K+a galaxies in the zCOSMOS Survey: Physical properties of systems in their post-starburst phase

0.72 MB·English

by  D. Vergani

Astronomy&Astrophysicsmanuscriptno.0000 (cid:13)c ESO2010
January21,2010
⋆
K+a galaxies in the zCOSMOS Survey:
Physical properties of systems in their post-starburst phase
D.Vergani1,2,G.Zamorani2,S.Lilly3,F.Lamareille4,C.Halliday5,M.Scodeggio6,C.Vignali2,P.Ciliegi2,
M.Bolzonella2,M.Bondi7,K.Kovacˇ3,C.Knobel3,E.Zucca1,K.Caputi3,L.Pozzetti1,S.Bardelli1,M.Mignoli1,
A. Iovino8,C.M.Carollo2,T.Contini4,J.-P.Kneib9,O. LeFe`vre9,V.Mainieri10,A.Renzini11,A.Bongiorno12,
G.Coppa2,O.Cucciati9,S.delaTorre8,L. deRavel9,P.Franzetti6,B.Garilli6,P.Kampczyk2,J.-F.LeBorgne4,V.Le
Brun9,C.Maier2,R. Pello4,Y. Peng2,E. PerezMontero4,E. Ricciardelli11,J.D.Silverman2,M. Tanaka10,
0 L. Tasca6,L. Tresse9,U.Abbas13,D. Bottini6,A. Cappi1,P. Cassata9,A. Cimatti2,L. Guzzo8,A.M.Koekemoer14,
1 A. Leauthaud15,D. Maccagni6,C. Marinoni16,H.J. McCracken17,P. Memeo6,B. Meneux12,18,P. Oesch2,
0
2 C. Porciani19,R. Scaramella20,P.Capak21,D.Sanders22,N.Scoville21,andY.Taniguchi23

n (Affiliationscanbefoundafterthereferences)
a
J Received30June2009;accepted9September2009

1
2
ABSTRACT

]
O Aims. Theidentitiesofthemainprocessestriggeringandquenchingstar-formationingalaxiesremainunclear.Akeystageinevolution,however,
appearstoberepresentedbypost-starburstgalaxies.Toinvestigatetheprevalenceofthesegalaxiesandtheirimpactongalaxyevolutioningeneral,
C
weinitiatedamultiwavelengthstudyofgalaxieswithk+aspectralfeaturesinthewell-studiedCOSMOSfield.
. Methods.Weexamineamass-selectedsampleofk+agalaxiesintheCOSMOSfieldatz=0.48−1.2usingthespectroscopiczCOSMOSsample.
h
Toclassifygalaxiesintheirk+aphase,weuseaspectroscopiccriterion,basedontheamplitudeoftheHδabsorptionlineandtheabsenceof
p
the[OII]emissionline.Wedevelopouranalysisforawell-definedsamplebyimposingstringentconfidencelevelsonthespectroscopicredshifts
-
o andspectralmeasurements.Wecompareourresultsfortwomass-selectedsamplesofstar-formingandquiescentgalaxiesselectedusingapurely
r spectralclassificationschemefromthe10,000zCOSMOScatalogue(i.e.,basedonmeasurementsof4000ÅbreakandEW[OII]).
t
s Results.In our mass-limited sample, k+a galaxies occupy the brightest tail of the luminosity distribution. They are as massive as quiescent
a galaxiesandpopulatethegreenvalleyinthecolourversusluminosity(orstellarmass)distribution.Asmallpercentage(<8%)ofthesegalaxies
[ haveradioand/orX-raycounterparts(implyinganupperlimittotheSFRof∼8M yr−1).Overtheentireredshiftrangeexplored,theclassofpost-
⊙
starburstgalaxiesismorphologicallyaheterogeneouspopulationwithasimilarincidenceofbulge-dominatedanddiskygalaxies.Thisdistribution
2
doesnotvarywiththestrengthoftheHδabsorptionlinebutinsteadwithstellarmassinawayreminiscentofthewell-knownmass-morphology
v
relation.TheresultsabouttheincidenceofasymmetriesandtheconcentrationofthelightdistributionderivedfromHST/ACSimagesimplythat
8
thisgalaxypopulationpossiblyrepresentsanintermediatestageofgalaxyevolution.Althoughk+agalaxiesarealsofoundinunderdenseregions,
6
they appear to reside typically in a similarly rich environment as quiescent galaxies on a physical scale of ∼ 2−8 Mpc, and in groups they
9
showamorphologicalearly-to-latetyperatiosimilartothequiescentgalaxyclass.Withthecurrentdataset,wedonotfindevidenceofstatistical
1
significantevolutionineitherthenumber/massdensityofk+agalaxiesatintermediateredshiftwithrespecttothelocalvalues,orthespectral
.
9 properties,althoughmoresolidresultsonthisandotheraspectswillbeobtainedfollowingthecompletionofthesurvey.
0 Conclusions.Severalmechanismsrelatedandunrelatedtotheenvironmentareatworkinquenchingstar-formationactivityingalaxiesonshort
9 timescales(<1Gyr).Thosegalaxies,whichareaffectedbyasuddenquenchingoftheirstar-formationactivity,mayincreasethestellarmassof
0 thered-sequencebyuptoanon-negligiblelevelof∼10%.
:
v Keywords.Galaxies:formation-galaxies:evolution-galaxies:fundamentalparameters-galaxies:massfunction-cosmology:observations
i
X
r
a 1. Introduction Brinchmann&Ellis 2000), observational evidence exists that
manygalaxieshaveexperiencedmorphologicaltransformations
Numerous studies have demonstrated that a bimodal popula-
at various redshifts. Brinchmann&Ellis (2000) reported that
tion of galaxies exists up to high redshift (e.g., Franzettietal.
a decline has occurred in the mass density of irregular galax-
2007; Mignolietal. 2009). The first population consists of
ies between z ≃ 1 and today, in contrast to an increase for
galaxies that are actively star-forming, gas-rich, rotationally-
regular galaxies. Further support of this redistribution between
supported, and morphologically disk-dominated. The second
galaxy types has been provided by study of the stellar mass
population represents typically quiescent galaxies that are
function by many different surveys, e.g., K20 (Fontanaetal.
gas-poor, pressure-supported, and have spheroidal morpholo-
2004), COMBO-17 (Borchetal. 2006), DEEP2 (Bundyetal.
gies. As noted by many studies (e.g., Boweretal. 1998;
2006), SWIRE-VVDS-CFHTLS (Arnoutsetal. 2007), SDSS
(Martinetal. 2007), and VVDS (Verganietal. 2008). It is im-
Send offprint requests to: D.Vergani, e-mail: portant to assess the role of environment in these transforma-
[email protected]
tionsofgalaxytype.Forinstance,ithasbeenshownthatgalaxies
⋆ Based on data obtained with the European Southern Observatory
in denseregionshavemodifieddistributionsof HI,the primary
VeryLargeTelescope,Paranal,Chile,program175.A-0839.
