Morphologies and stellar populations of galaxies in the core of Abell 2218

26 Pages·2007·4.08 MB·English

Mon.Not.R.Astron.Soc.376,125–150(2007) doi:10.1111/j.1365-2966.2007.11335.x
Morphologies and stellar populations of galaxies in the core of Abell 2218
(cid:2)
S. F. Sa´nchez,1 N. Cardiel,1,2 M. A. W. Verheijen,3 S. Pedraz1,2 and G. Covone4
1CentroAstrono´micoHispanoAlema´n,CalarAlto(CSIC-MPI),C/Jesu´sDurba´nRemo´n2-2,04004-Almer´ıa,Spain
2DepartamentodeAstrof´ısica,FacultaddeF´ısicas,UniversidadComplutensedeMadrid,28040Madrid,Spain
3KapteynAstronomicalInstitute,POBox800,9700AVGroningen,theNetherlands
4INAF–OsservatorioAstronomicodiCapodimonte,Naples,Italy
Accepted2006November20.Received2006November14;inoriginalform2006June21
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ABSTRACT ad
e
d
Wepresentastudyofthestellarpopulationsandmorphologiesofgalaxiesinthecoreofthe fro
galaxyclusterAbell2218.Integralfieldspectroscopy(IFS)observationswereperformedusing m
PMASinthePPAKmodecoveringafieldofviewof∼74×64arcsec2centredonthecoreofthe http
cluster,inordertoobtainspectroscopyofanunbiasedflux-limitedsampleofclustergalaxies. ://m
n
43objectsweredetectedintheIFSdata,31ofthemwithenoughsignal-to-noiseratiotoderive ra
s
the redshift, all of them brighter than I < 21.5 mag. 28 are at the redshift of the cluster (17 .ox
withpreviouslyunknownredshift).Individualspectraoftheclustermemberswereextracted ford
and compared with single stellar population models to derive the luminosity-weighted ages jou
rn
and metallicities. In addition, deep Hubble Space Telescope (HST)/ACS F475W-, F555W-, a
ls
F625W- and F850LP-band images centred on the cluster core were obtained from the HST .org
archive(z ∼28mag).Adetailedmorphologicalanalysisofallthegalaxieswithinthefield a/
ofviewoflitmheseimagesdowntozlim<22.5magwasperformedclassifyingthemaslate-type, t UN
intermediateandearly-type,onthebasisoftheirSe´rsicindices.Theliteraturewasscannedto IV
E
R
lookforspectroscopicallyconfirmedclustermemberslocatedwithinthefieldofviewofthe S
ID
ACSimage.Thefinalsampleof59galaxiescomprisesourreportedsampleof28galaxiesin A
D
the core, and 31 additional galaxies in the outer regions. In addition, multiband broad-band C
O
photometrywasextractedfromtheliteratureforallobjects. M
P
Themorphologicallysegregatedcolour–magnitudediagramshowsthattheearly-typegalax- L
U
iescovertherangeofbrighterandreddercolours(theso-called‘redsequence’).Alargefraction T
E
ofspiralgalaxies(∼50percent)isfound,mostofthemfainterthanthelimitofpreviousstudies. NS
E
Theycoverawiderangeincolours,frombluecolourstypicalofButcher–Oemlergalaxiesto D
E
redcolourssimilartothoseofearly-typegalaxies.Thisresultindicatesthatearly-typegalaxies M
A
aremoremassiveandhaveolderstellarpopulations,whilelate-typegalaxiesarelessmassive D
R
and have a wider range of stellar populations. The distribution of luminosity-weighted ages ID
asafunctionofmetallicitiesandluminosity-weightedmasses,andthedistributionofSe´rsic on
J
indices as a function of the luminosity-weighted masses confirm these results. They in fact un
e
agreewithaproposedtwo-stepscenariofortheevolutionofgalaxiesinclusters,wherethestar 1
4
formationisquenchedfirstintheinfallingspirals,afterwhichamorphologicaltransformation , 2
0
1
followsthatrequireslargertime-scales.Thisscenarionaturallyexplainsthepopulationfaint 3
late-typegalaxieswitholdstellarpopulationsobservedinthiscluster.Inaddition,anextremely
bluemerginggalaxysystemisfoundatthecore,withthenominalredshiftofthecluster.
Keywords: techniques:spectroscopic–galaxies:clusters:individual:Abell2218–galaxies:
ellipticalandlenticular,cD–galaxies:fundamentalparameters–galaxies:spiral–galaxies:
stellarcontent.
1 INTRODUCTION
Galaxyclustershavebeenusedfordecadestostudytheevolution
ofgalaxies.Beingtracersofthelargestdensityenhancementsinthe
(cid:2)E-mail:[email protected] Universe, they are considered the regions where galaxies formed
(cid:3)C 2007TheAuthors.Journalcompilation(cid:3)C 2007RAS
126 S.F.Sa´nchezetal.
first.Atlowredshifttheyaredominatedbylargeearly-typegalaxies trographmultislitsormasks.Inparticular,theblendingofdifferent
(E/S0),whichcoloursareconsistentwithabulkformationoftheir components in the core of the clusters is a problem, impossible
starsathighredshift.Classically,itwasthoughtthattheevolution tohandlewithclassicalslitspectroscopy.Duetothat,mostofthe
ofthisdominantpopulationthroughdifferentcosmologicalepochs spectroscopicsurveysarebasedonpre-selectionoftheclustercan-
waslargelypassivewithoutsignaturesofimportantdeviations(i.e. didates,mostofthembasedoncolourormorphologicalselections
periodsofmorerecentstarformation).However,thissimplepicture (e.g.Baloghetal.1999;Dressleretal.1999;Kelsonetal.2000;
haschangedinthelastdecades.Butcher,Wells&Oemler(1983) Ziegler et al. 2001; Fritz et al. 2005). Alternatively, they are fo-
andButcher&Oemler(1984)showedanincreaseinthefraction cusedongalaxiesinthelesscrowdedouterregions.Theseselections
ofbluegalaxiesinclusterswithredshift.High-resolutionground- mayaffectsubstantiallytheconclusions.Forthesereasonswehave
basedandHubbleSpaceTelescope(HST)imageshaveshownthat startedanobservationalprogrammeaimedatobtainingintegralfield
theycorrespondtogalaxieswithlate-typemorphologies,oftendis- spectroscopy(IFS)ofthecoreofsomeoftherichestgalaxyclusters
turbedorwithclosecompanions(e.g.Thompson1986;Thompson inthelocaluniverse,includingAbell1689(ESO73.A-0605)and
1988;Couchetal.1994;Dressleretal.1994,1997).Oneobserves Abell2218.ForbothofthemdeepHST/ACSimagesareavailable.
