Star formation, starbursts and quenching across the Coma supercluster

1.2 MB·English

by  Smriti Mahajan

Mon.Not.R.Astron.Soc.000,1–17(2010) Printed29January2010 (MNLATEXstylefilev2.2)
Star formation, starbursts and quenching across the Coma
supercluster
0
Smriti Mahajan⋆, Chris P. Haines, Somak Raychaudhury
1
0 SchoolofPhysicsandAstronomy,UniversityofBirmingham,BirminghamB152TT,UK
2

n
a
J

9 ABSTRACT
2

WeanalyseSpitzerMIPS24µmobservations,andSloanDigitalSkySurvey(DR7)opti-
]
O calbroadbandphotometryandspectra,toinvestigatethestarformationpropertiesofgalaxies
residingintheComasuperclusterregion.We findthatstarformation(SF)indwarfgalaxies
C
isquenchedonlyinthehighdensityenvironmentatthecentreofclustersandgroups,butthat
h. passively-evolving massive galaxies are found in all environments, indicating that massive
p galaxiescanbecomepassiveviainternalprocesses.TheSF-densityrelationobservedforthe
- massive galaxies is weaker relative to the dwarfs, but both show a trend for the fraction of
o star-forminggalaxies(fSF)decliningto∼0intheclustercores.WefindAGNactivityisalso
r
t suppressedamongmassivegalaxiesresidingintheclustercores.
s
We present evidence for a strong dependence of the mechanism(s) responsible for
a
[ quenchingstar formationin dwarfgalaxiesonthe clusterpotential,resultingin two distinct
evolutionary pathways. Firstly, we find a significant increase (at the 3σ level) in the mean
1
equivalentwidthofHα emissionamongstar-formingdwarfgalaxiesinthe infallregionsof
v
theComaclusterandthecoreofAbell1367withrespecttotheoverallsuperclusterpopula-
4
tion,indicativeoftheinfallingdwarfgalaxiesundergoingastarburstphase.Weidentifythese
0
starburstgalaxiesastheprecursorsofthepost-starburstk+Agalaxies.Extendingthesurvey
0
0 ofk+Agalaxiesoverthewholesuperclusterregion,wefind11.4%ofalldwarf(zmag>15)
. galaxies in the Coma cluster and 4.8% in the Abell 1367 have post-starburst like spectra,
2
whilethisfractionisjust2.1%whenaveragedovertheentiresuperclusterregion(excluding
0
the clusters). This points to a cluster-specific evolutionaryprocess in which infalling dwarf
0
galaxiesundergoastarburstandsubsequentrapidquenchingduetotheirpassagethroughthe
1
denseICM.Ingalaxygroups,thestarformationininfallingdwarfgalaxiesisinsteadslowly
:
v quenchedduetothereducedefficiencyofram-pressurestripping.
Xi Weshowthatinthecentral∼ 2h−701MpcoftheComacluster,the(24−z)near/midinfra-
redcolourof galaxiesiscorrelatedwith theiroptical(g−r)colourandHα emission,sepa-
r
a ratingall mid-infrared(MIR) detectedgalaxiesinto two distinctclasses of ‘red’and‘blue’.
Byanalysingthespatialandvelocitydistributionofgalaxiesdetectedat24µminComa,we
find that the (optically) red 24µm detected galaxies follow the general distribution of ‘all’
the spectroscopicmembers,buttheir (optically)bluecounterparts(i) are almostcompletely
absent in the central∼ 0.5h−1 Mpc of Coma, and (ii) have a remarkablepeak in their ve-
70
locitydistribution,correspondingtothemeanradialvelocityofthegalaxygroupNGC4839,
suggesting that a significant fraction of the ‘blue’ MIR galaxies are currently on their first
infalltowardsthecluster.TheimplicationsofadoptingdifferentSFRtracersforquantifying
evolutionarytrendsliketheButcher-Oemlereffectarealsodiscussed.
Keywords: galaxies:clusters:general,galaxies:evolution,galaxies:fundamentalparameters,
infrared:galaxies
1 INTRODUCTION sisting of two rich Abell clusters, separated by 30h−1 Mpc, but
70
connected byaprominent filamentofgalaxiesandpoorer groups
The Coma supercluster is the nearest rich supercluster of galax-
(e.g.Fontanelli1984),whichispartof thesupercluster identified
ies (Chincarini&Rood 1976; Gregory&Thompson 1978), con-
as the “Great Wall” in the first major redshift survey of galaxies
in the nearby Universe (Geller&Huchra 1989). At a distance of
⋆ e-mail:[email protected] ∼ 100h−701 Mpc,itaffordsacloserlookatthepropertiesofindi-
2 Mahajan,HainesandRaychaudhury
vidualgalaxies(1kpc≃2.1arcsec),butitslargeangularscaleon Abell 1367, along withthe associated filament of galaxies as the
thesky presents observational challenges for the narrow fieldsof ComaSupercluster.Inthispaper,weusetheSpitzerMIPS24µm
viewofmostinstruments. observations,whereveravailable,alongwiththebroadbandcolours
It is interesting to note that even though they are the two and spectral star formation indicators from the Sloan digital sky
mostsignificantstructuresinthesupercluster,thetwoAbellclus- survey (SDSS).Weadopt thedistance modulus of theComa Su-
ters are remarkably different in every respect. The optical lumi- perclustertobem−M=35.0,andusecosmologicalparameters
nosityfunctionofgalaxiesintheComacluster,forinstance,hasa ΩΛ =0.70,ΩM =0.30, and H0 = 70 km s−1 Mpc−1 for cal-
muchsteeperfaint-endslopethanthatofitsneighbourAbell1367 culatingthemagnitudesanddistances.Wenotethatattheredshift
(Iglesias-Pa´ramoetal. 2003). Early studies (e.g. Dressler 1980) ofComa(z=0.023),ourresultsareindependent ofthechoiceof
revealed that the fraction of spiral galaxies in the Coma clus- cosmology.Inthenextsection,wepresentourdataandreduction
ter is anomalously low compared to Abell 1367. In addition, methodology,andsummariseourmainresultsin§3and§4.Wedis-
galaxies in the core of the Coma cluster were identified to be cusstheirimplications,and comparethepropertiesofgalaxiesin
unusually deficient in neutral hydrogen (e.g. Sullivan&Johnson variouspartsofthesuperclusterin§5,summarisingin§6.