2 D.Verganietal.:Post-starburstgalaxiesinCOSMOSfield
source of fuel for star-formation, and truncated star-formation vational constraints, the majority of studies cover small areas
activity (e.g., Giovanelli&Haynes 1985; Cayatteetal. 1990; and might probe specific types of environments (large-scale
Koopmann&Kenney 2004). However, several other mecha- structures in pencil surveys, or studies in clusters). In addition
nisms may also contribute (see below). Thus, important ques- to the observational limitations, finding post-starburst galaxies
tionsingalaxyformationandevolutionarewhetherstar-forming is also challenging because of the short duration of the post-
galaxies are the precursors of the quiescent galaxy population starburst phase (1–1.5 Gyr) (see Couch&Sharples (1987) and
observedatthepresentday,orwhethertherearealternativepaths Bargeretal.(1996)).Becausetheyrepresentasmallfractionof
in galaxyformation,and, if so, whatare the primaryprocesses thetotalgalaxypopulation,thesystematiceffectscausedbythe
involvedandhowaretheycontrolled? aforementioned problems require particular attention when in-
Themainlimitingfactorinansweringthisquestionisthatwe terpretingtheresultsinageneralframework.
still have notrue understandingof whattriggersstar formation In this paper, we present for the first time a mass-
ingalaxiesandwhythisactivityissuppressedinsomeofthem. selected sample of post-starburst galaxies at intermediate red-
There is a class of galaxiesthat might shed light on this issue. shift in a wide variety of different environments. This mass-
Theyarecalledk+aorE+Agalaxies(dependingontheauthor), selected sample and its parent galaxies are extracted from
orsimply galaxiesin the post-starburstphase.Inthis work,we the largest multiwavelength survey existing up to now, the
adoptthespectralterminologyk+ainsteadofthemorphological Cosmic Evolution Survey, or COSMOS (Scovilleetal. 2007)
E+Anomenclature. imagedwithHST/ACS(Koekemoeretal.2007).Weprovidean
Post-starburst galaxies, discovered by Dressler and Gunn overviewofthemainphysicalpropertiesofthesegalaxiesusing
(1983), exhibit peculiar spectral characteristics, i.e., strong data of unprecedented quality and completeness in this study.
Balmerabsorptionlines,indicativeofanintensestar-formation We presentafirstattempttoquantifytheircosmicevolution.A
epochinthepastbillionyears,andanabsenceofemissionlines, moredetailedstudyofthissubjectanditsrelationtothegeneral
asignatureofalackofongoingstar-formation.Onthebasisof pictureofgalaxyevolutionoverthepast8Gyrwillbepresented
these spectral signatures, they have been assumed to represent oncompletionofthesurvey.
galaxies in a transitional stage between being a star-forming, Thepresentworkisorganizedasfollows:thedataandsam-
late-typegalaxyandapassive,early-typesystem. ple selection are presented in Sect.2; results are described in
Variousmechanismshavebeenproposedtoexplainthechar- Sect.3, and a summary is given in Sect. 4. Throughout this
acteristic spectral features of post-starburst galaxies. Cluster- work,weassumeastandardcosmologicalmodelwithΩM =0.3,
relatedmechanisms,suchasram-pressuregasstripping,harass- ΩΛ = 0.7,and H0 = 70kms−1Mpc−1.Magnitudesaregivenin
ment, or strangulation (Gunn&Gott 1972; Larsonetal. 1980; theABsystem.
Balogh&Morris 2000) are efficient in suppressing the star-
formation activity of a galaxy. Events such as galaxy mergers
2. Sampleselection
and interactions (Toomre&Toomre 1972; Barnes&Hernquist
1992), commonly found in both the field and in clusters (al- 2.1.Thespectroscopicsurvey:zCOSMOS
though more efficient in the field), have been proposed. The
large fraction of morphological irregularities (of similar phys- ThezCOSMOSproject(Lillyetal.2007)isaredshiftsurveyof
ical timescale as a post-starburst phase) and galaxy compan- galaxiesintheCOSMOSfieldconsistingoftwoobservingpro-
ions in the vicinity of these galaxies, support the hypothesis grams.The “deep part”focuseson high-zgalaxies(1.4 < z <
thateitheradynamicalinteractionoramergerwithexchangeof 3.0) using a combined flux-limited (BAB < 25.25) and colour
material may have suppressed star formation. Strong AGN/SN selection(theseobservationsarenotusedinthepresentstudy).
feedback might also contribute to star-formation quenching The “bright part” of zCOSMOS project aims to acquire spec-
(Springeletal. 2005; Hopkinsetal. 2007), depending on the traofabout20,000galaxies(IAB < 22.5)witharatheruniform
stellarmass(Kavirajetal.2007). sampling rate (about 60–70%) across the 1.7 deg2 COSMOS
This galaxy population was first discovered, and has been field, a superb spectroscopic redshift success rate (> 97% at
primarilystudiedsince,insideclusters(Dressler&Gunn1983; 0.5 < z < 0.8), and high velocity accuracy (∼ 100kms−1).
Couch&Sharples1987;Tranetal.2003;Poggiantietal.1999, The brightpartis observedwith the red R ∼ 600VIMOSMR
2009), even if post-starburst galaxies have also been found grism covering the spectral range 5500–9500Å (2.55Å disper-
in the field. Numerous studies have been undertaken to study sion), which provides an ideal configuration for detecting key
the properties of field post-starburst galaxies at low red- spectralfeaturesingalaxiesouttoz∼1.2.Thispaperdealswith
shift(Liu&Kennicutt1995;Zabludoffetal.1996;Changetal. the first “10k” galaxydata set completed to date for the bright
2001; Nortonetal. 2001; Goto&SDSScollaboration 2003; partofthezCOSMOSsurvey,whichwerefertohereafterasthe
Quinteroetal. 2004; Yangetal. 2004), and at z > 0.3 zCOSMOSsurvey.Alldetailsofdataacquisition,reduction,red-
(Hammeretal. 1997; Dressleretal. 1999; Poggiantietal. shiftmeasurements,andtheirqualitycanbefoundinLillyetal.
1999; Baloghetal. 1999; Tranetal. 2004; Yangetal. 2008; (2007)andLillyetal.(2009),whichalsodescribesotherimpor-
Poggiantietal. 2009). These studies commonlyconcludedthat tantaspectsofthissurvey.
post-starburstgalaxiesareaheterogeneousparentpopulation.
Post-starburst galaxies appear to be rare at any epoch and
2.2.Thespectralmeasurementsandsampleselection
discrepantresults abouttheir nature are found in the literature.
The identifiable sources of discordance are numerous. Besides Wemeasurethespectralfeaturesinour10ksamplewithanau-
differencesinselectioncriteriaandspectralresolution,themain tomatedsoftwarepackage(platefit VIMOS).Hereweprovidea
source of discrepancy may be the different signal-to-noise ra- briefoutlineofthefittingprocedure.WerefertoLamareilleetal.
tio achieved at different redshifts. This implies that we should (2006) for full details, and both Tremontietal. (2004) and
also investigate the dependency of the underlying properties Brinchmannetal. (2004) for further details of the original
on the host galaxies (e.g., the sampling of only the most mas- pipelinedevelopedforthehigherresolutionSDSSspectra.The
sive or brightest systems). Furthermore, because of the obser- best-fit stellar continuum derived from a grid of stellar pop-
D.Verganietal.:Post-starburstgalaxiesinCOSMOSfield 3
In this paper, we use measurements of the spectral in-
dex Hδ as defined by Worthey&Ottaviani (1997) and
A
Baloghetal. (1999), the equivalent width of the [OII]λ3727
doublet (EW[OII]), the equivalent width of the [OIII]λ5007
(EW[OIII]), and the amplitude of the 4000 Å break (D 4000)
n
(Baloghetal. 1999). A careful treatment of spectral fitting
is mandatory because both emission and absorption lines are
present in the Balmer lines. The index Hδ and the equivalent
A
width of the lines are defined such that positive valuesrefer to
absorptionfeatures.