D
amorphologicalevolutionofthegalaxiesinclusters,inthesense AsimilarapproachhasbeenusedbeforebyCovoneetal.(2006), o
w
thatthefractionofE-typegalaxiesremainsconstantasafunctionof tostudythedynamicalmassoflowredshiftgalaxyclusters. nlo
differentcosmologicalepochs,whilethefractionofS0’sincreases Abell2218isoneoftherichestclustersintheAbellcatalogue ad
e
a2t0t0h0e).expenseofL-typestowardslowerredshift(e.g.Fasanoetal. (aArbeedlslh,iCftoorwfizn∼&0O.1lo7w, ainnd19a8v9e),lowciittyhadirsipcehrnseiossncolfas1s3o7f04k.mIths−as1 d from
The presence in clusters of a significant population of post- (Kristian,Sandage&Westphal1978;LeBorgne,Pello´&Sanahuja h
starburstgalaxiesknownas‘E+A’or‘k+a’galaxies(Dressler 1992).Theclustershowsasetofarclenses.Detailedhigh-resolution ttp://m
& Gunn 1983; Couch & Sharples 1987), indicates that star for- X-raymaps(McHardyetal.1990)andmass-concentrationstudies n
mation was active in the recent past and sharply halted at some basedonthepropertiesofthegravitationallenseshaveshownthat ras
.o
point during the last 1–1.5 Gyr (Poggianti 2003; Poggianti et al. theclusterhastwodensitypeaks,thestrongestofthemdominatedby x
fo
2004). Most of these galaxies are late-type, and they show a ra- alargecDgalaxy.Severalobservationalprogrammeshaveprovided rd
dialdistributionwithintheclustersthatisintermediatebetweenthe uswithalargedatasetofwell-deblendedslitspectraforthegalaxies jou
passiveandtheemission-linegalaxypopulations(Poggiantietal. intheouterpartofthecluster(Ziegleretal.2001),aswellasvery rna
ls
1999).Theseresultsleadtothehypothesisofatwo-stepevolution goodmultibandground-basedandHSTimaging(LeBorgneetal. .o
rg
ofgalaxies.Aspiralgalaxyiscapturedbytheclusterand,during 1992;Smailetal.2001;Ziegleretal.2001).Wefocusedoursurvey a/
itsinfall,suffersfromgas-strippingduetothetidalforcesinvolved onthecentralarcminregionofthecluster,aroundthecDgalaxy. t U
N
(e.g.Verheijen1997).Provokedbythelackofgas,thestarforma- Theoutlineofthisarticleisasfollows.Section2describesthe IV
tionishalted,andthegalaxyevolvespassively,beingobservedasa observationaldataset,includingadescriptionoftheIFSdataandthe E
R
‘k+a’post-starburst.Aninitialshortstarburstprocess,previousto HST/ACSimages.Section3includestheanalysisofthedatasetand S
ID
thegas-stripping,maybealsopresent(Poggiantietal.2004).After presentstheresults.Here,Sections3.1and3.2describetheauto- A
D
thisthegalaxyevolvesmorphologicallyoverlargertime-scales.The maticmorphologicalanalysisthatweperformedasopposedtoclas- C
discdimsduetothelackofanewgenerationofstars,thatotherwise sicalmethods.Section3.3showsthemethodusedtodeblendandex- O
M
willdominateitsluminosity,andthegalaxytransformsitselfinto tracttheintegratedspectraofthedifferentdetectedobjectsfromthe P
L
alenticulargalaxy.Drymergersbetweenlenticularandspheroidal IFSdata,withtheredshiftdeterminationexplainedinSection3.4. U
T
galaxiesbuildupnewlargespheroidals(e.g.Belletal.2006). Theanalysisofthemorphologicallysegregatedcolour–magnitude E
N
The proposed scenario suggests that a fraction of the observed (CM)diagramisincludedinSection3.5.Theluminosity-weighted SE
S0’s were previously star-forming spirals. For these galaxies the ages and metallicities of the galaxies, and their relation to their D
E
starformationwastruncatedatsometimebetween2and5Gyrago massesandmorphologiesarediscussedinSection3.6and3.7.The M
anditisexpectedthattheirluminosityfadedby∼0.5–1.5magand resultsarediscussedinSection4,andtheconclusionsarelistedin AD
becomeredder(Poggiantietal.2001),populatingtheredsequence Section 5. Throughout this article we have used a (cid:3)-cosmology, RID
atmagnitudesfainterthanthebrightest1or2mag(Poggianti2003). withH0=70kms−1Mpc−1,(cid:4)m=0.3and(cid:4)(cid:3)=0.7. on
Thesameprocessmayaffectdwarfgalaxiescapturedbytheclus- J
u
n
ter,whichwillsuffermorestronglytheeffectsoftidalinteraction e
2 OBSERVATIONAL DATA SET 1
andgalaxyharassment,whichinducesashortbutintensestarburst 4
, 2
(Mooreetal.1996;Rakos,Dominis&Steindling2001).Mostofthe 0
fielddwarfgalaxiesaregas-rich,bluegalaxieswithlate-typemor- 2.1 IFSobservationsanddatareduction 13
phologiesatanyredshift(Cairo´setal.2001;Barazzaetal.2006). Observations were carried out on 2005 June 30 and July 6 at the
Oncecapturedbythecluster,consumedorstrippedofitsgas,itis 3.5-mtelescopeatCalarAltoObservatorywiththePotsdamMul-
expected that their luminosity fades and they become redder too, tiAperture Spectrograph, PMAS, (Roth et al. 2005) in the PPAK
populating the faint-end of the red sequence. In both cases ones mode(Verheijenetal.2004;Kelzetal.2006).TheV300grating
expectstofindapopulationofredlate-typegalaxies,fainterthan wasused,coveringawavelengthrangebetween4650–8000Åwith
theoriginalearly-typegalaxiesinthecluster.However,mostofthe aresolutionof∼10Åfullwidthathalf-maximum(FWHM)anda
previousstudieshavefoundthatlate-typegalaxiesaremostlyblue samplingof3.2Åpixel−1.ThePPAKfibrebundleconsistsof382
(e.g.Thompson1986). fibresof2.7arcsecdiametereachone.Ofthem,331fibres(thesci-
Totestthishypothesis,completespectroscopicsurveysofclus- encefibres)areconcentratedinasinglehexagonalbundlecovering
tergalaxiesandhigh-resolutiondeepimagesforapropermorpho- afieldofviewof72×64arcsec2,withafillingfactorof∼65per
logicalanalysisofthefaintestpossibletargetsarerequired.Com- cent.Theskyissampledby36additionalfibres,distributedinsix
pletespectroscopicsurveysaredifficultandtimeconsuming,due bundlesofsixfibreseachone,locatedfollowingacirculardistribu-
totherequirementofdeblendingandplacingobjectsinthespec- tionat∼90arcsecfromthecentreandattheedgesofthecentral
(cid:3)C 2007TheAuthors.Journalcompilation(cid:3)C 2007RAS,MNRAS376,125–150
ThegalaxiesinthecoreofAbell2218 127
hexagon.Thesky-fibresaredistributedbetweenthescienceonesin
apseudo-slit,inordertohaveagoodcharacterizationofthesky.The
remaining15fibresareusedforcalibrationpurposes,asdescribed
below.
Foreachnight,threeditheredexposuresof1heachweretaken,
followingapatternof(cid:5)RA=+1.56arcsecand(cid:5)Dec.=±0.78
arcsec, in order to have a complete coverage of the science field
of view and to increase the spatial resolution. Each pointing was
dividedintoshorterindividualframesforapropercosmicrayre-
moval.Atotalof6hintegrationtimewasachievedaftercombining
theexposuresofthetwonights(3hpernight).
Data reduction was performed using R3D (Sa´nchez & Cardiel
2005;Sa´nchez2006),incombinationwiththeIRAFpackage(Tody D
1993)1andE3D(Sa´nchezetal.2004).Thereductionconsistsofthe ow
standardstepsforfibre-basedintegral-fieldspectroscopy.Amaster nlo
biasframewascreatedbyaveragingallthebiasframesobserved Figure1. Averagenight-skyemissionspectrumderivedbycombiningthe ad
e
dcautriionngothfethneigshptecatnrda isnubthtreacCteCdDfrwomastdheetesrcmieinnceedfurasimngesa. Tchoentlion-- snpigehcttr.aobtainedthroughthe36sky-fibresforthedifferentpointingsofeach d from
uumilluminatedexposuretakenbeforethescienceexposures.Each h
ttp
spectrumwasextractedfromthescienceframesbyco-addingthe ://m
flux in 5 pixels in the perpendicular direction of each fibre, and 2.2 HST/ACSimagesandliteraturedata n
stored in a row-staked-spectrum file RSS (Sa´nchez et al. 2004). ras
WeobtainedpubliclyavailableHST/ACSimagesoftheclusterin .o
WavelengthcalibrationwasperformedusingaHelampexposure x
theF475W,F555W,F625W,F775WandF850LPbandsfromthe fo
taken at the beginning of the night, and corrected for distortions rd
usingThArexposuresobtainedsimultaneouslywiththescienceex- HSTarchive.Table1summarizestheirmaincharacteristics,includ- jou
posuresthroughthecalibrationfibres(indicatedabove).Thefinal ingthecentralwavelengthandwidthoftheirbands(obtainedfrom rna
Sirianni et al. 2005). They basically correspond to images in the ls
accuracyofthewavelengthcalibrationwasestimatedbycomparing .o
classicalB,V,R,Iandzbands.Theimageswerecalibratedtothe rg
thecentralwavelengthoftheskyemissionlineswiththeirnominal Vega system using the corresponding magnitude zero-points and a/
values,findinganagreementwithinarangeof∼0.3Å.Differences t U
prescriptionslistedinSiriannietal.(2005).Table1alsoliststhe N
inthefibre-to-fibretransmissionthroughputwerecorrectedforby numberofindividualframesacquired,eachonewithadifferentori- IV
comparingthewavelength-calibratedRSSscienceframeswiththe E
entationandaslightlydifferentpointing,duetotheobservingstrat- R
correspondingcontinuumilluminatedimages.Fluxcalibrationwas S
egyoftheHST/ACScamera(inordertocoverthegapinbetween ID
performedusingstandardcalibrationstarsobservedthroughoutthe A
night (BD33 + 2642 and Hz44). The uncalibrated spectra of the tcheesstewdobyditfhfeerHenStT/CACCDSsr)e.dTuhcetioancqpuipireeldinfer,awmheischwienrceluadleresabdiyasparnod- D C
starsextractedfromthereducedframeswerethencomparedwith O
darkcurrentcorrection,flat-fielding,quantumefficiencycorrection, M
publiclyavailablecalibratedspectra,determiningafluxcalibration P
field-distortioncorrection,thederivationoftheastrometricsolution L
correctionthatwasappliedtothescienceframes,oncecorrectedby U
and flux calibration. We correct each individual from for cosmic T
thedifferencesinexposuretime.Theaccuracyofthespectropho- E
rays,realignedtothestandardnorth-eastandfinallycombinedinto N
tometric calibration was ∼0.15 mag across the wavelength range SE
coveredbyourdataset,asdeterminedbycomparingwithhighac- asingleimageperfilterafterasky-backgrounddeterminationand D
subtraction.Thefinalexposuretimeandlimitingmagnitudeofeach E
curatebroad-bandphotometry,aswewilldiscussbelow.Anight-sky M
image,estimatedbylookingforthefaintestobjectdetectedat∼15σ A
emissionspectrumwasobtainedforeachpointingbycombiningthe D
overthebackground,arelistedinTable1.Alltheimagesaremuch R
36spectraobtainedthroughthesky-fibres,andsubtractedfromthe deeperthanourIFSdata,aswewillseebelowindetail.Inparticu- ID
331science-fibrespectra.Fig.1showstheaveragenight-skyspec- o
larthisisthecasefortheF850LP-bandimage.Thisimageismuch n
trumobtainedaftercombiningallofthem.Severalstrongnight-sky J
deeperthantherest,beingasdeepasthecorrespondingimageof un
emissionlinescontaminateourspectra,inparticularinthereddest e
part(λ>7200Å). UDF (e.g. Coe et al. 2006), and therefore one of the deepest im- 14
agesevertakenofthiscluster.Theclustercoreiscentredonone , 2
The three dithered exposures were then combined, creating a 0
singleRSSframewith993sciencespectraandthecorresponding of the ACS chips for this particular image, and the field of view 13
of our IFS data is completely covered by it. This high-resolution
combinationofthepositiontable.Afinaldatacubewith1arcsec
ACSF850LP-bandimagewillbeusedforadetailedmorphological
pixel−1 sampling was created for each night by interpolating the
analysis.
