Dynamical state and star formation properties of the merging galaxy cluster Abell 3921

20 Pages·2008·1.85 MB·English

A&A430,19–38(2005) Astronomy
DOI:10.1051/0004-6361:20041811 &
(cid:1)c ESO2005 Astrophysics
Dynamical state and star formation properties of the merging
(cid:1),(cid:1)(cid:1)
galaxy cluster Abell 3921
C.Ferrari1,2,C.Benoist1,S.Maurogordato1,A.Cappi3,andE.Slezak1
1 LaboratoireCassiopée,CNRS/UMR6202,ObservatoiredelaCôted’Azur,BP4229,06304NiceCedex4,France
e-mail:[email protected]
2 InstitutfürAstrophysik,Technikerstraße25,6020Innsbruck,Austria
3 INAF,OsservatorioAstronomicodiBologna,viaRanzani1,40127Bologna,Italy
Received6August2004/Accepted31August2004
Abstract.WepresenttheanalysisandresultsofanewVRIphotometricandspectroscopicsurveyofthecentral∼1.8×1.2Mpc2
regionofthegalaxyclusterA3921(z=0.094).Wedetectthepresenceoftwodominantclumpsofgalaxieswithamassratioof
∼5:amainclustercentredontheBrightestClusterGalaxy(BCG)(A3921-A),andanNWsub-cluster(A3921-B)hostingthe
secondbrightestclustergalaxy.Thedistortedmorphologyofthetwosub-clusterssuggeststhattheyareinteracting,whilethe
velocitydistributionof104confirmedclustermembersdoesnotrevealstrongsignaturesofmerging.Byapplyingatwo-body
dynamicalformalismtothetwosub-clustersofA3921,andbycomparingouropticalresultstotheX-rayanalysisofA3921
based on XMM observations (Belsole et al. 2005), we conclude that A3921-B is probably tangentially traversing the main
clusteralongtheSW/NEdirection.Thetwosub-clustersareobservedinthecentralphaseoftheirmergingprocess(±0.3Gyr),
withacollisionaxisnearlyperpendiculartothelineofsight.BasedonthespectralfeaturesofthegalaxiesbelongingtoA3921
weestimatethestarformationpropertiesoftheconfirmedclustermembers.Substantialfractionsofbothemission-line(∼13%)
andpost-star-formingobjects(socalledk+a’s,∼16%)aredetected,comparabletothosemeasuredatintermediateredshifts.Our
analysisrevealsalackofbrightpost-star-formingobjectsinA3921withrespecttohigherredshiftclusters,whilethefraction
ofk+a’sincreasestowardsfaintermagnitudes(M > −20).SimilarresultswereobtainedintheComaclusterbyPoggianti
RAB
et al. (2004), but at still fainter magnitudes, suggesting that the maximum mass of actively star-forming galaxies increases
withredshift(“downsizingeffect”).Thespatialandvelocitydistributionsofk+agalaxiesdonotshowsignificantdifferencesto
thoseofthepassivepopulation,andtothewholecluster.MostoftheseobjectsshowrelativelyredcoloursandmoderateBalmer
absorptionlines,whichsuggestthatstarformationhasceased∼1−1.5Gyrago.Theirpresenceisthereforedifficulttorelate
totheon-goingmergingevent.Wefindthatstar-forminggalaxiesshareneitherthekinematicsnortheprojecteddistributionof
thepassiveclustermembers.Moreover, mostemission-linegalaxiesareconcentratedinA3921-B andintheregionbetween
thetwosub-clusters.Wethereforesuggest thattheongoingmergermayhavetriggeredastar-formationepisodeinatleasta
fractionoftheobservedemission-linegalaxies.
Keywords.galaxies:clusters:general–galaxies:clusters:individual:Abell3921–galaxies:distancesandredshifts–
cosmology:observations
1. Introduction stillformingatthepresentepoch(e.g.Jones&Forman1992;
West etal. 1995; Donnellyet al. 2001),mergingclusterspro-
In the standard cosmological scenario of hierarchical struc-
vide a unique tool to test or analyze the physics of structure
tureformation,boundobjectsformfromthecollapseofinitial
formationandevolution.
density fluctuations that grow under the influence of gravity
throughmergingofsmallerstructuresthathaveformedbefore. Major cluster-cluster collisions are the most energetic
AsopticalandX-raystudiesrevealthatclustersofgalaxiesare eventsthathaveoccurredin the Universesince theBig Bang,
astheyreleasetotalenergiesupto3×1064erg(Sarazin2003),
(cid:1) Based on observations collected at the European Southern
and their effects on all cluster components are far from be-
Observatory,Chile.Proposalnumbers:67.A-0494(A),67.A-0495(A)
ing fully understood. The intra-cluster gas experiences com-
and70.A-0710.
(cid:1)(cid:1) TablesA.1–A.5areonlyavailableinelectronicformattheCDS pression, rarefaction and shock waves (Schindler & Müller
via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) 1993), and shows characteristic features such as cold fronts
orvia (Markevitch et al. 2000). Additionally, as regards galaxy dis-
http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/19 tribution, the velocity dispersion of the cluster members can
Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20041811
20 C.Ferrarietal.:DynamicalstateandstarformationpropertiesofthemerginggalaxyclusterAbell3921
Table1.Imaging:summaryoftheobservations.