AuthorsusuallyadoptHδAandEW[OII]featuresintheircri-
teriaforselectingpost-starburstgalaxies,and/orHα depending
on the spectral coverage, although some exceptions exist. For
example, Tranetal. (2004) used a combination of Hδ and Hγ
indices,in additionto [OII],Yanetal. (2006) addedanempiri-
callimit to EW(Hβ) in constrainingEW[OII],and someothers
used, in addition to a threshold on emission lines, the relative
abundance of A-type to K-type stars, i.e., the ratio A/K (e.g.,
Quinteroetal.2004).Inthispaper,weadoptaconventionaldef-
initionusingboththe[OII]emissionlineandHδA,tobeableto
comparewiththemajorityofotherworks,althoughanycompar-
isonisdifficultandonlypossiblequalitatively.Inafuturepaper,
basedonthe20kgalaxysample,wewillexaminetheeffectsof
different selection criteria on a larger sample of Hδ-enhanced
galaxies(e.g.,LeBorgneetal.2006)andthepossiblecontribu-
tionfromobscuredAGNs(Yanetal.2006).Inthepresentpaper,
ourpost-starburstcandidatecriteriaarethattheequivalentwidth
ofHδ islargerthan+3Åandtheequivalentwidthoftheemis-
A
sion line [OII] is larger than −3Å. This is illustrated in the top
panelofFig.1,wheretheequivalentwidthofHδ isplottedver-
A
susEW[OII]forthe10k-parentgalaxiesintheredshiftinterval
0.48 ≤ z ≤ 1.2(theredshiftbinexploredinthiswork,whichis
imposedbyourinstrumentalset-up).Thisplotshowsthespec-
troscopic criteria adopted to select the post-starburst candidate
galaxies.AsisevidentinFig.1,theregioncorrespondingtoour
k+acandidatesisnotcompletelyseparatefromthatofthe10k-
parent population as it is in the SDSS data (cf. the left panel
of Fig.2 by Yanetal. (2009)). This is mainly due to the errors
associatedwiththespectralmeasurements.Toevaluatetheirim-
pact on the classification scheme, we tested differentempirical
thresholds and applied a confidence level to the spectral fea-
tureswhendefiningoursamples.Thisapproachaimstoexclude
(include) residual signatures of ongoing star formation and to
Fig.1.(Top)DistributionofequivalentwidthsofHδvs.EW[OII]
testthereliabilityofourspectralmeasurements.Whilethenum-
for all galaxies in the 10k-parent sample in the redshift inter-
bersofobjectschangedependingonthethresholdsadopted,of
val 0.48 ≤ z ≤ 1.2. This plot shows the criteria adopted to
course, we obtained an optimal compromise between the abil-
select the post-starburst candidate galaxies, i.e., the region be-
ity to completea robust,statistical analysis and reliable results
tween HδA> +3Å and EW[OII]> −3Å marked with dashed by including only galaxies with EW[OII] larger than -3Å and
lines. Isophotal contours are in steps of 10% with the faintest
EW(Hδ)largerthan3Åatthe2σconfidencelevel.
isophotelevelstartingatthe10%level.(Bottom)Templatespec-
trum of a typical galaxy in a post-starburst phase obtained by To maintain strong control on occasional poor wavelength
stacking 69 zCOSMOS post-starburst galaxies more massive calibration that can affect in particular the bluest part of our
than log(M/M )> 10.0 (corrected by selection effects as ex- spectra, we visually inspected each spectrum of potential can-
⊙
plainedinSect.2.5). didates. For spectra with doubtful measurements of EW[OII],
we imposed an additional limit on the equivalent width of the
EW[OIII]line.Weusedasubsampleofhighsignal-to-noise10k
spectra and high confidence spectral measurements in which
ulation synthesis models (Bruzual & Charlot 2003, hereafter bothEW[OII]andEW[OIII]linesweredetectableinthespectra,
BC03)wassubtractedfromtheobservedspectrum.Anyremain- to define this limit on the [OIII] lines. From the best-fit model
ing residual was removed by fitting a low-order polynomialto solution, we inferred a threshold for the EW[OIII] of −6.9Å.
thecontinuum-subtractedspectrum,andallemissionlineswere Furthermore,fromallofourcatalogues,weremovedbothstars
then fitted simultaneously with a Gaussian profile. Finally, ab- and broad-line AGNs and adopted a high-quality flag scheme
sorption features and spectral breaks were measured after sub- for the spectroscopic redshifts (> 95.5% confidencelevel) (for
tractingemissionlinesfromtheoriginalspectrum. details see Lillyetal. (2007) and Lillyetal. (2009)). Although
4 D.Verganietal.:Post-starburstgalaxiesinCOSMOSfield
weinspectedeachspectrumindividually,ourconfidencelevelin
theredshiftmeasurementsis100%forourpost-starburstsample.
We also performed quality checks of our post-starburst candi-
datespectratoverifytheimpactofresidualskylinesonspectral
features.WeexaminedthereducedVIMOSspectrumfortheim-
pactoffringingeffects,whichmaybesignificantatwavelengths
above λ > 8,500Å because of the thinned, back-illuminated
CCDsusedinVIMOS(Scodeggioetal.2005).
The final sample presented in Sect.3 contains spectra that
passedalloftheaforementionedtests.
Usingtheconstraintsdescribedabove,weobtainasampleof
74 post-starburstgalaxiesin the flux-limited zCOSMOS bright
sampleintheredshiftintervalz∈[0.48−1.2].Threeofthemare
atredshiftshigherthanz>1.Foroneobject,thecomputationof
a precise absolute magnitudeis impossible because of the lack
of sufficientphotometriccoverage,and it is excludedfrom our
statisticalanalysis(fordetailsseeinZuccaetal.(2009)).
2.3.Post-starburstspectraltemplate
Giventhedifficultiesinselectingthisrarepopulationofgalaxies,
weprovideatemplatespectrum1 tothecommunityofthepost-
starburstgalaxypopulationobtainedbystacking69zCOSMOS
post-starburstgalaxiesmoremassiveoflog(M/M )>10.0(cor-
⊙
rectedbyselectioneffectsasexplainedinSect.2.5).Adescrip-
tion of the steps performed to obtain this template spectrum is Fig.2. Stellar massand B-bandabsolute magnitudeas a func-
giveninthefollowingtext. tion of redshift of the zCOSMOS bright sample (points, gray
The bottom panel of Fig.1 shows the template spectrum, symbols). The (violet) triangles show the location of post-
which exhibits the distinctive spectral features of galaxies that starburstgalaxies.Thelightanddarkgrayareasshowtheselec-
have recently experienced a starburst before its star-formation tioninstellarmassadoptedinsampleMCORandsampleMST,
wasquenchedpriortoobservations.Thesefeaturescanbesum- respectively. The dashed (black) line in the upper panel illus-
marizedasfollows: tratesthecompletenessinmass-to-lightratiofortheglobalpop-
i) The strength of the [OII] nebular emission line, which is ulation(for comparison,it is plotted with a dotted,red line the
emitted in HII regions around O and B stars, is almost absent completeness limit for the early-type population). The dashed
(black) line in the bottom panel is plotted as reference to il-
(EW[OII]<−2.5Å)inourtemplatespectrum.Giventhat[OII]is
anindicatorofrecentstarformation(withlifetimeof107yr),this lustrate that our galaxies of sample MST are brighter than the
evolving-luminositylimitatM <−20.1−z.
propertyindicatesthatthestarformationduringtheepochofthe B
observationhasstopped.TheHαemissionlineisanotherrobust
indicatorofongoingstarformation,butcannotbedetectedwith
ThecombineduseofthetwoSFRestimators(Hδand[OII]),
our adopted R ∼ 600 VIMOS MR-grism observational setup,
which are sensitive to stars of differentstellar masses and life-
whichcoversthespectralrange5500Å<λ<9500Å.
times,allowsustoobtaininthenextsectionaquiteprecisecos-
ii) Another typical spectral feature of this galaxy populationis
micclockoftheirrecent(<1Gyr)star-formationhistory,andto
thestrongabsorptionfeaturesofBalmerlines.AlthoughHαand
characterizetheirphysicalproperties.