combined RSS frame using E3D (Sa´nchez et al. 2004). This cor-
respondsto2.85kpcpixel−1 attheredshiftofourtarget.Finally,
Table1. SummaryoftheHST/ACSimages.
thetwodatacubesobtainedforeachnightwererecentredateach
wavelength,correctingfordifferentialatmosphericrefraction,and
Filter Pivotλ Width Number Exposure maglim
combinedusingIRAFtasks.Thefinaldatacubecomprises4828in- (Å) (Å) offrames time(s) (15σ)
dividual spectra, covering a field of view of ∼71 × 68 arcsec2.
F475W 4744.356 418.80 3 4267.0 27.42
F555W 5359.547 375.19 4 5601.0 28.04
F625W 6310.454 415.31 5 7013.0 28.08
1IRAFisdistributedbytheNationalOpticalAstronomyObservatories,which F775W 7693.026 432.03 8 10732.0 27.46
areoperatedbytheAssociationofUniversitiesforResearchinAstronomy,
F850LP 9054.768 538.95 12 11776.0 28.22
Inc.,undercooperativeagreementwiththeNationalScienceFoundation.
(cid:3)C 2007TheAuthors.Journalcompilation(cid:3)C 2007RAS,MNRAS376,125–150
128 S.F.Sa´nchezetal.
Wealsocollectedbroad-bandphotometryandredshiftinforma- Fig. 2 shows a grey-scale of the combined HST/ACS F850LP-
tion of the cluster galaxies available in the literature. Le Borgne bandimage,togetherwithlabelsindicatingthelocationofthegalax-
etal.(1992)performedaphotometricandspectroscopicsurveyof iesstudiedbyLeBorgneetal.(1992)(orangestars),Ziegleretal.
thegalaxiesinafieldof4×4arcmin2centredonAbell2218.This (2001)(bluesquares)andSmailetal.(2001)(redcrosses).Theloca-
surveycomprisesbroad-bandphotometryinfivebands(B,g,r,iand tionoftheconfirmedclustermemberssampledbyourIFSdataare
zfilters)for729objects,andredshiftdeterminationfor66ofthem, alsoindicated(greencircles).Aswewillseebelow,mostofthem
51clustermembers.Theycalibratedtheirphotometricdatausing havenotbeenpreviouslytargetedbyanyspectroscopicsurvey.
twodifferentsystems,theJohnsononefortheBband(Johnson&
Morgan1953),andtheThuan–Gunnfortheg,r,iandzbands(Thuan
2.3 ThedepthofourIFSdata
&Gunn1976).Wedidnotperformanyrecalibrationoftheirdata
to a common photometric system. These data were initially used ToobtainaroughestimateofthedepthofourIFSdata,wecom-
tocomparewiththecorrespondingintensityoftheindividualspec- paredthemwithavailabledeepopticalimages.Fig.3showsathree-
traoftheobjectsdetectedinourIFUdata,andfordoingsowejust colourimagecreatedbyco-addingthefluxofthefinaldatacube
D
neededtousethecorrespondingJohnsonandThuan–Gunneffective throughthreebroad-bandscorrespondingapproximatelytoB,Rand o
w
flux(e.g.Fukugita,Shimasaku&Ichikawa1995).Theaccuracyof I(left-handpanel),togetherwithasimilarimagecreatedusingthe nlo
theirphotometryrangesfrom0.1to0.5magfromthebrighter(B∼ previouslydescribedHST/ACSdataset.Itisinterestingtonotethe ad
e
2la0ckmtahge)rteoqtuhierefadinqtuearl(itBy∼ne2e6demdatgo)poebrjfeocrtms.aTnheacscpuercattreosacnoaplyicsidsaotaf sciumbeilahraistieasfibneatlwFeWenHtMhefotwropoiimnta-gliekse,sdoeusrpcietesothfe∼f2ac.5tathrcasteocu,rmduactha d from
theage–metallicityindicators,liketheequivalentwidthsofHβand largerthantheonefromtheACS.BasedontheI-bandmagnitudeof h
ttp
MspgeZcbit,ergoblusectrothpeeticyaolp.bro(s2ev0rivd0ae1t)ihoirngeshpooqrfutea4dl8ityocnorleomdusurh-listfietslldeitcettmeedromdeieanrraalyttie-ot-nyresps.eolmuteimon- qtfhoueraolfiautiyrnItsFepsSetcddtareattaefcIotlriemdth∼oeb2pj2ero.c4pt,mowsaeegdr.osHtuuogdwhyleyvfoeerrs,toiwmbejaeotcentsltyhbegriedgtehhtteeigcrthitohenannoliuImg<iht ://mnras
bersofthecluster.Theyalsopresenthigh-qualitymultibandpho- 21.5mag(signal-to-noiseratio>5σ perspectralpixel),aswewill .ox
fo
tometry in four bands (U, B, V and I), derived from groud-based seebelow.Thislimitisfarbeyondpreviousspectroscopicstudies rd
oiebssebravsaetdioonns,HanSdT/aWmFoPrCp2hoolbosgeicrvaaltaionnasly(sFis70o2fW19boafntdh,e∼4R8bgaanladx)-. onthiscluster,aroundIlim∼18.7mag(Ziegleretal.2001). journa
ls
Finally,Smailetal.(2001)presentaphotometricstudyoftheages .o
3 ANALYSIS AND RESULTS rg
andmetallicitiesof81early-typeclustercandidates(33withcon- a/
firmedredshifts),basedonground-basedK-bandandHST/WFPC t U
3.1 Photometricanalysis N
multibandimages,intheF450W,F606WandF814Wbands.Inboth IV
cases,theirphotometricdatawerecalibratedintheVega-basedsys- WeusedthepreviouslydescribedHST/ACSimagestoobtainhomo- E
R
tem.Manyotherstudieshavetargetedthisclustersinceitsredshift geneousmultibandphotometryforallthedetectedobjectsinfieldof S
ID
confirmation by Kristian et al. (1978), and the early photometric Abell2218.Thishomogeneousphotometricdatasetisfundamental A
D
studiesbyButcheretal.(1983).However,inthispaperweonlyuse to estimate the accuracy of the spectrophotometric calibration of C
thephotometricandspectroscopicdataofthearticleslistedabove. ourIFSdataandtostudythestellarpopulationsofthegalaxiesin O
M
thecluster.WeuseSEXTRACTOR(Bertin&Arnouts1996)todetect PL
objects in the images up to 15σ over the sky (20 connected pix- U
T
elswithasignal-to-noiseratioofatleast2σ eachone).Thislimit E
N
is very relaxed, and most probably we are losing many real faint SE
objectsbyusingit.However,wearenotinterestedinthefaintest D
E
targets,mostprobablyhigh-zobjectsorarclenses(seeFig.1),but M
A
onlyintheclustermembers.Foreachdetectedtargetweobtained D
R
aperturephotometryineachband,usingtheMAG BESTparameter ID
derivedbySEXTRACTOR.ThelittlevariationsoftheHST/ACSpoint o
n
spreadfunction(PSF)comparedtothesizeofourtargetsofinter- J
u
n
estguaranteeshomogeneousphotometricdataset.Table2liststhe e
1
resultsfromthisanalysisforthe59galaxiesconfirmedtobecluster 4
, 2
members,usingthepublisheddatalistedaboveortheresultsfrom 0
1
our spectroscopic survey (see below). For each target it includes 3
theSEXTRACTORID,thecoordinates(RAandDec.),andthepho-
tometryineachbandpluserrors.Duetothechangeoforientation
andslightlydifferentpointingoftheindividualHST/ACSframes,
the F850LP band covers a slightly larger area than the images in
theotherbands,andsomeoftheobjectswereonlydetectedinthis
band.