1978; Giovanelli&Haynes 1985; Bernsteinetal. 1994), ion-
ized hydrogen (Raychaudhuryetal. 1997) and molecular gas
(H2/CO) (Bosellietal. 1997a). Bothun,Schommer,&Sullivan
2 OBSERVATIONALDATA
(1984)foundastrongHIgradientinComagalaxies-manyofthe
inner,HIpoorspiralsbeingquiteblue,suggestingthatsomegasre- Webase our work on the photometric and spectroscopic data ac-
movalprocesshasactedquiterecently,whileAbell1367wasfound quired by the SDSS Data Release 7 (Adelman-McCarthyetal.
tobeamixtureofHIpoorandrichgalaxies. 2006, (DR7)), which for the first time covers the entire region
The availability of more detailed analyses of star forma- of the Coma Supercluster presented inthis paper. Over asmaller
tion, based on multi-wavelength data, and of spectral indices area, wealso use archival 24µm mid IR (MIR) images, from the
sensitive to stellar ages, has made this field more interest- Multi-bandImagingPhotometer(MIPS)instrumentaboardSpitzer
ing. Estimates of star formation activity from mid-infrared (IR) (Riekeetal.2004),availableoverasignificantfractionofthetwo
(Bosellietal.1997b)andGalaxyEvolutionExplorer(GALEX)ul- mainclusters.
traviolet(Cortese,Gavazzi,&Boselli2008)photometryshowthat
star formation in Coma on the whole is substantially suppressed
comparedtothatinthefield,whileAbell1367hasanabundance 2.1 Opticaldata
ofbrightstar-forminggalaxies(Cortese,Gavazzi,&Boselli2008).
WeselectthegalaxiesbelongingtotheComasuperclusterfromthe
In a hierarchical model of the formation of structures in the
SDSSspectroscopiccatalogueonly,requiringthemembergalaxies
Universe,itisnotsurprisingthatarichcluster,witharelaxedap-
tobewithin170.06RA6200.0degand17.06Dec633.0degon
pearance in its X-ray image, such as Coma, would represent the
thesky,andwitharadialvelocitywithin2,000kms−1ofthemean
end product of the gradual assimilation of several galaxy groups
redshiftoftheComacluster(6,973kms−1)ortheAbell1367clus-
overtime.InspiteoftheclaimofDressler&Shectman(1988)that
ter(6,495kms−1)respectively(Rinesetal.2003).Allourgalax-
theComaclusterdoesnothavesignificantsubstructure,ithassub-
iesarebrighterthanSDSSmagnituder=17.77(∼ M∗+4.7for
sequentlybeenshowntohaveatleastthreemajorsubclustersinthe
Coma),whichisthecompletenesslimitoftheSDSSspectroscopic
optical(e.g.West1998)andX-raymaps(e.g.Davis&Mushotzky
galaxycatalogue.
1993; Adamietal. 2005). At the other end of the supercluster,
Abell 1367 is also found to be elongated with three major sub-
clusters, alongtheaxisof thefilament, withapopulation of star-
2.2 MIPS24µmdata
forminggalaxiesinfallingintotheSEcloudandpossiblytheother
two as well (Corteseetal. 2004). This has prompted a multitude For the mid-infrared study of Coma and Abell 1367, we use
of studies attempting to link the dynamical history of the super- archival 24µm Spitzer MIPS data covering 2×2deg2 in the case
clusterwiththepropertiesofindividualgalaxiestoinvestigateboth ofComa(PID:83,PIG.Rieke)and30′×30′forAbell1367(PID:
theprocessofbuildingclusters,aswellastheeffectoflarge-scale 25,PIG.Fazio).TheComa24µmdatasetconsistsoffourcontigu-
structureonthestarformationhistoryofgalaxies(e.g.Hainesetal. ousmosaics(seeFig.1)obtainedinmediumscanmode,withscan
2006). legspacingequaltothehalfarraywidth,producinghomogeneous
By employing the spectral analysis of galaxies in Coma, coverageoverthemosaic,withaneffectiveexposuretimeperpixel
together with the X-ray map of Neumannetal. (2003), of 88sec. Baietal. (2006), who used this data to determine the
Poggiantietal. (2004) found that the post-starburst (k+A) 24µm luminosity function of Coma, estimate the 80% complete-
galaxies,inwhichstarformationhasbeenquenchedwithinthelast ness limit to be 0.33mJy, corresponding to a star formation rate
1-1.5 Gyr, are associated with the X-ray excess attributed to the (SFR)of 0.02M⊙yr−1 at the redshift of Coma. TheAbell 1367
substructureinComa.However, thisworkwaslimitedto3fields 24µmdatasetconsistsofasinglemosaicobtainedinmediumscan
inthe∼2MpcregionsurroundingthecentreofComa. mode,withscanlegspacingequaltothefullarraywidth,producing
In addition to star formation indices measured from optical aneffectiveexposuretimeperpixelof40sec.
spectra, theavailabilityof Spitzer MIPS mid-IR observations has The SEXTRACTOR package (Bertin&Arnouts 1996) was
been recently utilised by several authors to characterise the star used to automatically detect sources, and obtain photometric pa-
formationpropertiesoftheobscuredcomponentingalaxiesinclus- rameters. The images were first background-subtracted, and then
tersandgroups(e.g.Saintonge,Tran,&Holden2008;Hainesetal. filtered with Gaussian functions, with full width half maximum
2009;Wolfetal.2009;Baietal.2010),sincethe24µmfluxcanbe (FWHM)matching the24µm point-spread function (PSF).Aper-
usedasarepresentationofthedust-reprocessedoverallIRflux. ture photometry was obtained for all objects having 8 contigu-
Hereafter, we will refer to the pair of clusters Coma and ous pixels above the 1σ rms background noise level. Following
Starformationand quenchingacrosstheComa supercluster 3
SWIRE,wemeasuredthefluxeswithincircularaperturesofdiam- Inordertogiveageneraloverviewoftheopticalstarforma-
eter21,30,60,90and120′′.Forthevastmajorityofsources,the tionproperties of galaxies along theentire supercluster, inFig. 1
6′′ FWHMpoint-spreadfunctionofMIPSleavestheobjectunre- we plot the positions of all the galaxies from the SDSS spectro-
solvedat24µm,andhenceweestimatethetotal24µmfluxofthese scopiccataloguefoundinourredshiftrange,andthemajorgalaxy
objectsfromthefluxcontainedwithinthe21′′ aperture,corrected groups obtained from the NASA Extragalactic Database (NED,
byafactor1.29. http://nedwww.ipac.caltech.edu/).Inordertoavoidmultipledetec-
However, at the redshift of the Coma and Abell 1367 clus- tions,weconsiderthegroupsonlyfromtheNGS,WBL,USGCand
ters,manyofthebrightergalaxies(spiralsinparticular)arelarger HCGcatalogues,andevenamongstthese,weidentifyduplication
than our nominal 21′′ diameter aperture, such that a significant bymerginggroupscloserthan1′andwithin±100kms−1ofeach
fraction of their 24µm flux ismissed. For these galaxies, we use other.