Hβ are often filled in by nebular emission that can make their
interpretation difficult, Hδ can be used to estimate the stellar
ages.Fromlate-typestarstoearlier-typestars, theHδstrength- 2.4.Stellarmasses
ens.ThiscanbeachievedifA-starsdominatethemain-sequence
Since we studied galaxies that are understood to have expe-
contributionneartheepochoftheobservation.
rienced a recent secondary burst of star formation, we used
iii) As noted originally by Rose (1984), the ratio of the equiv-
Charlot&Bruzualmodelswiththeinclusionofsecondarybursts
alentwidthsoftheCaIIHλ3968+Hǫ toCaIIKλ3934Åspectral
(Charlot&Bruzualinprep.,orCB07).Thesemodelshavebeen
linesisconstantinstarslaterthanaboutF02,butincreasesdra-
frequently used to model the complex star-formation history
maticallyforearliertypestarsastheCaII linesweakenandHǫ
of a galaxy more realistically than a purely exponentially de-
strengthens.Thisspectralpropertyisalsoillustratedinourtem-
clining law (Kauffmannetal. 2003a; Brinchmannetal. 2004;
plate spectrum, where the CaII Hλ3968+Hǫ is deeper than the
Salimetal.2005).
CaIIKλ3934Åabsorptionlines.
Stellarmasseswerederivedbyfittingstellarpopulationsyn-
iv)Wenotethelackofemissionfrom[OIII]andHβlinesinour
thesismodelsfromthelibraryofCB07tothebroad-bandoptical
templatespectrum,whichindicatesthatnarrow-lineSeyfertsdo
(CFHT:u,i,Ks;Subaru:B,V,g,r,i,z;Capaketal.2007)and
notrepresentanimportantcomponentofoursample,incontrast
near-infrared(Spitzer/IRAC:3.6µm,4.5µm;Sandersetal.2007)
tootherworks(e.g.,Yanetal.2009).
photometryby minimizingthe chi-squaretest statistic for each
galaxy.Theestimateofthestellarmassprovidedbythebest-fit
1 This template can be accessed from the web page modelspectralenergydistributionisthecurrentmasscontained
http://www.bo.astro.it/∼daniela/template.fits. instarsineachgalaxyattheepochoftheobservationobtained
D.Verganietal.:Post-starburstgalaxiesinCOSMOSfield 5
byintegratingthestarformationhistoryoverthegalaxyageand complex scheme that introducesa redshift dependenceof SSR
subtractingthecumulativemasslostduringstellarevolution. doesnotalterthefinalresultsappreciablyintheredshiftbinex-
The measurement of stellar mass included some assump- ploredhere(seeBolzonellaetal.2009).InthesampleMCOR,
tions.WeusedaChabrierinitialmassfunction(Chabrier2003) themasscompletenesslimit,whichdependsonbothredshiftand
withaloweranduppercutoffat0.1and100M ,adustattenua- mass-to-lightratio, also takes accountof the colour-magnitude
⊙
tiondescribedbythemodelofCharlot&Fall(2000),andmetal- relation(Pozzettietal.2009;Bolzonellaetal.2009).
licities between 0.1 and 2 Z⊙. The star-formation history was The second approach permits us to increase the number of
modelledbytwocomponents.Thefirstcomponentwasacontin- objects(from18to69)inouranalysisandexploreawiderred-
uousstar-formationmodelwith an exponentiallydeclininglaw shiftbin(z∈[0.48−1.2])thanthatofz∈[0.48−1.0]forsample
oftheformSFR(t)∝exp(-t/τ),wheretimescaleτandagetare MST. However, we always compare the results obtained with
intherangeτ/[Gyr]=[1,∞]andt/[Gyr]=[0.1,20].Thesecond thisMCORsamplewiththoseobtainedwiththecompleteMST
componentwasrepresentedbyrandomburstssuperimposedon sampletovalidatefurtherourconclusions.
the first component that had an equal probability of occurring Figure2showsthestellarmassofthezCOSMOSbrightsam-
throughoutthelifeofthegalaxy. pleasafunctionofredshiftand,overplotted,thedataofthepost-
Ofcoursetheresultsdependontheadoptedassumptionsand starburstgalaxiesinourmass-completesamples(withviolet,tri-
modelsused.Forexample,theassumptionofa Chabrierinitial anglesymbols).Wenotethattheconservativelimitimposedby
mass functionproducesa consistentdifference in the valuesof Mock catalogues agrees well with the completeness in the ob-
thestellarmassatfixedagethatiswithinafactorof1.7,butwith servedmass-to-lightratiofortheglobalpopulation.Thislimitis
a smalldispersion,of thosederivedwith the SalpeterIMFpre- similartothemoreconservativelimitobtainedfortheearly-type
scription (Pozzettietal. 2007). The inherent uncertainty in the population (defined using the spectrophotometricclassification
initial mass function adopted is not a global limitation for re- describedinZuccaetal.2009).Thestatisticalweightsandmass
sults based on stellar masses given the constancy of this rigid completenessadoptedarethevaluesusedintheluminosityand
shift over a wide range of star formation histories, although to mass function studies by Zuccaetal. (2009), Bolzonellaetal.
limitthevariousdegeneraciesalargesetofgridsofstellarpop- (2009), and Pozzettietal. (2009). In the same figure, we also
ulationsynthesismodelswereused.Furtherdetailsofthestellar plottheB-bandabsolutemagnitudeasafunctionofredshift.It
massmeasurementscanbefoundinBolzonellaetal.(2009)and isimportanttonotethatoursampleMST,whichiscompletein
Pozzettietal.(2007,2009). terms of stellar mass such that log(M/M )> 10.8, is brighter
⊙
thantheevolving-luminositylimitatM <−20.1−z(illustrated
B
withthedashed,blacklineoverplottedintheleftpanelofFig.2).
2.5.Post-starburstrepresentativesample
Sinceouroriginalselectionisbasedonaflux-limitedsurvey,we
2.6.Comparisonsamples
used subsamples of galaxies complete in terms of stellar mass
(adopting two independent approaches) to avoid selection bi-
We compiled two comparison catalogues of strongly star-
ases.
forming and quiescent galaxies. For these galaxies, we also
In the first approach, we adopted a conservative limit to
adopted consistently the same stellar mass completeness crite-
the stellar mass of log(M/M )> 10.8 in the redshift bin z ∈
⊙ ria,redshiftflags,andconfidencelevelsonthespectralmeasure-
[0.48 − 1.0] (from now on, this stellar Mass STringent crite-
mentsasusedindefiningtheclassofpost-starburstgalaxies.
rion refers to sample MST). This stringent mass limit was de-
Thestar-formingandquiescentgalaxyclasseswereselected
fined using Mock catalogues from the Millennium simulation
usingapurespectralclassification.Wecombinedmeasurements
(DeLucia&Blaizot2007)ofthezCOSMOSbrightsample(see
of the amplitude of the 4000 Å break, taken as an age esti-
Meneuxetal. (2009) for an exhaustiveexplanationon the sub-
mator of the underlying stellar population, and the equivalent
ject).Weselected18post-starburstgalaxiesforthisconservative
width of the [OII]λ3727 line, assumed to be an indicator of
mass-completesample,orsampleMST.
the star-formation activity. Thus, quiescent galaxies were de-
To maximize the quality of the statistical analysis, we also
fined to be galaxies with a zero, or negligible, star-formation
constructed a second sample, or sample MCOR, including
rate inferred using the EW[OII], i.e., EW[OII]> −3Å, and an
galaxies more massive than log(M/M )> 10.0 in the redshift
⊙
binz∈[0.48−1.2],andaccountingforunder-representedgalax- oldunderlyingstellarpopulationdescribedbytheirlarge4000Å
break (D 4000> 1.5) (e.g., Kauffmannetal. (2003b)). We de-
iesathighzbycorrectingwithstatisticalweightsandaV /V n
tot max
formalism(Schmidt1968).Thetotalvolumesampledbythesur- fined a sample of strong EW[OII] emitters by constructing a
veywithintheredshiftbinexploredisdenotedbyV .Thevol- catalogue of objects with EW[OII]< −15Å and small 4000 Å
tot
ume Vmax is the maximum volume in which each galaxy of a break (Dn4000≤ 1.5). From the 10k sample, we also compiled
givenI magnitudeisstillobservable.Weappliedthestatistical the 10k-parentcatalogue of galaxies following the selection in
AB
weightsadoptedinthestudiesofluminosityandmassfunctions stellar massadoptedforsample MCOR and sample MST, and
of the zCOSMOS sample (Zuccaetal. 2009; Bolzonellaetal. allothercriterialistedabove(fromnowonwhenweusetheter-
2009;Pozzettietal.2009).Theseweightsaretheproductofthe minology“allobjects”,werefertothe10k-parentcatalogue).