Figure2. Grey-scaleimageofthecombinedF850LP-bandimage,together 3.2 Morphologicalanalysis
withlabelsindicatingthelocationofthegalaxiesstudiedbyLeBorgneetal.
Oneofthegoalsofthepresentstudyistodeterminethestellarpopu-
(1992)(orangestars),Ziegleretal.(2001)(bluesquares)andSmailetal.
(2001)(redcrosses).Thelocationoftheconfirmedclustermemberssampled lationsofgalaxiesintheclusterasafunctionoftheirmorphologies,
byourIFSdataarealsoindicated(greencircles).Northisupandeastis inordertounderstandtheiroriginandevolution.Toachievethisgoal
towardsleft-handside. weperformedamorphologicalanalysisoftheHST/ACSF850LP
(cid:3)C 2007TheAuthors.Journalcompilation(cid:3)C 2007RAS,MNRAS376,125–150
ThegalaxiesinthecoreofAbell2218 129
D
o
w
n
lo
a
d
e
d
fro
Figure3. Left-handpanel:three-colourimagecreatedbyco-addingthefluxofthefinaldatacubethroughthreebroad-bandscorrespondingapproximatelyto m
B,RandI.Right-handpanel:similarimagecreatedusingHST/ACSdatadescribedinthetext.Anelectronicversionofthefullsizeimagecanbefoundinthe http
webpagehttp://www.caha.es/sanchez/abell2218/BRIbig.jpg. ://m
n
ra
s
images,thedeepestofourdataset,followingtheprescriptionspre- ofdecreasingbrightness,modellingandsubtractingeverygalaxyin .ox
fo
sented in Coe et al. (2006), where similar images were analysed. theSEXTRACTORcatalogue.GALFITisableofsimultaneousfitseveral rd
Thisanalysisrecovers,foreachgalaxy,theSe´rsicindex,theinte- targets,producingreasonableresults,inmostofthecases.However, jou
gratedmagnitude,theeffectiveradius,theellipticity,theposition inthiscasethefittingprocessisdominatedbythebrightertargets, rna
angleandtheasymmetryindex. andfluxofthefainteronesismorepoorlyrecovered.Weperformed ls.o
ForundisturbedgalaxiestheSe´rsicindexncorrelatesfairlywell extensivetestsbeforeadoptingthedescribedprocedure.Inaddition, arg/
withmorphologicaltype(e.g.Andredakis,Peletier&Balcells1995; wedeterminetheasymmetryindex(A),followingtheprescriptions t U
Bceenltlreatlaclo.n2c0e0n4tr)a,tiinonthse(is.ee.nbseultgheatdgoamlainxaietesdwgiathlahxiigehs)nthhaanvgeahliagxhieesr iinntChoeeFe8t50alL.P(2-b0a0n6d).imOfagtheebr8i0g2htoerbjtehcatnszde<te2c6te.5dmbyagS,EwXeTRaAppClTiOeRd NIVE
R
withlown(i.e.discdominatedgalaxies).Lesswellbehavedgalax- GALFITtothe241galaxiesbrighterthanz<22.5mag.Theerror S
ies, including mergers and irregulars, generally do not have well inthederivedSe´rsicindexisrestrictedtoarangeof±0.25,based IDA
definedSe´rsicindices.Fortunately,thesegalaxiescangenerallybe onthesimulationspresentedinCoeetal.(2006),andthereforea D C
weededout(orselectedfor)bymeasuringtheirlargeasymmetries morphologicalsegregationbasedonthatparameterisreliable.The O
M
(e.g.Conseliceetal.2003). remaining galaxies are far beyond the depth of our spectroscopic P
L
Insummarythemorphologicalanalysisconsistsofthefollowing data, and therefore excluded. Table 3 lists the resulting fitted pa- U
T
steps.First,weuseSEXTRACTOR(Bertin&Arnouts1996)todetect rameters and their uncertainties for the 59 galaxies confirmed to EN
galaxies in the combined F850LP-band image. For each detected be cluster members, using the published data listed above or the S
E
galaxywerecoverthephysicalandimagecoordinates(RA,Dec.,X resultsfromourspectroscopicsurvey(seebelow).Itincludes,for D
E
andY),theintegratedmagnitude(mag best),thescalelength(r50), each galaxy, the SEXTRACTOR identification, the χ2/ν of the fit- M
the position angle and the ellipticity. Then, we create a postage tingprocedure,thez-bandmagnitude,theeffectiveradius(R ),the A
e D
stamp5×r onaside,forthebrightestgalaxy.Withinthatpostage axisratio(a/b),thepositionangle(θ),theSe´rsicindex(n)andthe R
50 s ID
stamp,neighbouringgalaxiesaremaskedoutusingellipses,each asymmetryindex(A).Thelistederrorswereestimatedbasedonthe o
ellipsegivenaminoraxislengthb=2×r50forthatgalaxy.Using resultsofthesimulationspresentedinCoeetal.(2006).Theerrorin n Ju
GALFIT(Pengetal.2002),thebrightestgalaxyisfittedtoasingle theasymmetryindexisdifficulttoquantifyforindividualobjects, ne
componentSe´rsicmodel(cid:9)(R)=(cid:9) ×exp{−κ [(R/R )1/n−1]}, however,basedonthequotedsimulations,weestimateittobelower 14
where Re is the effective or half-lieght radius, (cid:9)n e is thee effective than∼0.1forthegalaxiesofoursample.Ofallthesegalaxies,only , 201
surfacedensity,(cid:9)(R)isthesurfacedensityasafunctionofradius onehasastrongasymmetryindex(∼0.8).Thisgalaxyiscovered 3
andκ =κ(n)isanormalizationconstant.Thefitisconstrainedto bythefieldofviewofourIFSdata,anditwillbediscussedlaterin
0.2<n<8and0.3<R <500pixels,andthecentroidisconfined detail.
e
towithin2pixelsofthepositionderivedbySEXTRACTOR.Asinitial
guessesfortheGALFITparameters,weusetheSEXTRACTORoutput
3.3 TheSe´rsicindicesandtheHubblesequence
parameters.LackingestimatesfortheSe´rsicindexfromSEXTRAC-
TOR,westartallfitswithn=1.5.GALFITconvolvesthemodelwith OurmorphologicalclassificationschemeisbasedontheSe´rsicin-
therequiredPSF.Priortothefit,wecreatedaPSFimagebycom- dexofthegalaxyprofile.Weconsiderthisautomaticclassification
bining postage stamps of unsaturated point-like objects extracted schememorestraightforward,easytoreproduce,andlesssubjective
fromthecombinedF850LP-bandimage. thantheclassicalby-eyeclassification(readWolfetal.2005,foran
Havingbeencalculatedforthebrightestgalaxy,theSe´rsicmodel illustrationoftheproblem).However,mostofthepreviousstudies
issubtractedfromtheF850LP-bandimage.Thissubtractionben- onthemorphologiesofgalaxiesinclustersarebasedonby-eyeclas-
efitsthesubsequentmodellingoffainternearbygalaxies.Wethen sificationsfollowingtheHubblescheme(Hubble1926),revisedby
proceedtomodelthesecondbrightestgalaxy,andcontinueinorder deVaucouleurs,deVaucouleurs&Corwin(1976).Normally,they
(cid:3)C 2007TheAuthors.Journalcompilation(cid:3)C 2007RAS,MNRAS376,125–150
130 S.F.Sa´nchezetal.
Table2. Summaryofthephotometricanalysis.