insteadoneofthelargerapertures,matchedtocontaintheoptical ThepositionsofAGNhostgalaxies,identifiedusingtheBPT
flux as parametrized by R90z (the radius which contains 90% of diagram (Baldwin,Phillips&Terlevich 1981, also see §4.2), and
thez−bandflux)asmeasuredfromtheSDSSimage.Finally,for non-AGNstarburstgalaxies(star-formingontheBPTdiagram,and
some of the bright cluster galaxies (9intotal), thevalue of R90z havingEW(Hα)>25A˚),areindicatedinFig.1.Wealsoshowthe
is under-estimated. To correct for this, SM eyeballed the 24µm limitedregionsatthecoresoftheComaandAbell1367clusters,
images of these objects, and estimated an appropriate radius, in overwhichthe24µmdataareavailable,andutilisedinthispaper.
each case, for extracting the‘total’ 24µm flux. Inthecase of the Plottedinthebottom panel of thesamefigure, isthedistribution
brighterspiralgalaxiesinComaandAbell1367,the24µmemis- of the AGN and starburst galaxies, as fractions of the 150 galax-
sionoftenshowssignificantstructureduetospiralarmsandstar- iescontributing to each bin. It is worth mentioning that although
formingregions, necessitating ahighvalue of thedeblending pa- thesedistributionsareafairrepresentationofthedata,thebinning
rameter, to prevent the galaxy being shredded by SEXTRACTOR. hasresultedinaspuriousfeatureatthepositionofAbell1367.As
Particularcarewasrequiredtocorrectlydeblendtheface-onspiral isalreadywellknownintheliterature,thecentralregionofAbell
NGC3861fromNGC3861B,whilekeepingNGC3861intact.We 1367issparselypopulated,themajorityofgalaxiesinthecorebe-
notethatfor ∼50%of our samplethetypical measurement error inglate-type.Inthisfigure,itappearsthatthefractionofstarburst
inthe24µmfluxis<0.1mJy(whichis.10%ofthemeasured galaxiesdoesnotdeclineinthecoreofAbell1367,butifthedatais
flux),andfor>95%theerroris<0.35mJy.Intotal,∼90%ofour re-binnedsoastohaveequally-spacedbins,thiswouldnotbethe
galaxieshave24µmfluxmeasuredwith<30%uncertainty. case.Havingsaidthat,thedeclineinthefractionofstarburstgalax-
iesatthecentreofAbell1367ismuchshallowerthanthatseenin
Coma.
2.3 MatchingtheSDSSspectroscopiccataloguewiththe
Ingeneral, theoptically-identified AGN seem tobemore or
24µmsources lessuniformlydistributedthroughoutthesupercluster(Figs.1and
Tomatch the SDSSspectroscopic catalogue for theComa super- 2),exceptforasharpdeclineinthecoresofboththeclusters.This
cluster with the MIR 24µm data, we choose the nearest optical latterresultiscounter-intuitive,becauseAGNhostsareknownto
counterpart within 5′′ of each 24µm source. This accounts for a beearly-typemassivegalaxieswhichmostlyresideindenseenvi-
displacementofnomorethan2.5h−1kpcbetweenthecentres.We ronments.Hence,thismayindicatethattheopticalemissionfrom
70
notethat&80%ofthematchesarefoundtohavecentreswithin2′′ the AGN present in this region is obscured and/or detectable at
ofeachother.16ofoursourcesabovethedetectionthresholdwere otherwavelengths(radio/X-ray).Thelatterisbeyondthescopeof
foundtohavemultiplematcheswithinthematchingradiusof5′′. thiswork,butourMIPS24µmdatasupportstheformerpossibil-
Our final dataset comprises of 3,787 galaxies (r 6 17.77) ity.AsseeninFig.11,notonlydoestherelativefractionofthered
withspectroscopicredshiftsfromSDSS,andradialvelocitywithin 24µmgalaxiesmarginallyincreasetowardsthecentre,around50%
±2,000 km s−1 of Coma or Abell 1367, in the ∼500 square de- ofthemarefoundwithin0.5h−701 MpcofthecentreoftheComa
gree region of the sky covered by the Coma supercluster. Out of cluster, indicating that optical emission from several AGN hosts
these,197within∼2h−1MpcofthecentreofComaand24within maybeobscured.Furtherevidenceinsupportofthisargumentmay
70
∼0.65 h−1 Mpc of thecentre of Abell 1367 arefound to havea bedrawnfromFig.8wherealargefractionofgalaxiesdetectedat
70
24µmcounterpart.TheprobabilitythatabackgroundMIPSsource 24µmdonotshow[OIII]and/orHβinemission,buttheirHα/[NII]
lieswithinthematchingradiusof5′′ bychance,dependsonlyon fluxratiosareconsistentwiththepresenceofanAGN(Milleretal.
thedensityofthe24µmsources.Forthecompletenesslimit(0.33 2003)ontheBPTdiagram(Baldwin,Phillips&Terlevich1981).