inverseofthetargetsamplingrate(TSR)andtheinverseofthe OurselectedsamplesaredisplayedinFig.3,wheretheselec-
spectroscopic sampling rate (SSR). The target sampling rate is tioncriteriausedtoclassifygalaxiesinourcomparativeanalysis
the ratio of the objects spectroscopically observed to the total areshown.Thisfigureillustrateshowthethreeclassesofgalax-
numberofobjectsintheparentphotometriccatalogue.Thespec- ies (highly star-forming, quiescent, and post-starburst galax-
troscopic sampling rate is the ratio of the number of observed ies) populate the plane of the amplitude of 4000 Å break and
objects with reliable redshifts to the total number of observed EW[OII].Smallpointsandisophotalcontoursin steps of15%,
objects,calculatedinbinsofapparentmagnitude.Theapparent with the faintest isophote level starting at 15%, show the 10k-
magnitude dependence takes into account the decline towards parentgalaxieslevel.UsingcriteriaadoptedforsampleMCOR,
faintermagnitudes,inourabilitytomeasurearedshift.Amore we obtain a comparison sample of 295 quiescent galaxies and
6 D.Verganietal.:Post-starburstgalaxiesinCOSMOSfield
amount of dust. Galaxies with these characteristics should be
dusty, starburst galaxies (and misclassified post-starburst ob-
jects). This is a hypothesis that should be verified especially
when dealing with galaxies understood to have experienced a
recent, strong burst of star formation. OB stars responsible for
the [OII] emission, as other younger stellar generations, are
more strongly obscured by dust than older stellar populations.
This age selective dust extinction enables the true strength of
the Balmer absorption lines to be measured, but only a frac-
tionoftheemissiontobedetectedinstrongerstarburstgalaxies
(Poggiantietal. 2001, 2009). To ascertain whether our sample
of post-starburst galaxies may be contaminated by dusty star-
burst galaxies, we performeddifferent tests. First, we explored
the dusty starburst scenario by computing both the extinction
valuesandstar-formationrate by SED fitting. Secondly,taking
advantageoftheavailablemultiwavelengthobservations,weex-
ploredthe X-ray and 1.4GHz radiodata sets (Schinnereretal.
2007;Cappellutietal.2009;Brusaetal.2009).
1) Dust extinctionandSED fitting SFR. We foundthatpost-
starburstgalaxieshaveingenerallowvaluesofextinctionasesti-
matedbyfittingCB07modelSEDs(medianvalue<τ >=0.6).
V
ThemedianSFRis0.6M yr−1,andthe25%and75%quartiles
⊙
Fig.3. The amplitude of 4000 Å break is plotted as a func- ofthedistributionrangefrom0.01M yr−1 to4.2M yr−1.The
⊙ ⊙
tion of EW[OII] to show the selection criteria used to classify distributionsofbothτ andSFRsforpost-starburstgalaxiesdif-
V
theclassesofstar-forming(square,bluesymbols)andquiescent
fer at a high confidence level from the same distributions for
(circle,orangesymbols)galaxies.Triangle(violet-coded)sym-
star-forminggalaxies(<τ >∼2and<SFR>∼21M ).While
V ⊙
bols show the post-starburst galaxies. Small points (in black)
post-starburst galaxies exhibit a similar cumulative τ distri-
V
show the 10k-parentcatalogue (isophotal contoursare in steps
bution to quiescent galaxies, the latter galaxies have an even
of 15% with the faintest isophote level starting at 15% level).
lower rate of star-formation activity, as expected when using
The4000Åbreakhistogramsofthe10k-parentsample(dashed, the used spectroscopic criteria described in Sect.2 to identify
gray line) and of the post-starburst galaxies (solid, violet line) them (< τ >∼ 0.4 and < SFR >∼ 0.07M ). For only one
V ⊙
are also shown along the x-axis. The verticallines indicate the post-starburst galaxy do we infer from SED fitting a high rate
locationofthecutinDn4000(at1.5).Thehorizontallinesindi- ofstar-formationactivity(∼ 140M⊙yr−1)andahighextinction
catethelocationofthetwoextremethresholdsat-3Åand-15Å (τ ∼2).Thisobjectisoneofthetwopost-starburstgalaxiesde-
V
inEW[OII].ArrowsareupperlimitstoEW[OII]values.Typical tectedatbothX-rayandradio1.4GHzfrequencies,evenifthe
errors in the spectral measurements are plotted as errorbars at radiodetectionisonlyatthe3σconfidencelevel(seebelow).
thebottomofthepanel.
2) 1.4 GHz radio frequency. We investigated the dusty star-
burst hypothesis further by cross-correlating our sample with
174 star-forminggalaxies, while we have 178 quiescentgalax- the 1.4GHz VLA-COSMOS survey (Schinnereretal. 2007;
iesand45star-forminggalaxiesinsampleMST.The10k-parent Bondietal.2008).IfitisnotproducedbyAGNemission,thera-
galaxiesnumber1508followingthecriteriaofsampleMCOR, diofluxisindicativeofthecurrentstar-formationactivity(over
and665forsampleMST. shorttimescalesof108 yr)unbiasedbydust(seeCondon1992,
forareview).
We found that one post-starburst galaxy has a counterpart
3. Results
at 1.4 GHz radio frequency with a detection confidence level
We present the main physical properties of the mass-selected of 7.5σ and five others have possible 1.4 GHz emission at
post-starburstgalaxiesidentifiedinthezCOSMOSsurveyinat- ≃3σconfidencelevel.Fortheentiresampleof74post-starburst
temptingtoobtainacoherentpictureoftheirorigininthecon- galaxies,we thereforehavea detectionrateof1.3±1.3%,if we
text of galaxy formation.Our analysis refers to galaxies of the adopt the threshold of 4σ for assuming a radio detection to
statisticallycorrected,mass-completesampleatz∈[0.48−1.2] be true (as adopted in Schinnereretal. (2007) and Bondietal.
(sample MCOR) and the conservative mass-limited sample at (2008)).Thisdetectionrate increasesto 8.1±2.7%,if we adopt
z∈[0.48−1.0](sampleMST). thelessconservativethresholdof3σforradiodetection.