ID RA Dec. B475 V555 R625 I775 z850
1 248.9537297 +66.2117906 18.09±0.02 17.29±0.02 16.52±0.01 15.85±0.01 15.44±0.01
247 248.9853560 +66.1981066 18.73±0.03 17.94±0.02 17.14±0.02 16.44±0.01 16.08±0.02
169 248.9870850 +66.1850276 18.86±0.03 18.09±0.03 17.34±0.02 16.70±0.02 16.19±0.02
512 248.9569254 +66.2059389 18.92±0.03 18.32±0.03 17.51±0.02 16.80±0.02 16.44±0.02
554 248.9546845 +66.2094133 19.53±0.04 18.87±0.04 18.26±0.03 17.56±0.03 17.00±0.03
172 249.0078962 +66.2089431 19.22±0.04 18.46±0.03 17.67±0.02 16.96±0.02 16.47±0.02
501 248.9646342 +66.2088796 19.40±0.04 18.67±0.04 17.93±0.03 17.12±0.02 16.61±0.03
561 248.9762849 +66.2328158 19.33±0.04 18.62±0.04 17.92±0.02 17.17±0.02 16.65±0.03
189 248.9961745 +66.2011969 19.57±0.05 18.75±0.04 17.93±0.02 17.27±0.02 16.78±0.03
103 249.0154109 +66.1939177 19.50±0.05 18.72±0.04 17.91±0.02 17.25±0.02 16.68±0.03
132 249.0082842 +66.1973755 19.57±0.05 18.76±0.04 17.95±0.03 17.27±0.02 16.73±0.03 D
o
555 248.9468602 +66.2111799 19.92±0.05 19.19±0.05 18.44±0.03 17.69±0.03 17.14±0.03 w
n
201 249.0162635 +66.2230941 19.61±0.05 18.83±0.04 18.09±0.03 17.39±0.02 16.89±0.03 lo
a
2 248.9453711 +66.2204800 19.89±0.06 19.10±0.05 18.35±0.03 17.65±0.03 17.12±0.03 de
255069 224488..99863032212262 ++6666..12906580612416 1290..8290±±00..0056 1199..2404±±00..0055 1188..4463±±00..0033 1177..8703±±00..0033 1177..2206±±00..0044 d from
349 248.9959951 +66.2142513 20.00±0.06 19.23±0.05 18.54±0.03 17.77±0.03 17.27±0.04 h
152760 224498..09265330610996 ++6666..22112964825146 1290..9101±±00..0066 1199..1325±±00..0055 1188..4612±±00..0034 1177..7933±±00..0033 1177..2461±±00..0044 ttp://m
n
523 248.9871643 +66.2387374 – – – – 17.39±0.04 ra
s
604 248.9528976 +66.2162360 19.41±0.04 18.95±0.04 18.35±0.03 17.94±0.03 17.49±0.04 .o
x
599 248.9437771 +66.2056763 20.23±0.07 19.40±0.05 18.65±0.04 18.00±0.03 17.46±0.04 fo
658 248.9415886 +66.2137478 20.29±0.07 19.53±0.06 18.80±0.04 18.11±0.04 17.59±0.04 rdjo
337 248.9878980 +66.2037120 20.53±0.08 19.76±0.06 18.97±0.04 18.35±0.04 17.64±0.05 urn
596 248.9468067 +66.2083220 20.68±0.08 19.83±0.07 19.07±0.05 18.37±0.04 17.77±0.05 als
462 248.9709853 +66.2100529 20.30±0.07 19.49±0.06 18.85±0.04 18.21±0.04 17.86±0.05 .o
471 248.9564109 +66.1951060 20.64±0.08 19.83±0.07 19.03±0.04 18.46±0.04 17.86±0.05 arg/
302 248.9999910 +66.2116528 20.68±0.08 19.91±0.07 19.18±0.05 18.53±0.04 17.79±0.05 t U
388 248.9849322 +66.2108143 20.41±0.07 19.74±0.06 19.08±0.05 18.48±0.04 17.97±0.05 N
449 248.9708088 +66.2082924 20.62±0.56 19.85±0.10 19.05±0.04 18.39±0.04 18.01±0.05 IVE
72 249.0309189 +66.1939739 – – – – 17.89±0.05 RS
408 248.9916711 +66.2220150 20.62±0.08 19.91±0.07 19.19±0.05 – 17.95±0.05 ID
A
95 249.0389514 +66.2155899 20.96±0.09 20.24±0.08 19.53±0.06 18.87±0.05 18.33±0.06 D
798 249.0240772 +66.1820752 – – – – 18.41±0.07 C
O
511 248.9579508 +66.2047618 20.35±0.11 19.72±0.10 19.25±0.06 18.68±0.06 18.51±0.07 M
493 248.9754372 +66.2174786 21.44±0.12 20.74±0.10 19.99±0.07 19.33±0.07 18.64±0.07 PL
643 248.9401830 +66.2084529 21.45±0.12 20.67±0.10 19.97±0.07 19.29±0.06 18.59±0.07 UT
329 249.0093949 +66.2250084 21.19±0.11 20.49±0.09 19.69±0.06 19.00±0.06 18.52±0.07 EN
549 248.9519752 +66.2020070 21.33±0.11 20.65±0.10 19.94±0.07 19.23±0.06 18.76±0.08 SE
632 248.9484012 +66.2157872 21.47±0.14 20.50±0.10 19.84±0.06 19.21±0.06 18.89±0.07 D
E
357 248.9817003 +66.2002660 21.85±0.14 20.54±0.09 19.73±0.06 19.05±0.06 18.53±0.07 M
560 248.9510744 +66.2127998 22.45±0.19 21.81±0.17 21.06±0.12 20.49±0.12 20.00±0.10 A
D
196 249.0143811 +66.2200223 21.71±0.13 20.99±0.12 20.26±0.08 19.57±0.07 19.01±0.09 R
384 248.9824979 +66.2061905 21.46±0.12 20.83±0.11 20.17±0.08 19.57±0.07 19.09±0.09 ID
411 248.9954237 +66.2254124 21.75±0.14 20.99±0.12 20.27±0.08 19.56±0.07 18.91±0.08 on
603 248.9619939 +66.2146043 21.83±0.14 21.10±0.12 20.37±0.09 19.73±0.08 19.13±0.09 Ju
n
533 248.9615077 +66.2088133 22.07±0.15 21.39±0.14 20.60±0.10 19.78±0.08 19.30±0.09 e 1
641894 224488..99462274550703 ++6666..22006184049761 2211..9844±±00..1154 2211..2214±±00..1133 2200..5582±±00..0099 1199..9973±±00..0099 1199..5300±±00..1100 4, 20
1
153 249.0133589 +66.2067467 21.83±0.14 21.16±0.13 20.41±0.09 19.79±0.08 19.24±0.10 3
513 248.9540625 +66.2052672 22.31±0.23 21.58±0.15 20.64±0.18 19.64±0.18 19.04±0.09
507 248.9578870 +66.2008693 22.40±0.20 21.68±0.18 21.03±0.12 20.39±0.11 19.84±0.13
431 248.9752130 +66.2068778 20.89±0.09 20.60±0.10 20.27±0.08 19.97±0.09 19.78±0.13
659 248.9460977 +66.2174923 23.69±0.34 22.91±0.29 22.14±0.20 21.46±0.18 20.81±0.17
575 248.9613337 +66.2169292 22.94±0.24 22.05±0.20 21.43±0.15 20.79±0.13 20.36±0.17
574 248.9534579 +66.2081824 23.14±0.32 22.48±0.28 21.74±0.16 21.02±0.14 20.47±0.16
subdividethegalaxiesintheirdifferentHubbletypes,oratleastin Asweindicatedabove,Andredakisetal.(1995)showthatthere
threegroups:purespheroidals(E-type),lenticulars(S0),andspirals isacorrelationbetweentheSe´rsicindexandmorphologicaltype.
(L-type)(e.g.Fasanoetal.2000;Poggiantietal.2001;Smailetal. However,hereweneedtodeterminehowtheseparameterscorrelate
2001;Ziegleretal.2001).Inordertocomparewithpreviousresults forourdataset.Fordoingso,wefollowFasanoetal.(2000)and
it is necessary to determine to which type our galaxies, based on createasetofsimulatedgalaxies.Theirmagnitudes,scalelengths,
theirSe´rsicindices,correspond. position angles and ellipticities are extracted from the previously
(cid:3)C 2007TheAuthors.Journalcompilation(cid:3)C 2007RAS,MNRAS376,125–150
ThegalaxiesinthecoreofAbell2218 131
Table3. Summaryofthemorphologicalanalysis.