mJy) adopted here, this density is ∼ 3,000deg−2 (see fig. 3 of WefurtherinvestigatetheenvironmentaltrendsforAGNand
Shupeetal. 2008). This gives the required probability as 0.018, starformationactivitywithintheComasupercluster,byestimating
implyingthatoursampleof19724µmdetectedgalaxiesinComa thelocalfractionofstar-forminggalaxies(fSF)andAGN(fAGN)
mayhave3–4interlopers.Asampleofthecatalogueofdataforthe over theentire supercluster, both for massive galaxies(z<14.5;
24µmdetectedgalaxiesisprovidedinTable1. Mz<M∗+1.8,assumingMz∗=−22.32fromBlantonetal.2001)
andthedwarfgalaxypopulation(z >15;M >M∗+2.3).Fol-
z
lowingHainesetal.(2007), wedefinethelocal projecteddensity
ofgalaxies(ρ(x))usingavariantoftheadaptivekernelestimator
3 STARFORMATIONANDAGNACROSSTHECOMA
(Silverman1986;Pisani1993),whereeachgalaxyiisrepresented
SUPERCLUSTER
by an adaptive Gaussian kernel κ (x). This is different from the
i
Located∼100Mpcfromus,theComasuperclusteroffersaunique algorithmofSilverman(1986),which,initspreviousapplications
opportunity for investigating the effect on individual galaxies of toastronomicaldata,requiresthekernelwidthσi tobeiteratively
thehierarchical formationof structuresintheUniverse, asgalax- settobeproportionaltoρ−1/2.Inthiswork,wefixthetransverse
i
iesprogressthroughvariousenvironmentstowardsthecoresofthe widthσitobeproportionaltoD3,whereD3isthedistancetothe
clusters. thirdnearestneighbourwithin500kms−1.
4 Mahajan,HainesandRaychaudhury
Table1.CatalogueofComagalaxiesdetectedat24µm(Thistableisavailableinfullonline.)
RA Dec z g r z 24µmflux flaga
(J2000) (J2000) mag mag mag (mJy)
194.4574 28.6243 0.0250 17.80 17.35 17.24 0.31 1
195.1548 28.6641 0.0236 17.47 16.89 16.49 1.25 1
194.5385 28.7086 0.0255 15.21 14.56 14.08 36.02 1
194.9172 28.6308 0.0179 15.67 15.47 15.30 7.90 1
194.3645 28.4397 0.0260 17.16 16.57 16.19 0.75 1
194.5628 28.5218 0.0231 17.00 16.56 16.32 1.84 1
194.3912 28.4823 0.0209 14.35 13.54 13.00 1.86 3
194.4747 28.4998 0.0244 15.94 15.18 14.56 0.04 0
195.2170 28.3661 0.0255 14.51 13.73 13.10 1.39 0
195.3123 28.5217 0.0281 17.48 17.11 16.98 1.06 1
a ClassificationonthebasisofBPTdiagram,where
0:unclassified
1:star-forming
2:AGN
3:AGNaccordingtoMilleretal.(2003)criteria(seetext)
Thedominantfactorsgoverningthestarformationproperties Abell 1367 clusters, while elsewhere, it is uniformly distributed.
of a galaxy are the mass of its DM halo, and whether it is the ThisappearstocontradicttheresultsofHainesetal.(2007),who
centralorasatellitegalaxyinahalo(e.g.Kauffmannetal.2004; foundtheAGNfractiontobeindependent ofenvironment,andto
Yangetal.2005;Blantonetal.2006).Weadopttheabovemethod, be amonotonically increasing function of stellar mass. However,
andchoosethedimensionsofthekernel,keepingthisinmind.In thevolumestudiedbyHainesetal.(2007)withinSDSSDR4did
thecaseofgalaxieswithingroupsorclusters,thelocalenvironment notcoverclustersasrichasComaorAbell1367.Thismayindicate
ismeasuredonthescaleoftheirhosthalo(0.1–1Mpc),whilefor thatonlytheverydenseenvironmentsaffecttheopticalAGNactiv-
galaxiesinfieldregionsthelocaldensityisestimatedbysmooth- ityofgalaxies.Therelationbetweenf andlocalenvironment
AGN
ingovertheir5–10nearestneighboursoronscalesof1–5Mpc(for isunclearintheintermediatedensitygroupenvironment.Whilethe
details,seeHainesetal.2007). f valuesdeclineinthecentreofsomegroups,inothersthey
AGN
We can then map the local galaxy density (ρ) and z- exceedthemeanvalueforthefield.Althoughthereisaslightindi-
band luminosity density (j ) as ρ(x)=P κ (x−x ) and cationthatthegroupsinwhichthef appearstobedeclining,
z i i i AGN
j (x)=P L κ (x−x ),whereL isthez-bandluminosity mostly lie on the filament connecting the Coma and Abell 1367,
z i z,i i i z,i
ofgalaxyi(seeFig.3).FollowingYangetal.(2005),weidentify orinthevicinityoftheclustersthemselves,whilethegroupswith
local maxima in the z-band luminosity density as galaxy groups highervaluesoff lieinunderdenseregions.However,itisnot
AGN
and clusters, whose masses correlate with the total z-band lumi- possibletodrawanyfirmstatisticalinferencesfromthisobserva-
nosity associated with the peak. By comparison with the Millen- tion.
niumsimulation,weexpectallgroupswith4ormoremembersto
beassociatedwithalocalmaximuminjz(x)(Hainesetal.2007). WhethertheAGNfractionfAGN varieswithenvironmentor
Thelocalz-bandluminositydensityisshownasblackcontoursin not depends on how theactivity of the galacticnuclei is defined.
Figs.2–4. Milleretal.(2003)donotfindanycorrelationbetweengalaxyden-
InFig.2weshowthespatialvariationoffAGN amongmas- sityandfAGN inalargesampleofgalaxies(Mr=−20;0.05<z<
sivegalaxies(z<14.5)acrossthesupercluster,wherewecalculate 0.095),wheretheAGNaredefinedintermsofopticalemissionline
f asP κ(x−x )/ρ(x),wherei⊂AGN isthesubset ratioscharacterisedbytheBPTdiagram.Otherstudiesfindhigher
AGN i⊂AGN i
ofgalaxiesclassifiedasopticalAGN,basedontheratiosofther- incidence of optical AGN in groups and clusters (Arnoldetal.
