxies(againwithasignificantshift resentsthez=0slopeoftheKormendyRelation(LaBarberaetal.
ofthezeropointcausedbylookbacktime).Asexpected,galaxies 2003).SampleS1(blackdots)–focussingonbrightgalaxiesoutto
withahigherstellarmasscontentappearbrighterthanlessmassive thehighestredshifts–showsaremarkableagreementwiththelo-
galaxieswiththesamesize,roughlyadirectconsequenceofanon- calobservations,andwithmoderateredshiftclusters(Ziegleretal.
homologous surface brightness distribution (Graham&Guzma´n 1999).WeextendtheconclusionofZiegleretal.(1999)ofaneg-
2003).Anestimateofthetypicalerrorbarisshownineachpanel. ligliblechangeintheslopeoftheKRtoawiderangeofredshifts
ForthesurfacebrightnessweusethescatteroftheK-corrections and toa fieldenvironment. Themost massive galaxies in sample
applied to obtain B-band magnitudes (0.4 mag). The true error S2(greytriangles)alsoagreewiththelocalobservations,whereas
should be most certainly smaller than this K-correction. The er- aselectionofthegalaxieswiththelowermassesinS2(greystars)
rorinthesizeistakenfromtheworstcasescenariowhenretriving havelargerscatterandsteeperslopes.
half-lightradiifromsimulations(seeappendix). TheK20sample Inthemiddlepanelwecomparetheobservedtrendoftheaver-
fromdiSeregoAlighierietal.(2005)(crosses)spananarrowred- agesurfacebrightnesswiththefadingofatypicaloldpopulationas
shiftrange(0.9···1.3),andarecompatiblewithourhigh-redshift labelledinthefigurecaption.Noticethattheleastmassivegalaxies
(left)ormassive(right)subsamples. ofsampleS2areconsistentlybrighterthanthisestimate,reflecting
Noticethemajordifferencebetweentheselectioncriterionof thepresenceofyoungerstellarpopulations.Itisinterestingtono-
thisworkandFerrerasetal.(2005).Inthatpaper,aKormendyse- ticethatthe∼0.5magarcsec−2scatteraboutthebestfit(bottom)
lectionwasimposedonthevisuallyclassifiedsample,sothatearly- doesnotchangesignificantlywithredshiftorsample.
typesthatwerefoundnottoevolveintothelocalKRwererejected Tosummarise,wefindtheKormendyrelationofz∼0.3−1.2
fromthesample.Inthatcase,alargenumberoflow-redshift,blue tobethesameasatz ∼0.Theslopeandscatterofthecorrelation
early-typeswererejected.Itwasdiscussedwhether misclassifica- doesnotchangeandthezeropoint,i.e.thetypicalsurfacebright-
tionofbrightknotsofstarformationoranactivenucleuscouldex- nesschangesinagreementwithpurepassiveevolution.Onlylower
plainthisrejection.Inthispaperweusevisualclassificationasthe massspheroidals(Ms <3.5·1010M⊙)showasignificantdeparture
only selection criterion. We later define volume-limited subsam- fromtheaveragez = 0Kormendyrelation,appearingbrighterin
ples.Inthisprocessweenduprejectingsimilartypeofgalaxiesas hµBi0andwithasteeperslope.Webelievethisresultiscausedbya
10 I. Ferreraset al.
toexploretheintrinsiccolourdistribution.Weperformatechnique
basedonaVoronoitessellation(alreadyappliedin Ferrerasetal.
2005)that allowsarobust determination oftheradial colour gra-
dientandscatter.Infigure6weshowthatthecolourgradientsare
stronglydependent ontheoverallphotometrictype, suchthatred
galaxies have mostly red cores and blue early-type galaxies have
bluecores.Thisisanon-trivialstatement,asonecouldimaginestar
formationintheouterpartsofthegalaxy,resultinginbluegalax-
ieswithredcores.Wefindcentralstarformationinotherwisered
galaxies isvery rare. On the other hand, themajority of galaxies
with(recent)starformationexhibit mostof thisstarformationin
their centers. These observations suggest an inside-out formation
processinthesegalaxies,rulingoutrecent attemptsatexplaining
elliptical galaxy formation with an outside-in formation scenario
(Pipinoetal.2006).Aninside-outscenarioimpliesthatregardless
of the way star formation is triggered – primordial collapse ver-
sus merging/interactions, gas is accumulated towards the centre,
startingstarformationinthecentralregions,subsequently“diffus-
ing”outwards. One could argue whether themechanism explain-
ing these blue cores reduces to minor mergers (Ellisetal. 2001;
Kavirajetal. 2007b). Robust confirmation of the actual cause of
thesebluecoreswillrequiretheanalysisofabundanceratiogradi-
ents(seee.g.Ferreras&Silk2002).