ID χ2/ν z850 Re(arcsec) a/b θ ns A
1 0.017 16.10±0.02 12.06±0.05 0.49±0.01 37±4 1.63±0.06 0.093
247 0.023 16.21±0.02 5.18±0.05 0.85±0.01 51±4 1.88±0.06 0.175
169 0.024 17.07±0.02 1.66±0.06 0.49±0.02 89±4 1.48±0.07 0.043
512 0.033 17.20±0.02 1.51±0.06 0.77±0.02 −37±8 2.03±0.07 0.156
554 0.034 17.37±0.02 0.63±0.06 0.80±0.02 −10±5 1.80±0.07 0.321
172 0.016 16.84±0.02 1.64±0.06 0.82±0.01 −46±9 2.95±0.06 0.056
501 0.026 15.92±0.02 0.81±0.05 0.84±0.01 50±5 2.86±0.05 0.045
561 0.020 17.25±0.02 1.52±0.06 0.69±0.02 −81±12 1.47±0.07 0.081
189 0.019 17.14±0.02 2.69±0.06 0.90±0.02 −59±10 3.93±0.07 0.088
103 0.011 16.93±0.02 1.17±0.05 0.94±0.01 −26±5 2.69±0.06 0.042
132 0.018 17.28±0.02 1.20±0.06 0.46±0.02 −6±5 2.78±0.07 0.058 D
o
201 0.027 17.49±0.02 0.89±0.06 0.50±0.02 −61±10 1.91±0.07 0.132 w
n
2 0.013 17.43±0.02 0.96±0.06 0.80±0.02 −16±5 2.63±0.07 0.050 lo
a
256 0.022 17.64±0.02 0.51±0.06 0.85±0.02 75±5 1.74±0.07 0.078 de
530499 00..002186 1177..4968±±00..0022 10..0896±±00..0067 00..6760±±00..0022 −3731±±74 32..1293±±00..0077 00..102797 d from
126 0.033 17.44±0.02 0.68±0.06 0.95±0.02 −83±12 2.00±0.07 0.159 h
557203 00..002108 1187..3728±±00..0032 10..1750±±00..0087 00..3577±±00..0032 −−6838±±1103 11..8714±±00..0087 00..005447 ttp://m
n
73 0.018 17.90±0.02 0.83±0.07 0.81±0.02 38±5 2.02±0.07 0.077 ra
s
604 0.021 18.09±0.03 1.13±0.08 0.44±0.03 84±4 0.72±0.08 0.129 .o
x
599 0.019 17.76±0.02 1.04±0.07 0.91±0.02 20±5 3.94±0.07 0.051 fo
658 0.016 17.87±0.02 1.01±0.07 0.88±0.02 51±6 2.69±0.07 0.081 rdjo
337 0.015 17.96±0.02 0.59±0.08 0.78±0.02 5±4 2.49±0.07 0.050 urn
596 0.015 18.96±0.03 0.65±0.09 0.64±0.05 58±6 2.13±0.08 0.067 als
210 0.041 18.19±0.03 0.47±0.08 0.59±0.03 −21±6 2.03±0.08 0.823 .o
462 0.021 18.21±0.03 1.35±0.08 0.84±0.03 −21±7 2.35±0.08 0.102 arg/
471 0.015 18.12±0.03 0.97±0.08 0.89±0.03 3±5 2.11±0.08 0.118 t U
302 0.014 18.14±0.03 0.48±0.08 0.93±0.03 5±6 2.01±0.08 0.048 N
388 0.015 18.25±0.03 0.71±0.08 0.76±0.03 45±5 1.68±0.08 0.065 IVE
449 0.021 18.57±0.03 1.60±0.08 0.87±0.04 89±6 1.03±0.08 0.316 RS
72 0.019 18.64±0.03 0.73±0.09 0.44±0.04 72±5 2.13±0.08 0.057 ID
A
408 0.017 18.59±0.03 0.72±0.08 0.74±0.04 −12±6 1.46±0.08 0.152 D
95 0.015 18.41±0.03 0.63±0.08 0.85±0.03 80±6 1.45±0.08 0.062 C
O
798 0.020 19.02±0.04 0.88±0.10 0.45±0.06 9±5 0.81±0.09 0.068 M
511 0.029 18.74±0.03 0.57±0.09 0.53±0.04 −67±12 2.35±0.08 0.169 PL
493 0.018 18.99±0.03 0.45±0.09 0.72±0.06 −42±9 1.56±0.08 0.058 UT
643 0.014 19.09±0.04 0.45±0.10 0.99±0.06 84±13 1.74±0.09 0.094 EN
329 0.015 18.71±0.03 0.76±0.09 0.77±0.04 41±6 1.46±0.08 0.071 SE
549 0.012 19.04±0.04 0.63±0.10 0.95±0.06 −35±9 1.77±0.09 0.067 D
E
632 0.017 19.03±0.04 0.37±0.10 0.73±0.06 −82±13 1.49±0.09 0.102 M
357 0.018 18.70±0.03 0.60±0.09 0.67±0.04 −9±6 1.07±0.08 0.184 A
D
560 0.049 18.39±0.03 0.17±0.08 0.87±0.03 −74±12 0.53±0.08 0.469 R
503 0.013 19.33±0.04 0.83±0.10 0.80±0.07 13±6 0.97±0.09 0.133 ID
196 0.016 19.14±0.04 0.51±0.10 0.67±0.06 −8±6 1.38±0.09 0.079 on
384 0.017 19.53±0.06 0.77±0.12 0.43±0.08 −45±10 1.29±0.09 0.067 Ju
n
411 0.010 18.86±0.03 0.39±0.09 0.96±0.04 −81±13 1.88±0.08 0.080 e 1
650333 00..001125 1198..5897±±00..0063 00..4297±±00..1029 00..9964±±00..0084 4635±±99 11..1494±±00..0098 00..005495 4, 20
1
619 0.015 19.55±0.06 0.41±0.12 0.74±0.08 5±6 1.29±0.09 0.069 3
484 0.017 19.71±0.06 0.75±0.13 0.51±0.08 36±5 1.18±0.09 0.078
153 0.013 19.69±0.06 0.69±0.13 0.72±0.08 −76±13 1.34±0.09 0.062
513 0.024 20.04±0.08 1.15±0.17 0.50±0.10 39±10 2.31±0.10 0.194
507 0.013 20.27±0.09 0.38±0.18 0.85±0.11 74±10 1.31±0.10 0.061
431 0.014 20.20±0.09 1.10±0.18 0.85±0.11 32±11 0.89±0.10 0.218
803 0.017 21.21±0.16 0.67±0.26 0.86±0.15 64±17 0.40±0.15 0.240
659 0.019 20.66±0.11 0.25±0.22 0.81±0.12 −83±17 1.68±0.12 0.085
575 0.010 20.86±0.09 0.75±0.24 0.85±0.12 −40±13 1.39±0.13 0.119
574 0.011 20.49±0.06 0.27±0.22 0.84±0.11 −89±18 1.15±0.12 0.073
(cid:3)C 2007TheAuthors.Journalcompilation(cid:3)C 2007RAS,MNRAS376,125–150
132 S.F.Sa´nchezetal.
describedmorphologicalcatalogueof241galaxies.Foreachentry nationsfrombothextremesubgroups.Anothersourceofcriticism
inthemorphologicalcatalogue20galaxiesweresimulated,eachone abouttheadoptedschemeisthefactthatsomedwarfellipticalgalax-
withadifferentbulge-to-discratio,rangingrandomlyfrom0.01(al- iesshownlowSe´rsicindices(e.g.Barazza,Binggeli&Jerjen2003).