mally excited and recombination lines used in the BPT diagram. 2009).IftheAGNwereselectedaccordingtotheirradio,mid-IR
Wedonotconsiderthelowermassgalaxies,asthefractionofAGN or X-ray properties, they would be found in differen hosts: radio
declinesrapidlytozerobelowM >M∗+2; AGNinearly-typegalaxies,IRAGNinbluergalaxies,andX-ray
z
For comparison, Fig. 3 shows the distribution of f (x) as AGNingalaxiesofintermediatecolour(Hickoxetal.2009).The
SF
P κ(x−x )/ρ(x), where i⊂SF is the subset of galaxies environment dependence of AGN activity is thus largely linked
i⊂SF i
classified as star-forming according to the line-ratios used in the to the distribution of the hosts. The incidence of X-ray AGN is
BPT diagram. Fig. 4 shows a map of the local mean equivalent higheringalaxygroupsthaningalaxyclusters(Martinietal.2006;
width(EW)ofHαofstar-forminggalaxiesashEW(Hα)(x)i = Shenetal.2007).Thefractionofradio-loudAGNisthesameinthe
P EW(Hα)κ(x−x )/P κ(x−x ),plottedinaman- brightestgalaxiesofgroupsandclustersandinthefield,buthigher
i⊂SF i i⊂SF i
ner analogous to that used by Hainesetal. (2006) for galaxy in non-central galaxies (Bestetal. 2005). DelPozzoetal. (2010)
coloursintheShapleysupercluster. showthatthemassfunctionofblackholesisindependentofenvi-
In Fig. 2 we see that the AGN fraction f , among mas- ronment,andthevariationinthedistributionofoptical,radioand
AGN
sive galaxies (z < 14.5) declines at the centre of the Coma and X-rayAGNcanbeunderstoodintermsoftheaccretionprocesses
Starformationand quenchingacrosstheComa supercluster 5
Figure1.Toppanel:ThesurfacedensityofgalaxiesintheComasuperclusterisshowningrey.Thepositionsofpassivegalaxies(blackdots),AGNhost
galaxies(openredcircles)andstarburstgalaxies(bluestars);EW(Hα)>25A˚∼logSSFR∼−10yr−1)areindicated.Thebiggreencirclesarethegroups
intheregion,fromtheNASAExtragalacticDatabase.TherectangularpinkregionsshowtheSpitzerMIPSfieldsofviewforthetwoclusters.Bottompanel:
ThesolidredlineandthedashedbluelineshowthefractionofAGNandofthestarburstgalaxies,amongallthegalaxiesshownintheupperpanel.The
binsarechosentohave150galaxiesineachofthem.ThesolidverticallinesrepresentthecentresoftheComaclusterandAbell1367respectively,whilethe
dot-dashedlineshowstheRApositionoftheNGC4839group(seetext).Allthedottedlinesrepresentthepositionsofgroups,shownintheupperpanel.
thatleadtothemanifestationoftheAGNinthevariousrangesof much stronger SF-density relation, with f rising rapidly from
SF
electromagneticradiation. the f < 0.1–0.4 seen in the cluster cores, to f > 0.95 in
SF SF
thefield.Thisresultisinagreement withliterature,where, using
Wemeasuredthesignificanceofthespatialvariationsseenin
colourorSFRindicators,ithasbeenshownthatdwarfgalaxiesex-
f acrossthesuperclusterbyperformingMonteCarlosimula-
AGN hibitmuchstrongerradialtrendswithenvironmentthantheirmas-
tions,inwhichwemaderepeatedmapsoff afterrandomlyas-
AGN
sivecounterparts(Grayetal.2004;Tanakaetal.2004;Smithetal.
signingthepositionsoftheAGNtothebrightgalaxies(i.e.testing
2006; Hainesetal. 2007, among others). The origin of the SF-
thenullhypothesisthatf isconstantacrossthesupercluster),
AGN densityrelationforthedwarfgalaxiescanbeattributedtothefact
and measuring the fraction of maps in which a given local value
that(i)thestarformationindwarfgalaxiescaneasilybequenched
off wasobtainedwithinoneoftheMonteCarlosimulations.
AGN
bythetidalimpactofamassiveneighbour and/ortheICMofthe
Theresultsofthesesimulationsarerepresentedbythewhitecon-
cluster(e.g.Larson,Tinsley&Caldwell1980),and(ii)unliketheir
tours,whichindicateregionsthathavef =1and2-σ below
AGN
massivecounterparts,dwarfs(oftheluminositiesconsideredhere)
themeanvalueacrossthesupercluster.Thisconfirmsthatthede-
do not become passive by internal mechanisms such as gas con-
clineinf seentowardsthecoreoftheComaclusterissignif-
AGN sumptionthroughstarformation,mergingetc..However,theeffects
icantatthe3σlevel,whilethatseeninAbell1367issignificantat
ofsupernovaewindblowoutsandfeedbackbecomesignificantfor
the2σlevel.
relativelyfaint (M∗.107 M⊙) dwarfs(e.g.MacLow&Ferrara
In Fig. 3 we show the general correlation between the frac- 1999; Marcolinietal.2006).Inthiswork wedo not findany ev-
tion of star-forming galaxies (fSF) and environment for massive idenceforthequenching ofstarformationingalaxiesonthefila-
galaxies(z<14.5;toppanel)andthedwarfgalaxies(z>15;bot- ment(s),butonlyinthecoresoftheembeddedgalaxygroups.
tom panel). For the massive galaxies we find an almost uniform
f <0.5 in all the environments. For dwarf galaxies, we see a The H emission traces the current SFR of a galaxy, while
SF α
6 Mahajan,HainesandRaychaudhury
Figure2.ThelocalAGNfraction(fAGN)amongmassivegalaxies(z<14.5),asafunctionofspatialpositionacrosstheComasupercluster.Themapis
colour-coded withf .Theblackcontoursindicate thelocalluminosity-weighted (z-band)galaxydensityacrossthesupercluster. Thewhitecontours
AGN
indicateregionswithAGNfractionfAGN =1and2-σbelowthemeanvalueoverthesupercluster.
the continuum flux under this linecan be used as anindicator of this,weseektotestthenullhypothesisthattheEW(H )isinde-
α
itspastSFR,makingthegloballyaveragedEW(H )fromagalaxy pendentofspatialpositionamongstar-forminggalaxies.InFig.4,
α
aneffectiveindicatorofthebirthrateparameter (b≡current SFR weshow,asoverlaidwhitecontours,theregionsinwhichthelocal
normalized by the SFR averaged over the lifetime of a galaxy). observed mean value of the EW(H ) is 2 and 3σ above that av-
α
Recently, Leeetal. (2009) have shown that EW(H )∼40A˚ cor- eragedover thewholesupercluster. Thisconfirmsthattheexcess
α
responds to b = 1. In Fig. 4 we show the variation in the mean starformationseenintheinfallregionsofComa,andtowardsthe
EW(H ) asafunction of environment for thestar-formingdwarf coreofAbell1367, isindeedsignificantat>3σ level.Inquanti-
α
galaxies.Thisispotentiallyaverypowerfultechniquefordissociat- tative terms, this excess in the infall regions of the Coma cluster
ingtheintrinsicallyactivestarformationhistory(SFH)oflow-mass is due to a population of ∼30 dwarf starburst galaxies (z > 15,
galaxiesfromastarburstcausedbytheimpactoflocalenvironment EW(H )>40A˚) located within or along the white2-σ contours
α
(seeHainesetal.2007,forinstance). inFig.4.Theanalogous excess inthecoreofAbell 1367 canbe
ascribedto∼10starburstdwarfgalaxies.