Toinvestigatethemeanradiopropertiesofthepost-starburst
galaxies,weperformedastackinganalysisofthe1.4GHzradio
3.1.Robustnessofoursample:dustystarbursthypothesis?
images. From the stacking analysis, we excluded the six post-
Galaxies with spectral characteristics similar to those used to starburstgalaxieswithapossibleradiocounterpartplusthreead-
define a post-starburstgalaxy are often interpreted to be a sys- ditionalsourcesthatexhibit,within2.5′′oftheiropticalposition,
tem in which star formation has suddenly and almost com- eitherradio emissionmoresignificantthan3σthatis notasso-
pletelystoppedwithinthepastfewGyrs.Thiscessationofstar- ciatedwiththepost-starburstgalaxy,oranegativeradiopeakof
formationisassumedtoexplainthelackofemissionlinesinthe highersignificancethan3σ.Fortheremaining65post-starburst
galaxyspectrum. galaxies,imagesof40×40pixels(correspondingto14′′×14′′)
An alternative explanation, however, is that these galaxies wereextractedaroundtheopticalposition.Finally,allthe65im-
do have emission, but that this is extinguished by a significant ageswerecombinedto obtaina medianimage with a 1σradio
D.Verganietal.:Post-starburstgalaxiesinCOSMOSfield 7
noiseof≈2µJy.Amarginaldetectionof≈ 3σconfidencelevel TheopticalspectraofthesethreesourceswiththehighestX-ray
was found at a distance of 1.4′′ from the centre of the stacked countsin thehardband,mightindicatethattheyareAGNhost
image with a radio flux of ≈7 µJy. To access the reliability of galaxies.
this detection,we repeated the stacking analysisfor 5 different
samples, each of them consisting of 75 random positions. The OfthesefiveX-raysources,twohaveaclearassociationwith
criterionusedtoexcludefromthestacking,post-starburstgalax- sub-mJysourcesintheVLAcatalog(Bondietal.2008),oneof
ies with a radio detection(ora negativeradio peak,see above) which has the highest SFR of all sources and dust extinction
was applied similarly to the simulated samples. All 5 stacked inferredfromSEDfitting.
images from the simulated sample had a 1σ radio noise in the
range 1.4-1.8µJy. No radio detection was identified for any of To place constraints on the properties of the X-ray unde-
thesimulatedstackedimages. tected post-starburst galaxies, X-ray stacking analysis was ap-
Therefore, assuming the marginal detection in the stacked pliedtotheremaining49sources,extractingsourcecountsfrom
imageto be realandassuming its fluxof 7 µJy to be the mean circles of 5′′ radius and background counts from a region at
radiofluxofradio-quietpost-starburstgalaxies,wecanestimate least10′′ awayfromthesourceposition.X-raysourcespresent
their mean star-formation rate, or more realistically its upper in the Chandra source catalog (Elvis et al. 2009) were care-
limit since some objects may be AGNs. We obtained an upper fully masked. To limit the problems with the PSF at large off-
limit of 8.2M yr−1 by adopting the calibration by Belletal. axisangles,weextractedsourcecountsusingonlyobservations
⊙
(2003). The radioluminosityL was calculatedby assum- wherethesourcepositionwaslocatedwithin8′ofthefieldcen-
1.4GHz
ingamedianredshiftofz =0.7014. tre(seeMiyajietal.(2008)andKimetal.(2009)foradescrip-
median
We then explored the physical properties of the six (> tion of the tool adopted here). We did not find any significant
3σ)radio-detectedpost-starburstgalaxies.Radio-detectedpost- signal from the stacked post-starburst galaxies using Chandra;
starburst galaxies are slightly more massive and brighter than the 3σ flux upper limit to the stacked 0.5–8 keV counts is
non-radio-detected post-starburst galaxies (logM/M = ≈ 2×10−16 cgs,correspondingtoarest-frame0.5–8keVlumi-
⊙1.4GHz
11.22 ± 0.09 versus log M/M = 11.08 ± 0.05; nosityof≈4.9×1041ergs−1attheaverageredshift(hzi≈0.73)
⊙no−1.4GHz
M =−22.64±0.37versusM =−22.44±0.14). ofthestackedsources.Thisupperlimittothe stackedsignalis
B,1.4GHz B,no−1.4GHz
Post-starburstgalaxieswithacounterpartat1.4GHzalsoshow consistentwithemissionfromnormalandstarburstgalaxies.
slightlybluercolours((U-B) =1.10±0.1)thannon-radio-
1.4GHz
detectedpost-starburstgalaxies((U-B) = 1.18±0.03). Dewanganetal.(2000)reportedadetectionofanactivenu-
no−1.4GHz
Although these differences are not statistically significant per cleusinonepost-starburstgalaxyselectedduringasearchforul-
se, their properties and low detection rate suggest that radio- trasoftsourcesinaROSATcampaign,butnoradioemissionwas
detectedpost-starburstgalaxiesmightrepresenttheinitialphase detected within a radiusof 15′′ in the NRAO/VLA and FIRST
in the evolution of post-starburst galaxies, close to the peak of sky surveys. In previous studies, similar counter-checks led to
thestarburst(thetimescaleofradioemissionisanorderofmag- a detection rate of the order of 1-15%. Miller&Owen (2002)
nitudeshorterthanthatofthek+aphase). using the galaxies selected by Zabludoffetal. (1996), detected
2 in 15 post-starburstsystemsin radiocontinuum(correspond-
3) X-ray counterparts. To characterize the multiwavelength
ing to a star-formation rate of about 2-6 M yr−1), and 1 in
propertiesof post-starburstgalaxies, we also searchedfor their ⊙
56objectsweredetectedbyBlakeetal. (2004) (at7 M yr−1).
possible X-ray counterparts using the available data provided ⊙
by XMM-Newton for the entire 2 deg2 of the COSMOS field UsingChandraobservationsoftheExtendedGrowthStripfrom
DEEP2,Georgakakisetal.(2008)foundanassociationbetween
(Cappelluti et al. 2009; Brusa et al. 2009) and Chandra over
the inner 0.9 deg2 (Elvisetal. 2009) at considerably deeper post-starburstgalaxiesat z ∼ 0.8 andAGN, supportinga mod-
eratelevelsofAGNobscuration.Althoughtheseresultsaredif-
X-ray flux levels. While only one post-starburst galaxy is de-
tectedbyXMM-Newtonforamatchingradiusof6′′,fiveX-ray ficulttocomparebecauseofdifferentselectioncriteriaanddif-
ferentmassandluminosityranges,theseresultshavebeenused
sources(four“new”plustheXMM-Newtonone)aredetectedby
Chandra(within4′′ because of the sharperPSF) at a detection toclaimthatatleastsome“so-calledpost-starburst”galaxiesare
still actively forming stars, implying that a selection based of
likelihood>10.8inatleastoneX-rayband(seeElvisetal.2009
the blue-part of the optical spectra is imprecise. Some authors
and Puccettiet al. 2009for furtherdetails) amongthe 54 post-
haveshownthatopticalspectratypicalofapost-starburstgalaxy
starburstgalaxieslocatedinthefieldofChandramosaic.Visual
inthebluepartoftheirspectrum,havesubstantialHαemission
inspection of all of these sources supports their likely physi-
(e.g.,Liu&Kennicutt1995).Wefullyagreewiththestatement
calassociationwiththe post-starburstgalaxiesunderinvestiga-
by e.g., Miller&Owen (2002) that the minimum requirement
tion.Allofthefivepost-starburstgalaxiesdetectedbyChandra
fordefiningasystemintheirk+a,orpost-starburstphaseisthe
are in regions of moderate-to-high exposure time (>100 ks),
coverageofbothblue-andred-partsoftheopticalspectrum,or
wheremorethanoneChandrapointingisavailable(Elvisetal.
complementingtheopticaldatawithotherwavelengthdatasets.
2009). However, the limited number of counts (≈ 9–40) in the
0.5–7keVbandpreventsusfromperforminganadequateX-ray
spectral analysis. X-ray counts were converted into fluxes (us-
ing a power-lawmodelwith photonindexΓ = 1.4, as adopted
in the Chandra-COSMOScatalog; Elvis et al. 2009), and then
into0.5–10keVrest-frameluminosities(giventhesourcespec- Basedonourcomplementaryradio-continuumanddeepX-
troscopicredshiftsof0.67–1.10)of1.1–8.6×1042ergs−1. raydata,wefoundthatanegligiblefractionofourpost-starburst
At leastforthethreesourceswiththe highestX-raycounts galaxies,selectedbyusingbluediagnosticsoftheopticalspec-
inthehardband(above2keV),thisluminosityrangesuggestsa tra, have possible residual star-formation activity, and a small
significantcontributionfromAGNemission.Theassumptionof percentageshowsacontributionfromAGNemission.Thus,the
aphotonindexofΓ= 1.9,whichistypicalofunobscuredAGN mainconclusionofourpresentstudyisthatwehaveselecteda
emission,woulddecreaseX-rayfluxesandluminositiesby25%. robustsampleofpost-starburstgalaxies.