mostapuredisc)to1000(almostapurespheroidal).Eachgalaxy Althoughingalaxysurveyswithoutmorphologicalpre-selections
image was created using GALFIT, with a two components Se´rsic mostofthegalaxieswithlowSe´rsicindicesarespiralgalaxiesAn-
model,onewithn =4forthebulgeandanotherwithn =1for dredakisetal.(1995),thecontaminationfromdwarfellipticalscould
s s
thedisc,convolvingthemwiththesamePSFusedinthemorpho- beapotentialprobleminthestudyofgalaxyclusters.Toaddress
logicalanalysis.Finally,noisewasaddedfollowingtheprescription thisproblemweperformavisualinspectionofthe29galaxieswith
presentedinCoeetal.(2006)andSa´nchezetal.(2004a)forsim- n <1.75listedinTable2and3(i.e.theonesclassifiedasspirals)
s
ulatingACSimages,toresembleasmuchaspossibletheoriginal usingthepreviousdescribedHST/ACSimages.Onlytwoofthese29
F850LP-bandimage. galaxies(∼7percent)couldbeclassifiedasdwarfspheroidalsbased
Thesamemorphologicalanalysisdescribedintheprevioussec- onthisvisualinspection.However,bothofthemarenearbybrighter
tionwasperformedforeachofthesimulatedgalaxyimages,fitting galaxies,andtheirlowsurfacebrightnessareas(likediscstructures)
D
themtoasinglecomponentSe´rsicmodel.Finally,weendupwith arenotclearlydistinguished.Inanycase,thepossiblecontamina- o
w
∼5000simulationscomprisingtherecoveredSe´rsicindexforacer- tionbydwarfspheroidalgalaxiesisexpectedtobemarginal.The nlo
tainbulge-to-discratio.Simien&deVaucouleurs(1986)showthe remaininggalaxiesshowclearevidenceofdiscstructures.Taking ad
e
ttyhpeirceavlibseudlgHe-utob-bdliescserqautieonscfeo.rItnhepadritfifceurleanrt,fthameyilpiersesoefngtathlaeximeseainn isncthoeamcecoisunvtatlhidesweictahvineatthsewceocnotenxstidoefrtthhiaststthuedayd.optedclassification d from
fractionalluminosityofthespheroidalcomponentofthegalaxies h
ttp
anditscorrespondingmagnitudedifferencefor eachgalaxytype. 3.4 Spectraextraction ://m
Fig.4showstheSe´rsicindicesofthesimulatedgalaxiesasafunc- n
tionofthesemagnitudedifferences((cid:5)mag).Thedifferencesfor To deblend and extract the integrated spectra of each individual ras
.o
various families of galaxies along the Hubble sequence are indi- galaxyinthefieldweusedatechniquefor3Dcrowdedfieldspec- x
fo
cated (extracted from Simien & de Vaucouleurs 1986). Based on troscopy developed by ourselves (Sa´nchez et al. 2004b; Garc´ıa- rd
this figure we adopted the following scheme to morphologically Lorenzo et al. 2005; Sa´nchez et al. 2006a). The technique is an jou
acrlaescsoifnysgidaelarexdiesspbhaesreodidoanlsth(Ee-Stye´rpsei)c,ignadliacxeise:sgwaliathxi1e.s7w5i<thnnss<>22..55 eFIxTt3eDns.iIotnctoonIsFisStsooffGaALdFeITbl(ePnedninggetoaflt.h2e00sp2e),cttrhuamtwoefneaamchedobGjAeLc-t rnals.org
areconsideredintermediatetype(mostlyS0’s,butstronglycontam- in the data cube by fitting of analytical models. IFS data may be a/
inatedbyotherfamiliesofbulge-dominatedspirals,likeSaandSb), understoodasasetofadjacentnarrow-bandimages,withthewidth t U
andgalaxieswithns<1.75areconsideredspirals(L-type). ofthespectralpixel.Foreachnarrow-bandimageitispossibleto NIV
This classification scheme is based on the fair correlation be- applymodellingtechniquesdevelopedfor2Dimaging,likeGALFIT, E
R
tweenthebulge-to-totalluminosityratioandtheHubbletypeofa andextractthemorphologicalandfluxinformationforeachobject S
ID
galaxySimien&deVaucouleurs(1986).However,wemayadmit inthefieldateachwavelength.Thespectraofalltheobjectsareex- A
D
that some degree of contamination is expected among the differ- tractedafterrepeatingthetechniquethroughthedatacube.Wehave C
entmorphologicalsubgroupsduetothedispersioninthiscorrela- already shown that the use of additional information to constrain O
M
tion.Althoughwecanclearlydistinguishbetweenpurespheroidals the morphological parameters increases the quality of the recov- P
L
(E-type)andspirals(L-type),thesubgroupofintermediatetypeis eredspectra(Sa´nchezetal.2006a).Withthatpurposeweusedthe U
T
mostprobablylessaccuratelydefined,withconsiderablecontami- morphological parameters previously derived from the HST/ACS E
N
F850LP-bandimage. SE
Wecreatea2Dimagebyco-addingthefluxthroughtheuseful D
E
wavelengthrange(4650–8000Å)coveredbyourdatacube.After M
A
this,wecross-checkedbyeyethisimage,lookingforthosegalaxies D
R
detectedintheF850LPbandforwhichwehadperformedamor- ID
phologicalanalysis.Wedetect41objects(40galaxiesandonestar) o
n
in the collapsed 2D image, out to z < 23 mag. The gravitational J
u
n
arclenseswereexcludedsincetheirstudyisoutthescopeofthe e
1
currentwork.Fig.5showsagrey-scaleoftheco-added2Dimage 4
, 2
derivedfromthedatacube,togetherwithanidentificationlabelat 0
1
thelocationofeachofthedetectedobjects. 3
Aswementionedbefore,weusethepreviouslyderivedmorpho-
logicalparametersforeachgalaxytoextractitsintegratedspectrum
bymodellingeachgalaxyinthedatacubewithGALFIT3D.Weuse
thesamemodelthatwasusedtofittheF850LP-bandimage,withall
themorphologicalparametersfixed,andfittingonlythefluxateach
wavelength.Foreachwavelength,wecreatedaPSFusingpostage
stampimagesoftheonlyfield-starinthefieldofviewofourIFSdata.
Asinthepreviousmorphologicalanalysis,the3Dfitwasperformed
inasequentialway.Wefirstextractedthespectrumofthebrightest
galaxy, the central cD, masking the neighbouring galaxies. Once
Figure4. Resultsfromthesimulations.SoliddotsshowtherecoveredSe´rsic
indicesofthesimulatedgalaxiesasafunctionofthemagnitudedifference extracted, its 3D model was subtracted to the original data cube,
betweenthebulgeandtotalcomponents((cid:5)mag).Overplottedcontoursshow andthespectrumofthesecondbrightestgalaxywasextracted.The
thedensitydistributionoftheplottedpoints. processcontinuediterativelyuntilthefaintestgalaxyspectrumwas
(cid:3)C 2007TheAuthors.Journalcompilation(cid:3)C 2007RAS,MNRAS376,125–150
ThegalaxiesinthecoreofAbell2218 133
& Charlot (2003). We will describe later in detail which models
wereused.Wemanuallytunedtheredshiftofthemodelmatching
thedifferentabsorptionfeatures,includingtheBalmerlines(Hβ,
Hγ, Hδ), the different Fe and Mg lines covered by our spectral
range,aswellasspectralfeatures,likethe4000-Åbreak.Dueto
thespectralresolutionofourIFSdata,andthesignal-to-noiseratio
of our targets, we do not expect to have an accuracy better than
0.001intheredshiftdetermination.Wederivedrealisableredshifts
for31galaxies,28ofthemattheredshiftofthecluster(z=0.17
±0.2),oneisaforegroundgalaxy(z=0.104),andothertwoare
backgroundgalaxies(bothatz∼0.426).Table4liststhederived
redshiftstogetherwiththeidentificationsshowninFig.5(thatwe
willusethroughoutthispaper)andtheSEXTRACTORidentification D
listedinTable3toallowfordirectcomparison. o
w
11 of the 28 galaxies were studied before by Le Borgne et al. nlo
(1992).Theredshiftsderivedfromourdataarecoincidentwiththe ad
e
rdeepriovretendeiwntrheidssahritfitcslefowrit1h7ingtahlaexeixeps,ecatleldofertrhoerms(σcozm∼p0at.0ib0l1e).wWithe d from
theredshiftofthecluster.Thesenewgalaxieshavebeenincluded h
ttp
in Table 3. In total, the final sample of cluster members (28) is ://m
comparablewiththatoneofprevioussimilarstudies,butwiththe n
Figure5. Grey-scalerepresentationofthe2Dimageobtainedbyco-adding advantagethatnosamplehasbeenselected:oursampleiscomplete ras
thefluxfromthedatacubethroughallthecoveredwavelengthrange(4650– .o
withinthefieldofviewofourIFSdatadowntoourdetectionlimit. x
8000Å).Thecirclesindicatethelocationofthedetectedgalaxiesinthedata fo
cube,togetherwithanumberindicatingthelabelcodeusedtoidentifythem. Oqufatlhiteysespgeaclatrxoiescs,oopnylytosidxehriavveetbheeenprporpeevritoieusssotfudthieedirwstiethllaernopuogph- rdjou
ulations(Ziegleretal.2001).Therefore,oursamplewillincrease rna
ls
extracted.TheindividualspectraareshowninAppendixA.Allof significantlyourknowledgeofthecoreofthecluster. .o
rg
themarebackgroundlimited.Thus,thesignal-to-noiseratiocanbe a/
estimatedfromthebackground-noiselevel.Weusedthevariations t U
N
intheskyspectrumineachsky-fibretodeterminethelevelofback- 3.6 TheCMdiagram IV
groundnoise.Inthisway,weestimatedthatthesignal-to-noiseratio E
oftheextractedspectrarangebetween∼1000and∼5perspectral Fig.6showstherest-frameB−VversusMV CMdiagramforthose RS
pixelforthebrightestandfaintesttargets.Thesignal-to-noiseratio clustermemberslistedinTable2,derivedfromtheHST/ACSbased IDA
isaffectedstronglybytheskyemissionlines(seeFig.1),whereit photometricdatadescribedabove.Theabsolutemagnitudeswere D C
dropstolessthan∼3σ evenforthebrightesttargets,respectively. corrected for cosmological effects and we applied a k-correction O
M
Theeffectismoresevereinthewavelengthrangeλ>7200Åwhere based on the same SSP synthetic model. For each synthetic SSP P
manyskyemissionlineshappentobeblended. model (described below) we derived the expected B − V colours LU
T
Theprocedurereliesontheassumptionthatthemorphologicalpa- attheaverageredshiftoftheclusterandintherestframe,deriving E
N
rametersdonotchangesubstantiallyasafunctionofthewavelength, atthesametimetheV-bandk-correction.Anaveragerelationbe- SE
somethingwhichisnotcompletelytrue,sincecolourgradientsare tweentheobservedandrest-framecoloursandthek-correctionwas D
E
expectedinallthegalaxies.But,iftheychange,weexpectthisef- derived,andappliedtothedatashowninFig.6.Redsolidcirclesin- M
fecttohaveaminimimpactinthephotometryderivedwithGALFIT. dicategalaxieswithSe´rsicindexn>2.5(E-type),whilegreenstars AD
Thus,ifweusethemorphologicalparametersderivedbyapplying indicatethosewithn<1.75(L-type).Orangesolidsquaresindicate RID
GALFITtotheF850LPbandandweforcethefittingproceduretouse galaxieswithSe´rsicindicesinbetweenthosevalues,corresponding o
themforderivingthephotometryatanyotherband,therecovered tointermediategalaxies.ThecDgalaxyisindicatedwithanorange n J
u
pflaurxamsheoteurlsdfbreeeblya.siAcaltlhlyouthgehsoaumrepraesviwouhsenexwpeerliemteGnAtLsFsIhTofiwtnthtehsaet oitpwenithcraosdsi,ffbeerienngtsthyembbroiglhsitnescteoibtjiescwteinlltkhneofiwenldt.hWatecpDregfaelratxoielasbdeol ne 14
thisisthecase(e.g.Garc´ıa-Lorenzoetal.2005),wehavealsotested notfollowaSe´rsicprofileofanyindex,showingaflatterandmore , 20
1
thishypothesishere,asdescribedinAppendixB.Inanycase,the extendedlightprofileintheouterregions(e.g.Oemler1976).Open 3
method is not strongly sensitive to this assumption, as described circlesindicatethegalaxieswithinthefieldofviewofourIFS,with
inSa´nchezetal.(2006a).Inprinciple,itispossibletoperformthe redshiftslistedinTable4.Thebigopensquareindicatestheonly
extractionprocedureusingtheparametersderivedfromthemorpho- galaxywithahighasymmetryindex.