Ifgalaxiesareslowlyquenchedbyinteractionswiththeiren-
vironment (Baloghetal. 2004a), a decline in the mean EW(H )
α
is expected in the regions of denser environment. On the other
hand, if star formation is triggered due to the impact of envi- 4 OPTICALANDMID-IRANALYSISOFCOMAAND
ronment, the mean EW(H ) should increase. Unlike the slow ABELL1367GALAXIES
α
quenchingofdwarfsinthedenseenvironments(Tanakaetal.2004;
Theinfrared(IR)emissionfromnormalgalaxiesaroundλ=24µm
Hainesetal. 2007), here, in the Coma supercluster, we find that
isdominatedbyhotdustintheHIIregionsofmassivestars,inaddi-
thedwarfgalaxiesfollowdifferentevolutionarypathsinthegroups
tionto(usually)minorcontributionfromasymptoticgiantbranch
andinthedenserclusterenvironments.Ingalaxygroups,adwarf
(AGB) stars and the general interstellar medium (ISM). This re-
galaxy is slowly quenched via interactions with the other group
cycledemission,togetherwiththeopticalemission,canthuspro-
members and/or the tidal field of the group, while in the clus-
videagoodestimateofthetotalstarlightofagalaxyandbeused
ters, an infalling dwarf experiences a starburst in the intermedi-
to better constrain the current and past star formation rate (SFR)
atedensityenvironmentattheclusterperiphery(Porteretal.2008;
ofagalaxy(Calzettietal.2007;Kennicuttetal.2009;Riekeetal.
Mahajan,Raychaudhury&Pimbblet 2010), and is then rapidly
2009,amongothers).WhileIRastronomersusuallystudylate-type
quenched via cluster-related environmental mechanisms, such as
galaxies,early-typegalaxieshavereceivedsporadicattentiononly
ram-pressurestripping.
in the context of the warm dust component detected in a few of
InanalogytoFig.2,wemeasurethesignificanceofthespatial them(e.g.Knappetal.1989).
variationsseeninthemeanEW(H )viaMonteCarlosimulations, Inthispaper,wecombinetheopticalphotometricandspectro-
α
this time by making repeated maps after randomly swapping the scopicdatatakenbytheSDSSDR7,withthe24µmSpitzer/MIPS
valuesofEW(H )amongthedwarfstar-forminggalaxies.Indoing data,whichissensitivetoprocessedopticalemissionfromstars,to
α
Starformationand quenchingacrosstheComa supercluster 7
Figure3.Thelocalfractionofstar-forminggalaxies(fSF)asafunctionofspatialpositionacrosstheComasupercluster,formassivegalaxies(z<14.5;
toppanel)anddwarfgalaxies (z>15;lowerpanel).ThecoloursindicatefSF,andthesamecolourscaleisusedinboththepanels.Overlaidareblack
contoursindicatingthez-bandluminosity-weightedgalaxydensityacrossthesupercluster.ThisfigureshowsthatintheComasupercluster,starformationin
themassivegalaxies(z<14.5)seemstobesuppressedindependentoftheirlocalenvironment,whilethedwarfgalaxies(z>15)arestar-formingeverywhere
exceptinthedenseenvironmentinthevicinityofrichclustersandgalaxygroups.
8 Mahajan,HainesandRaychaudhury
Figure4.ThelocalmeanHαequivalentwidthofstar-forming(EW(Hα)>2A˚)dwarf(z>15)galaxiesacrosstheComasupercluster,indicatedbycolour.
Overlaidareblackcontoursindicatingthez-bandluminosity-weightedgalaxydensityacrossthesupercluster,andwhitecontoursshowingregionswherethe
localobservedmeanvalueoftheEW(Hα)is2and3-σabovethataveragedoverthewholesupercluster.
studythestarformationactivityofgalaxiesresidinginthedenser
regionsoftheComasupercluster.

1.0
24-z
SDSS magnitude limit
4.1 OpticalandMIRcolours -8.0
0.8 -6.0
In Fig. 5, we show the colour magnitude relation (CMR) for all -3.0
the spectroscopic galaxy members (r 6 17.77) found in this re- -1.0
gionintheSDSSDR7,andthosethatarealsodetectedat24µmin 0.6
the Spitzer/MIPS observations. For comparison, the 24µm galax- g-r
ies found in Abell 1367 are also shown. We only use galaxies
brighter than r =15.5 to fit the CMR. The CMR is of the form 0.4 CMR fit to Coma
g−r=1.244−0.032rforalltheComagalaxies,wherethemean CMR+/-MAD
A1367
absolutedeviationfromtherelationis±0.064mag.
0.2 Coma
We repeat thisanalysis in Fig. 6 for the near-infrared (NIR)
Optically detected Coma galaxies
band of SDSS (z-band), and the 24µm MIPS band. The galax-
12 14 16 18
iesclassified as red and blue on the optical colour-magnitude di-
r
agram,splitintotwoseparateclassesaround(24−z) = −6mag
ontheplotof thenear/midIRcolour and magnitudeaswell.Al-
Figure5.The(g−r)vsrcolour-magnituderelationforthe24µmbright
thoughweoverplotthelinesofconstantSFRinFig.6,according
galaxiesdetectedintheComacluster(circles)andAbell1367(bigstars).