8 D.Verganietal.:Post-starburstgalaxiesinCOSMOSfield
Fig.4.Stellarmass,B-bandabsolutemagnitude,andstellarmass-to-lightratiohistogramsareshownforeachclassofgalaxies.The
cumulativedistributionsofeachsamplearealsoplotted.Medianvaluesofpost-starburstgalaxiesand10k-parentsampleareplotted
as solid and dashed lines, respectively. Blue and red arrows are median values of star-forming and quiescent galaxies. The bars
assigned to each median value correspondto 1σ error computedwith resamplingtechnique.Galaxies classified as post-starburst
(violet),quiescent(red),star-forming(blue),andthe10k-parent(black)samplefollowthedefinitiondescribedinSect.2.5.
3.2.Basicproperties:luminosities,stellarmasses,and acombinationofahighstellarmassandrelativelylowmass-to-
colours light ratio. Among the most massive galaxies (criterion MST),
there is a trend for post-starburst galaxies to be the most mas-
The B-band absolute magnitude distribution plotted in Fig.4
sive objects, more massive than quiescentgalaxies (at 1σ) and
shows that post-starburst galaxies of the conservative sam-
star-forminggalaxies(at∼ 2σ).Sinceourpost-starburstgalax-
ple MST populate the brightest tail of our 10k-parent galaxy
iesare alreadyso massive,we mayconcludethattheirprogen-
population. The post-starburst galaxies more massive than
itorscannotbe commonstar-forminggalaxiesthathaveassem-
log(M/M )> 10.8 are brighter by ∼ 0.7 − 0.8 mag than the
⊙ bled their stellar mass by means of a continuous, smooth star-
galaxiesinbothourcomparisonclasses.Asimilaranalysisofthe
formation history and that have stopped, for whatever reason,
sampleMCORgalaxiesconfirmsthatthepost-starburstgalaxy
forming stars. Indeed to justify the observed stellar mass as-
sampleissystematicallybrighterby∼0.4−0.5magthantheme-
sembly of a post-starburstgalaxywith a typicalstellar mass of
dianbrightnessescomparedtomedianvaluesoftheothergalax-
log(M/M )= 10.58 (see Fig.4), a specific-SFR (SFR/M) of
⊙
ies. Both these results are statistically significant at more than ∼1.2×10−9(orSFR∼20M yr−1)isrequiredforagalaxywith
⊙
the 3σlevel.One of the reasonsfor thisB-bandbrightnessex-
astellarmassoflog(M/M )=10.23(where10.23isthemedian
⊙
cessisprobablyarecentburstofstarformationinpost-starburst
stellarmassfoundforourstar-forminggalaxies).Thisspecific-
galaxycandidates,ascommonlyconcludedinpaststudies.
SFRvalueistypicalofanobjectwithahighstar-formationef-
Post-starburst galaxiesalso populatethe massive tail of the
ficiency.Furthermore,wecanalsoexcludethedrymerginghy-
10k-parentgalaxies(see Fig.4). Their mass distribution differs
pothesis, i.e., the mergingof galaxyprogenitorsin the absence
fromthatofthe10k-parentgalaxiesat∼ 91%confidencelevel
ofgas,asthestrengthoftheHδabsorptionlinesimpliesarecent
usingtheKolmogorov-Smirnovtest(Pressetal.1992),andour
burstofnewstars,i.e.,thatfreshgasshouldhavebeenrecently
post-starburst candidates are as massive as quiescent galaxies.
available. Therefore, the possible progenitors might be the al-
Our different samples selected using the MCOR criteria show
readymassive,lessnumerousstar-forminggalaxiesateachcos-
thetypicalincreasingtrendinstellarmassandmass-to-lightra-
mictime,or/andthemostactivelystar-formingsourcesthatina
tioexpectedonthebasisoftheirgalaxyclassificationfromstar-
shortperiodofcosmictimecanassemblysufficientstellarmass:
forminggalaxypopulationtoquiescentgalaxies.Giventhesim-
in other words, the k +a’s progenitors might be galaxies with
ilar magnitudesandstellar masses spannedbythe star-forming
thehighestvaluesofspecificstar formationratesatanyepoch,
andquiescentgalaxiesselectedusingtheMSTcriteria,thistrend
assumingthatthe mass assemblyis a processsustained by star
isnotvisibleinthecorrespondingpanelsofFig.4.Post-starburst
galaxiesappeartohaveabrighterB-bandluminositybecauseof
D.Verganietal.:Post-starburstgalaxiesinCOSMOSfield 9
formationactivityattheobservedrate,i.e.,nomergereventsare
invoked.
Quiescent and star-forming galaxies populate the red se-
quence and the blue cloud, respectively, as shown in Fig.5 for
sampleMCOR.Thepeakofthedistributionforquiescentgalax-
iesisat(U−B)∼1.3,andstar-forminggalaxiesaredistributed
around a locus of (U − B) ∼ 0.8 in sample MCOR (colours
areslightlyredderforstar-forminggalaxiesinsampleMSTby
∼ 0.2 mag, due to the higher stellar mass selection). The un-
derpopulatedregionin-betweenthetwo distributions,i.e.,a re-
gioncalledthegreenvalley,isthelocuswithinwhichourpost-
starburstgalaxiesarelocated.Inthecolour-magnitudediagram,
they are on average (U − B) ∼ 0.25 redder than star-forming
galaxies,and∼ 0.1bluerthanquiescentgalaxies.Althoughthe
median distribution of colours appears to imply that the green
valleyispopulatedbypost-starburstgalaxies,theyspana wide
rangeofcolours.ThispropertyisconfirmedinFig.3,wherethe
4000 Å break histograms of the 10k-parent sample and post-
starburstgalaxiesareshownalongthex-axis(withasolid,vio-
letlineanddashed,grayline,respectively).The4000Åbreakof
post-starburstgalaxiesrangesbetweenquitesmallvalues(typi-
calofstar-forminggalaxies)andquitehighones(characteristics
of quiescent, passive galaxies), or 1.2 < D 4000 < 2, with a Fig.5. Distributionofthe(U-B) colourversusthestellar mass
n
peakatD 4000∼1.5. for post-starburst galaxies (triangle, violet colour-coded sym-
n
Based on this result, selecting post-starburst-candidate bols) and comparison samples (quiescent galaxies, circle, red-
galaxies such as those that exhibit stronger Balmer absorption coded symbols, and star-forming galaxies, square, blue-coded
symbols). Isophotal contours are in steps of 10% with the
linesthanexpectedfortheir4000Åbreak,e.g.,inawaysimilar
faintestisophotelevelstartingatthe10%level.Theverticalsolid
tothecriterionadoptedbyWildetal.(2009),shouldunderesti-
(dashed)lineindicatesourmass-completenessatlog(M/M )>
mate their truefraction.Ourresultmightjustify the low detec- ⊙
10.0(>10.8).
tionofthisclassofgalaxiesintheVVDSfield.Wildetal.find
only 5 post-starburst galaxies in the VVDS field, scaling from
their quoted 18 Hδ strong galaxieswith nebular emission lines
late-type divisions is independent of their Hδ strength shown
to 5 properly-definedpost-starburstgalaxies(i.e., those objects
in panel A of Fig.6. There is, instead, an increase with Hδ
amongthe18detectedoneswithoutnebularemissionlines).
in the fraction of post-starburst galaxies in irregular systems.