logicalanalysisasaninitialguess,andlettingGALFITtoderivethe
correctonesforeachwavelength.Inthiscase,therecoveredspectra
arenoisierandintheresidualdatacubethereisclearevidenceof
3.6.1 Morphologicalsegregation
notcorrectlyextractedstructure.
Toourknowledgethisisthefirsttimethatamorphologicallyseg-
regated CM diagram is shown for confirmed cluster members. In
3.5 Redshiftdetermination
previous works (e.g. Smail et al. 2001) only cluster galaxy can-
We determined the redshift of each detected galaxy in the field didateshavebeenshown.Themorphologies,basedontheSe´rsic
ofviewofourIFSdatabycomparingitsspectrumwithsynthetic indiceslistedinTable3,breakdownas:11E-type,19intermediate
modelsforsinglestellarpopulations(SSP)obtainedfromBruzual and29L-type.IfweconsiderasE-typegalaxiestobealltheobjects
(cid:3)C 2007TheAuthors.Journalcompilation(cid:3)C 2007RAS,MNRAS376,125–150
134 S.F.Sa´nchezetal.
Table4. Summaryoftheparametersderivedfromthespectra.
IFU SEXTRACTOR z Hβ Mgb Age Metallicity Mass
ID ID (Gyr) Z (1010M(cid:4))
1 1 0.174 1.6±0.1 3.0±0.1 9.8±2.8 0.019±0.011 102.1±2.6
2 512 0.179 2.1±0.1 3.4±0.1 12.0±3.4 0.025±0.014 74.6±2.8
3 501 0.163 1.1±0.1 4.2±0.2 12.9±3.6 0.046±0.026 39.5±2.0
4 554 0.175 2.8±0.1 3.6±0.1 12.1±3.4 0.025±0.014 67.9±3.3
5 555 0.181 1.4±0.1 4.1±0.2 11.6±3.2 0.032±0.018 36.2±2.2
6 2 0.181 1.5±0.6 6.1±0.9 12.7±3.6 0.045±0.025 75.7±4.6
7 509 0.184 1.0±0.1 3.0±0.1 9.9±2.8 0.019±0.011 17.2±1.2
8 570 0.184 1.5±0.2 2.3±0.7 12.7±3.6 0.045±0.025 39.9±2.7
9 599 0.163 1.5±0.1 4.4±0.2 9.6±2.7 0.019±0.011 10.5±0.8
11 658 0.172 2.2±0.1 3.9±0.1 9.5±2.7 0.018±0.010 11.5±0.8 D
o
12 596 0.174 1.4±0.1 4.9±0.1 12.4±3.5 0.024±0.013 13.6±1.2 w
n
13 462 0.179 3.2±0.1 2.7±0.2 4.5±1.3 0.018±0.010 7.8±0.6 lo
a
14 449 0.178 2.7±0.1 2.8±0.2 5.2±1.4 0.009±0.005 4.5±2.3 de
1165 654131 00..116672? 11..20±±00..12 02..75±±00..32 99..75±±22..77 00..001091±±00..001011 42..12±±00..53 d from
17 632 0.179 2.8±0.3 3.0±0.2 3.5±1.0 0.009±0.005 2.1±0.3 h
2108 556409 00..118702 2.2±0.2– 03..75±±00..34 59..27±±12..47 00..000199±±00..000151 21..88±±00..43 ttp://m
n
21 533 0.171 1.2±0.3 1.8±0.3 11.5±3.2 0.031±0.018 4.5±0.7 ra
s
22 603 0.171 – 2.7±0.3 5.8±1.6 0.003±0.002 1.0±0.2 .o
x
24 619 0.159 3.1±0.2 2.8±0.5 11.4±3.2 0.006±0.003 1.5±0.2 fo
25 484 0.174 1.2±0.5 0.9±0.3 4.2±1.7 0.007±0.004 1.0±0.2 rdjo
26 513 0.180 3.8±0.4 – 7.1±2.0 0.019±0.011 2.1±0.5 urn
27 659 0.168 – – 12.8±3.6 0.045±0.025 1.8±0.6 als
28 575 0.168 – – 12.7±3.6 0.044±0.025 3.6±0.9 .o
29 574 0.168 2.3±0.7 – 9.7±2.7 0.008±0.004 0.6±0.2 arg/
38 431 0.170 – 1.2±0.4 0.3±0.1 0.007±0.004 0.1±0.1 t U
41 507 0.168 – – 5.8±1.6 0.007±0.004 0.6±0.1 N
IV
E
Uncertainvaluesarewritteninitalic. R
S
ID
A
withn>1.75,theyrepresentonlya∼50percentofthetotal.Forthe D C
completesubsampleofgalaxiescoveredwithintheIFUfieldofview, O
M
themorphologiesbreakdownas:7E-type,8intermediateand13 P
L
L-type.Contrarytopreviousresults(e.g.Smailetal.2001;Ziegler U
T
etal.2001)thisindicatesthattheclusterpopulationisnotdominated E
N
byearly-typegalaxies,andthereisanalmostparitybetweenearly- S
E
andlate-typegalaxies. D
E
Ourapproachtoderivethemorphologiesissubstantiallydiffer- M
enttoinpreviousattempts,basedonvisualclassifications.There AD
isaconsiderabledebateoverthereliabilityofvisualclassifications RID
of early-type galaxies (e.g. Dressler et al. 1997; Andreon 1998; o
n
Fabricant,Franx&vanDokkum2000),andtheabilitytocompare J
u
themwithclassificationsbasedonprofileanalysis,likeconcentra- ne
1
tion and/or Se´rsic indices. However, since there are no previous 4
similar studies, we must compare with the existing ones. Ziegler , 20
1
etal.(2001)showvisualmorphologicalclassificationsof19oftheir 3
48clustermembers,basedonWFPC/HSTimaging.Theirclassifi-
cation was performed by W. Couch, who classified them as 8E,
1E/S0,5S0,3SB/0,1Saand1Sab.TakingtogetheralltheE-type
galaxies(E,E/S0andS0),theyaccountfora∼73percentofthe
Figure6. Rest-frameB−Vcolourdistributionasafunctionofabsolute
totalsample.Smailetal.(2001)presentedmorphologicalclassifi-
mcleasgninitduidceateMgValfaoxriethsowseithclSue´srtesircminedmebxenrs>lis2t.e5d(Ein-tTyapbel)e,w2.hiRleedgrseoelnidstcairrs- cationfor81clustercandidates(33withconfirmedredshifts),based
correspondtoobjectswithn<1.75(L-type).Orangesolidsquaresindi- onthesameanalysisasZiegleretal.(2001).Theirmorphologies
catelenticulargalaxies,withSe´rsicindicesinbetweenbothvalues.ThecD breakdownas:31E,3E/S0,28S0,5Saand14late-typespirals.
galaxyisindicatedwithanorangeopencross,beingthebrightestobjectin TakingtogetheralltheE-typegalaxies,theyaccountfora∼76per
thefield.Opencirclesindicatethe28galaxieswithinthefieldofviewofour centofthetotalsample.Eventakingintoaccountthedifferentmeth-
IFS.Thebigopensquareindicatestheonlygalaxywithahighasymmetry odsusedtoderivethemorphologies,andthedifficultiestocompare
index. them,itisclearthatbothZiegleretal.(2001)andSmailetal.(2001)
foundfewerL-typegalaxiesthanwhatwearefinding.
(cid:3)C 2007TheAuthors.Journalcompilation(cid:3)C 2007RAS,MNRAS376,125–150