to the empirical relation given by Calzettietal. (2007), it is im-
The solid line shows the colour-magnitude relation (CMR) fitted to the
portant for the reader to consider that the 24µm flux alone is an
Comadatapoints,whilethedot-dashedlinesmarkthemeanabsolutedevi-
accurate SFR indicator only for the late-type, dusty star-forming ation(MAD)boundariesoneitherside.WeadoptthelowerMADboundary
galaxies (we return to this issue below). The SDSS z-band, cen- in(g−r)forsegregatingtheredsequencefromthebluecloud.Notethat
tredat∼9000A˚,isagoodmeasureofthelightfromevolvedstars, wedonotuseanyupperboundfortheredsequencebecausefittingisdone
and hence the stellar mass of a galaxy. The top axis shows the usingspectroscopicallyconfirmedclustermembersonly.Thesymbolsare
stellar mass of galaxies estimated using the relation logM∗ = colour-codedaccording tothe(24−z)colouroftheMIRbrightsources
−0.306+1.097(g − r)−0.1−0.4(M −5log h−4.64), from (Fig. 6).Allthe other Comagalaxies are shownas greystars. Note that
r (24−z)=-6magseparatestheredsequencegalaxiesfromtheblueonesin
Belletal.(2003).Ontheotherhand,the24µmMIPSbandinthe
az−(24−z)plane(Fig.6).
MIRisagoodproxyforthedust-processedlight.Thismakesthe
(24−z)colouranexcellentapproximationforthevalueofthespe-
cificstarformationrate(SSFRorSFR/M∗)ofgalaxies.
Figs. 5 and 6 together show that the 24µm detected red se-
Starformationand quenchingacrosstheComa supercluster 9
Figure7.Thiscolour-colourdiagramshowsthatthegalaxiesidentifiedas
AGN(greencircles)andstar-forming(bluestars),onthebasisoftheBPT
Figure 6. The (24−z) colour of all galaxies in the Coma supercluster
diagramaccordingtotheiropticalspectra(seeFig.8),occupydifferentre-
sample,dividedintobluecloud(bluestars)andredsequence(red)(Fig.5)
gionsonthenear/midIRmagnitude-colour diagram.Thesolidandopen
galaxies,areplottedagainsttheirz-bandmagnitudes.Thedottedlineshows
green circles represent AGN classified on the BPT diagram and by the
the80%completenesslimitofthe24µmdata.TheconstantSFRlinesare
Milleretal. (2003)criterion respectively (see text). The passive galaxies
drawnusingtheconversionfactorgivenbyCalzettietal.(2007).Thehori-
(redpoints)detectedat24µmareconcentratedinasmallregiononthetop
zontaldashedlineat(24−z)=−6magisourempiricallychosencriterion
rightofthediagram.Thehorizontallineat(24−z)=−6magmarksthe
forselectingstar-forminggalaxies.Thetopaxisshowsthestellarmassof
boundarybetweenthesetwoclassesinthez-(24−z)IRcolourmagnitude
galaxiesestimatedusingarelationfromBelletal.(2003).
space(Fig.6).
quencegalaxiesinComahaveconsistentopticalandMIRcolours.
Wenotethattheoptical-MIRcoloursoftheredsequencegalaxies
arenotconsistentwiththoseexpectedfromphotosphericemission
from old stellar populations, with an excess emission always ap-
parentat24µm.TheSpitzerInfraredSpectrographobservationsof
early-typegalaxiesinVirgoandComaclustersshow thatthedif-
fuse, excess emission, apparent at 10–30µm in these galaxies, is
duetosilicateemissionfromthedustycircumstellarenvelopesof
mass-losingevolvedAGBstars(Bressanetal.2006;Clemensetal.
2009).Thestrengthofthissilicateemissionisaslowlydeclining
function of stellar age (Piovan,Tantalo&Chiosi 2003), and per-
sists even for very evolved stellar populations (> 10Gyr). The
optical-MIRcoloursofVirgoandComagalaxieshavebeenfound
to be consistent with such old stellar populations (Clemensetal.
2009).
ThebluegalaxiesinFig.5haveawidespreadinbothcolour Figure 8. The BPT diagram (Baldwin,Phillips&Terlevich 1981) for
and magnitude. The inhomogeneity of this class of galaxies be- the 24µm detected galaxies in the Coma cluster (filled circles) and in
comesevenmoreclearinFig.7,whereweplottheoptical(g−r) Abell1367(opentriangles),colourcodedbytheir24µmflux.Wealsoplot
colouragainstthenear/mid-IR(24−z)colour.JustasinFig.5,the allthegalaxieswithin±2,000kms−1ofComaorAbell1367,inthe500
passiveredgalaxiesclusterinasmallregionofthecolour-colour squaredegreesuperclusterregion,butnotdetectedat24µm,asgreycrosses
space,butthebluegalaxiesand(optical)AGNsspanawiderange .Thepointsplottedinalineatlog[OIII]5007/Hβ =−1.5arethegalax-
ieswhichhavenodetected[OIII]and/orH emission.Itisinterestingto
along boththeaxes. Wenote theclear separationof star-forming β
observe that a large fraction ofthe Comagalaxies without [OIII] and/or
(blue stars) and passive (red points) galaxies in (24−z) colour.
ThehorizontaldashedlineinFig.7indicatesourempiricallycho- Hβ havetheir[NII]/Hα fluxratios asexpected forAGN,suggestingthat
thesegalaxiesmayhavetheirnuclearemissionobscured.Weclassifysuch
sencriteriontoseparatethetwopopulationsabout(24−z)=−6. galaxiesasAGNiftheyhaveLog([NII]/Hα)>-0.2(Milleretal.2003).
Interestingly, although theAGN andstar-forminggalaxies (green
andbluesymbols respectively) areclassifiedonthebasisof their
opticalspectraontheBPTdiagram(Baldwin,Phillips&Terlevich
1981, also see §4.2), they occupy distinct regions in this plot of
opticalvsoptical-IRcolour,aswell.