Ourresultindicatesthatpost-starburstgalaxies,whichpop-
In panel C, we show the relative fractions of the three dif-
ulate the green valley for a very short time, provide a suitable
ferentmorphologicaltypes(bulge-dominated,disky-dominated,
galaxypopulationtooccupythegapbetweenblueandredgalax-
and irregulars) that are post-starburst galaxies compared to
ies in the colour-magnitude relation observed at any redshift
the same fractions for the samples of quiescent and star-
fromthelocalUniverseuptoz∼1.−1.5.Althoughk+agalax-
forminggalaxies.Asexpected,star-forminggalaxiesaremostly
iesmayrepresentsomeofthegalaxiesinthegreenvalley,wedo
disky-dominatedwith ∼ 20−30% having irregular morpholo-
nothoweverproposethattheyconstitutetheentirepopulationof
gies, while the majority of the quiescent galaxies are bulge-
green-valleygalaxies.
dominated. Morphologically, post-starburst galaxies are a het-
erogeneouspopulation,althoughwenotethatthemostmassive
3.3.Morphologiesandstructuralparameters post-starburstgalaxies(sampleMST)arepredominantlybulge-
dominatedsystems(seelowerpanelsofFig.6)andhaveasim-
We investigated the morphological properties of post-starburst ilardistributionamongmorphologicaltypesasquiescentgalax-
galaxiesusingthehighspatialresolutionHST/ACSF814Wim- ies.ItisinterestingtonotethatinthesampleMST,thereisno
ages(Koekemoeretal.2007).Galaxymorphologiescontainthe post-starburstgalaxywithanirregularmorphology,butwealso
imprintsofstellarpopulations,gascontent,anddynamicalstruc- notethelownumberstatisticsinthecurrentsampleMST.
tures.Thus,ifaneventsuchasmerginghasoccurredinthere- In some studies, post-starburst galaxies are found to be
centhistoryofagalaxy,itssignatureshouldbeobserved,given mostlybulge-dominatedand/orhavea morphologythatis con-
thatthetimescaleofgravitationaldisturbancesinducedbymerg- sistentwithbeingearly-type(Goto&SDSScollaboration2003;
ingissimilartothatofthepost-starburstphase. Quinteroetal. 2004; Tranetal. 2004; Blakeetal. 2004; Goto
We used the ZEST morphological classification of 2005; Baloghetal. 2005; Poggiantietal. 2009), and in some
Scarlataetal. (2007a) based on five non-parametric diag- othershaveahighincidenceofdisksandspiralarms(e.g.,Franx
nosticstodefineearly,late,andirregularmorphologicalclasses 1993;Couchetal. 1994;Dressleretal. 1994;Caldwell&Rose
of all studied galaxies in this work. We also explored in more 1997;Dressleretal.1999).Forexample,Quinteroetal.(2004)
detail properties such as the asymmetry index, the degree of visually inspected a sample of 160 galaxies from the SDSS at
irregularities,andtheconcentrationofthelight. 0.05<z<0.20,andclassified90%ofthemasbulge-dominated
The upper panel of Fig.6 illustrates that a similar frac- systems. Yangetal. (2004) used the term “diverse morpholo-
tion of post-starburstgalaxiesin the sample MCOR are bulge- gies”todescribefivehigh-resolutionHSTimagesofk+agalax-
dominated (E) and disky-dominated (L) (see panel B). How iesselectedfromthesampleofZabludoffetal.(1996),although
the galaxies are distributed between the so-called early- and we note that their selection did favour the bluest systems, i.e.,
10 D.Verganietal.:Post-starburstgalaxiesinCOSMOSfield
the ones with smallest D 4000 that have experienced most re-
n
centlyasignificantstarburstepisode.Theirdetailedstudyusing
bulge-to-disktwo-dimensionalfitting decompositionof four of
them identified one barredS0, two disky systems, and one un-
classifiablegalaxy.Theirinferredcentrallightdistributionswere
foundtohavea profilesimilarto,butmoreluminous,thanthat
ofatypicalearly-typepower-law.Athigherredshifts,Tranetal.
(2004)foundmorebulge-dominatedsystemsintheirk+agalaxy
sample than in field galaxies. The sample was selected from
a program designed to study galaxy evolution in clusters, and
k+a galaxieswere selected using a conservativeconstrainton
boththeHδandHγBalmerindicesandaccordingtotheabsence
of[OII]emissionline.Astudybasedonvisualclassificationof
ACS/HST imagesby Poggiantietal. (2009) identified a higher
fractionof S0 and Sa types amongfield post-starburstgalaxies
than Dressleretal. (1999) who instead found a higher fraction
oflatertypesamongpost-starburstgalaxies.
Basedonthemorphologicalresultsobtainedforthepresent
post-starburst galaxy sample, we agree with the widely estab-
lished result that post-starburst galaxies are morphologically a
heterogeneous population. We also found an increasing frac-
tion of early-type morphologies at increasing stellar mass in
agreementwiththewellknownmass-morphologyrelation(e.g.,
Tascaetal. 2009). We mightfind in the future with larger data
sets spanning a wider range of galaxy properties, that the flux
limits and other selection effects might play a critical role in
themorphologicalclassification,e.g.,favouringanearly-typeas-
signmentforbrighter/massivesystemsasfoundinthiswork.
Apart from being heterogeneousin their morphologies,the
second common feature used to describe post-starburst galax-
ies by most authors is the large fraction of asymmetries and
clumpinesspropertiesassociatedwiththem(e.g.,finestructure,
and tidal tails). We quantify the degree of asymmetry in the
light distribution using the asymmetry coefficient computed as
the difference between the image rotated through 180 degrees
and the original frame following the usual prescription (e.g.,
Conseliceetal. 2000) (for details see Scarlataetal. (2007b)).
Figure7 shows that for post-starburst galaxies in the sample
MCOR, the median and range of the asymmetry index are in-
termediatebetweenvaluesforquiescentgalaxies(andthe 10k-
parentsample)andstar-forminggalaxies.Comparingtheasym-
metrydistributionsofpost-starburstgalaxieswiththoseofstar-
forming and quiescent galaxies, they both differ at higher than
the99.9%KS-testconfidencelevel.Similarresultsareobtained
forthelightconcentration,whichisdefinedtobetheratioofthe
radiusincluding80%to thatincluding20%of the galaxylight
andmeasuresthecentraldensityofthegalaxylightdistribution.
In Fig.7, we show that the light concentration indices of post-
starburst galaxies are typically in-between the values for qui-
escent and star-forminggalaxies. These differencesin both the
Fig.6.Morphologicalclassificationofpost-starburstgalaxiesfor
morphological classification and the parameters of asymmetry
sampleMCOR(top)andsampleMST(bottom).InpanelAwe
andlightconcentrationofpost-starburstgalaxiesandthatofboth
show the fraction of the three different morphologicaltypes in
star-formingand quiescentgalaxies, suggest that post-starburst
post-starburst galaxies (bulge-dominated (E), disky-dominated
galaxiesrepresentanintermediatestageofgalaxyevolution.
(L)andirregulars(I))asafunctionofthestrengthofHδ.Similar
UsingtheZESTscheme,thedistortionofthelightdistribu-
plot is shown in panel B for post-starburst galaxies (indepen-
tion is quantified by the clumpiness index. This index is avail-
dently by the Hδ strength). Panel C shows the fraction of the
able forgalaxiesclassified as early-and late-types,i.e., notfor
three different morphological types in quiescent galaxies (or-
irregular galaxies, which by definition are very distorted and
ange)comparedtothesampleofpost-starburst(violet)andstar-
for which the index might be associated with only the max-
forming galaxies (blue), respectively. The error bars represent
imum degree of clumpiness. The flags for the clumpiness in-
thePoissonianerrors.
dexrangefromzero,indicativeofasmoothlightdistribution,to
three,correspondingtoaclumpylightdistribution(seeTable2,
Scarlataetal.(2007a)).Wefoundthatthemajorityofquiescent
galaxies have a smooth light distribution (86% have flags of 0 or1),whereasstar-forminggalaxiestendtohaveaclassification