Figure 11. Redshift evolution of the Kormendy relation. Sample S1 is
shownasblackcircles,sampleS2(grey)hasbeensplitwithrespecttothe Figure8showsthatthelocalobservationsofradialcolourgra-
medianstellarmasswithinthatsample(3.5·1010M⊙).Thebinsinredshift dientsinellipticalgalaxies(e.g. Peletier1990)mostlycorrespond
arechosensuchthatthenumberofgalaxiesperbinisconstantforeachset.
toarangeofmetallicities.Thiscanbethroughtofastheintrinsic
AKormendyrelationisfit,andthisplotgivestheslope(top),zeropoint
equivalentofthepuremetallicitysequenceofKodama&Arimoto
(middle;inmagarcsec−2)andbiweightscatteraboutthebestfit(bottom;
inmagarcsec−2). Thearrow in the toppanel is the z∼0 slope obtained (1997), used in that paper to explain the colour-magnitude rela-
tionamonggalaxies.Ourresultsareinagreementwiththesample
byLaBarberaetal.(2003)andthecrossesaretheclusterobservationsof
Ziegleretal.(1999).Thelineinthemiddlepanelcorrespondstotheevolu- of cluster galaxies at low and moderate redshift (Tamura&Ohta
tionofanexponentiallydecayingstarformationratestartedatzF=5,with 2000;LaBarberaetal.2004),butoursampleincreasesbyanorder
timescaleτ =0.5Gyrandsolarmetallicity. of magnitude thenumber andaccuracy of thecolour gradient es-
timates.LaBarberaetal.(2004)onlyfindopticalcolourgradients
compatiblewithzeroslope.Wedofindnon-zero,albeitsmall,opti-
combinationofyoungerstellarpopulationsandasignificantlydif- calgradientsandasignificanttrendwithstellarmass,suchthatthe
ferentsurfacebrightnessprofilecomparedtomoremassiveearly- mostmassivegalaxieshavealmostalwaysredcores.
typegalaxies,similartothez=0case(Graham&Guzma´n2003).
Defining volume-limitedsamples, wefindthat blue galaxies
only contribute 4% to the total number of bright (MV < −21)
early-typegalaxiesouttoz<1.4.Mostoftheblueearly-typegalax-
6 SUMMARYANDCONCLUSIONS ies in our – visually classified – sample correspond to low-mass
systems,andareremovedwhentakingvolume-limitedsamples.At
Wepresentasampleofvisually-classifiedearly-typegalaxiesover
the360arcmin2areacoveredbytheACS/HSTimagesoftheGreat facevalue,wecouldstatethatsignificant starformationinearly-
Observatories Origins Deep Survey (GOODS, Giavaliscoetal. typegalaxiesonlytakesplaceforgalaxieslessmassivethanMs ∼
2004), including both the North and South fields. The classifica- 1010M⊙evenouttoredshiftsz∼1.AsshownbyBelletal.(2007),
starformationinasubstantialfractionofbluecloudgalaxiesneeds
tion is only limited in flux (iAB < 24), i.e. no colour cuts or
to be shut off between z=1 and z=0, making them evolve to the
surfacebrightnessprofileconstraintsareimposedintheselection.
red sequence, thereby explaining the significant growth in mass
Thesamplecomprises910early-typegalaxies,forwhichwecom-
there. However, in addition to considering the contribution with
pilespectroscopicqualityredshiftsfrompubliclyavailabledatasets
respect to mass and star formation history, it is also well known
for 541 galaxies (i.e. ∼ 60% of the total sample). For the rest
thatbluecloudandredsequencediffermorphologically,withspi-
weestimate photometric redshifts from availabledata (bothHST
ralandirregulargalaxiesdominatingthebluecloudwhereasredse-
and ground-based). We find the comoving number density to de-
quencegalaxiesaremostlyearly-types.Sinceoursampleofearly-
pend strongly on stellar mass, such that the number of galaxies
with Ms ∼> 1011M⊙ only increases a factor 2 between z=1 and types was selected on pure morphological grounds, those early-
z=0, whereas the number of galaxies with stellar mass 1010 < typeswith(recent)starformationconfirmthescenarioofBelletal.
Ms/M⊙ <1011increaseafactor∼50inthesameredshiftrange. (2007),inthattheseobjectswillnot onlyevolveonto theredse-
quenceintermsofcolour,butalsointermsofmorphology.
Inthefull,flux-limitedsample,wefind26%ofourearly-typesare
blue(i.e.correspondingtoastarformationhistorywithtimescale The Kormendy relationwas explored intwo volume-limited
longerthat1Gyr,orwithasignificantamountofyoungstars).This samplesandaremarkableconsistencyisfoundwiththeKRatz=0,
fractionissimilartothenumberoflow-redshiftearly-typeswitha withnosignificantchangeoftheslopeoftheKormendyrelationout
significantamountofrecentstarformation(Kavirajetal.2007). toz∼1.2.Thescatteraboutthebestfitdoesnotchangesignificantly
ThedepthandsuperbresolutionoftheACSimagesenableus either.Onlytheaveragesurfacebrightnessisfoundtochangeinthe