10 Mahajan,HainesandRaychaudhury
4.2 Opticaland24µmstarformationindicators
The understanding of the formation of stars in galaxies requires,
amongotherthings,themeasurementoftherateatwhichtheinter-
stellargasisconvertedintostars.Withthedevelopmentofappro- -12 CAo1m36a7 R90/R50
Linear fit to Coma data
priatetechnology,radio,IRandUVphotometryandspectroscopy 3.0
areincreasinglybeingemployed,inadditiontothetraditionalopti- -10 star-forming 2.6
calobservations,forthepurposeofmeasuringtherateofstarfor- 2.2
mation.Thedustclouds surrounding theyoung stellarconcentra- 4-z -8 1.5
tionsabsorbstarlightandre-radiateitatIRwavelengths.Thus,the 2
star-forming
SFRmeasuredbyIRmeasurementsisaccurateonlyintheoptically
thicklimit.TheobservedUVradiationescapedfromthemolecular -6
cloudswhichblocktheUVlightatearlierages,comesmostlyfrom
starswithages107-108 yrs(Calzettietal.2005).Hαemissionis -4
producedonlyinthefirstfewmillionyearsfromthemostmassive
stars(Leithereretal.1999).Indeed,acombinationofUV,H and 0.1 1 10 100
α
IRobservationsisrequiredtogiveafullmeasureoftheobscured H EW
andunobscuredstarformation.Inthissection,weattempttocom-
bine the direct (optical) and the obscured (IR) radiation fromthe Figure9.The(24−z)colouras afunction ofHα EWforComa(filled
circles)andAbell1367(stars)galaxiesrespectively.Thestraightlinerep-
Coma and Abell 1367 galaxies to understand the process of star
resentsthelinearfittogalaxiesintheComacluster.Thoughthereisacon-
formationacrossthissupercluster.
siderablescatter,thecorrelation betweenthetwoquantities issignificant.
The24µmIRemissioningalaxiescanresultfromdustheated Thedottedlinesrepresentthelowerlimitsadoptedtoselect star-forming
by young massive star clusters as well as the AGN. Since our galaxiesusing(HαEW=4A˚)and(24−z)colour=−6.0(seeFig.6).For
MIPS data comes from the densest regions at the core of the completeness,wealsoshowthe24µmdetectedgalaxies thatdonothave
Comacluster(Fig.1),whichisafavourableenvironmentforAGN HαemissioninSDSSspectraatEW(Hα)=0.2A˚ (opencircles).The24µm
hosts, it is important to investigate the origin of the 24µm emis- emissioninthesegalaxiesislikelytocomefromtheevolvedAGBstars.
sion in these galaxies. To do so, in Fig. 8 we plot the usual ra-
tios of optical emission linewidths ([OIII], H , [NII] and H ),
β α
known to distinguish star forming galaxies from those domi- Petrosian radii R90r/R50r from the SDSS photometric catalogue,
nated by AGN, in what is popularly known as the BPT diagram which is an indicator of morphology). Passive spirals often have
(Baldwin,Phillips&Terlevich1981). concentrationindicesconsistentwithearly-types(R90r/R50r>2.6;
also see Mahajan&Raychaudhury 2009). But Fig. 9 shows that
InFig.8,weshowalltheemission-linegalaxiesfoundinthe
±2,000kms−1 redshiftslicearoundComaand/orAbell1367,in both the spiral and spheroidal galaxies are uniformly distributed
along both the axes, implying that the emission-line galaxies de-
the ∼500 square degrees Coma supercluster region (see Fig. 1),
tectedat24µmatthecoreofComaandAbell1367arenotdomi-
alongwiththegalaxiesdetectedat24µm(MIPS)inComa(circles)
natedbygalaxiesofanyparticularmorphologicaltype(asquanti-
and Abell 1367 (open triangles). Although we find a significant
fiedbytheconcentrationindex).
numberof24µmdetectedgalaxieswith[NII]/H ratiossuggestive
α
Duetothedominanceofearly-typegalaxiesinour24µmsam-
of AGN, this does not mean that the 24µm emission is predom-
ple,itisnotsurprisingthatonlyasmallfractionofgalaxiesclas-
inantly due to AGN. Goulding&Alexander (2009) have tried to
sifiedasstar-formingonthebasisoftheir(24−z)colour(Fig.6),
estimatethe contribution of the AGN component to theIR emis-
shownosignsofcurrentstarformationintheiropticalspectra.For
sionfromgalaxiesusingSpitzer/IRSspectroscopy.Althoughonly
completeness, at EW(H )=0.2A˚ we plot all the 24µm detected
a few galaxies in their sample have IR fluxes produced predomi- α
galaxies that do not show optical emission in H . As expected,
nantlyby AGN,theyalsofindthat asubstantial fractionof AGN α
most of these galaxies have (24−z) colours and concentration
areopticallyobscured,consistentwiththeresultspresentedhere.
parametersconsistentwiththoseofquiescent early-typegalaxies.
Wefindthatmostofthegalaxieswhichhavemeasuredvalues
Thissuggeststhattheir24µmemissionisprimarilyduetothecon-
ofallfouremissionlinesaredustystarforminggalaxies.However,
tributionfromAGBstars,which(unlikeyoung stars) donot pro-
interestingly,alargenumberofgalaxiesdetectedat24µminComa
duceH emission.Thegoodoverallcorrespondence betweenthe
thatdonotshowemissionin[OIII]and/orH ,dohavea[NII]/H α
β α photometric(24−z)colourandspectroscopicEW(H )makesthe
flux ratio consistent with the presence of an AGN. Such galax- α
(24−z)colouragoodcandidateforcomparingthestarformation
iescanbeclassifiedasAGNiflog ([NII]/H )>-0.2(Milleretal.
α activityinnearbygalaxies,intheabsenceofopticalspectra.
2003).
TheEWoftheBalmerlines,especiallythatoftheH emis-
α
sion line, has been extensively used to estimate the current opti-
4.3 Varyingfractionsofstar-forminggalaxieswithSFR
cal SFR of galaxies. It has been shown in the literature that the
tracer:implicationsfortheButcher-Oemlereffect
24µm flux is a good measure of dust-processed star light (e.g.
Calzettietal.2007).But,the24µmobservationsusedinthiswork PhotometriccoloursandEWsofemissionlineslike[OII]andH
α
covertheverycentralregionsoftheComacluster,andAbell1367, have been extensively used for studying the evolution of galax-
whichcomprisesmostlyofearly-typegalaxies.Inordertotestthe ies in time and across the sky. In one such pioneering work,
correspondence between the direct and dust-processed SFR trac- Butcher&Oemler (1984, BO84) found that clusters at moderate
ers,inFig.9weplotthe(24−z)colourasafunctionofspectro- to high redshifts contain an ‘excess’ of blue galaxies, relative to
scopicallymeasuredEW(H ).Wealsocolour-codethesymbolsac- their local counterparts. They estimatedtheblue fractionbycon-
α
cordingtotheconcentrationparameterofthegalaxies(ratioofthe sidering galaxies found within R30 (radius containing 30% of all