Filter ESOid No.ofexp. Totalexp. Seeing Mag.limit(AB)
(s) (arcsec) (5σ,2×FWHM)
V V/89 5 750 1.35 22.5
R Rc/162 5 750 1.15 22.7
I Ic/Iwp 10 1800 1.25 21.5
increase up to a factor of two during the merger event telescope and with WFI at the 2.2 m ESO telescope respec-
(Schindler& Böhringer1993). While the effectson the intra- tively. Section 2 briefly describes the observations, the data
clustermediumandonclusterinternaldynamicshavebeenan- processing technique and the completeness level achieved by
alyzed in some detail, the effects on the galaxies are still de- the new spectroscopicsample.In Sect. 3 cluster membersare
bated. Caldwell et al. (1993) showed that strong Balmer-line identifiedandin Sect. 4the opticalmorphologyof thecluster
absorption galaxies are distributed between the main Coma is studied. In Sect. 5 we perform a kinematical and dynami-
cluster and the merging SW sub-cluster, suggesting that star calanalysisofthecluster.Theopticalmassesofthetwomain
formationistriggeredbythemergingevent.However,various subclustersareestimatedinSect.6,andwesolvethetwo-body
physical mechanisms have been shown to affect the process problem for these two systems. The photometric and spectral
of star formation within clusters. Changes of the tidal gravi- properties of the cluster members are investigated in Sect. 7,
tational field during cluster merging could affect star forma- making it possible to define several subsamples whose spa-
tioningalaxies(e.g.Bekki1999).Theincreasingexternalpres- tial and velocity distributions are analyzed. The main results
surefollowingtheinfallofgalaxiesintothedenseintra-cluster andtheirinterpretationaresummarizedinthefinalSect.8.All
gasmaytriggerstarformation(Dressler&Gunn1983;Evrard numbersareexpressedasafunctionofh ,theHubbleconstant
75
1991),whilegasstrippingingalaxiesduetoram-pressureex- inunitsof75km−1s−1Mpc.We haveusedtheΛCDM model
ertedbytheICMcouldweakenthestarburstphenomenondur- with Ωm = 0.3 and ΩΛ = 0.7, thus 1 arcmin corresponds to
ing cluster-cluster collisions (Fujita et al. 1999). So far it is ∼0.097h −1Mpcinthefollowing.
75
notclearwhichofthesecompetingeffectsisthedominantone,
sinceresultsinfavouroftheformer(e.g.Abrahametal.1996;
Wang et al. 1997; Moss & Whittle 2000; Flores et al. 2000; 2. Thedata
Gavazzietal.2003;Poggiantietal.2004)andofthelatter(e.g.
Tomitaetal.1996;Baloghetal.1997,1998;Baldietal.2001) 2.1.Imaging
wereobtained.
TheopticalobservationsofAbell3921werecarriedoutusing
CombinedopticalandX-raystudieshavebeenparticularly theWideFieldImager(WFI)mountedattheCassegrainfocus
successfulinrevealingthecomplexdynamicsofmergingclus- oftheMPG/ESO2.2mtelescopeatLaSillaobservatory.WFI
ters (e.g. Davis et al. 1995; Lemonon et al. 1997; Roettiger isamosaiccamerawith4×2CCDchipscoveringatotalarea
et al. 1998; Durret et al. 1998; Arnaud et al. 2000; Donnelly of 34×33 arcmin2. To coverthe gapsbetween the eightindi-
etal.2001;Ferrarietal.2003).TheX-raypropertiesofmerg- vidualchipsofthecameraweadoptedastandardditheringse-
ing clusters are currently being investigated through a set of quence.Thefieldcentredonα = 22h49m44s δ = −64◦22m15s
XMM observations (XMM guaranteed time, Sauvageot et al. wasobservedinservicemodeintheV,RandIpassbands.The
2001),togetherwiththepropertiesoftheirgalaxydistribution data,includingphotometriccalibrationimages,wereobtained
through optical observationsby our group, hence providinga betweenAugust20thand23rd,2001.InTable1wesummarize
unique combined analysis. In this paper we will concentrate theobservations.
ontheopticalanalysisofthemergingclusterAbell3921,and The data reduction was performed using the package
ourresultswillbecomparedtoconclusionsobtainedfromthe “alambic” developed by B. Vandame based on tools avail-
X-rayanalysisofXMMdatabyBelsoleetal.(2005). able from the multi-resolution visual model package (MVM)
Abell 3921 is a R = 2, BM II Abell cluster at z = 0.094 by Bijaoui and collaborators (Bijaoui & Rué 1995; Rué &
(Katgert et al. 1998). Previous ROSAT and Ginga observa- Bijaoui 1997). The photometric calibration was obtained us-
tions revealed the presence of a main cluster and a substruc- ing several standard stars from Landolt (1992) over a large
ture with a very perturbed morphology, interpreted as falling range of airmasses, leading to an accuracy of the photometry
onto the main component (Arnaud et al. 1996). Following of∼0.05mag.
XMM-Newton/EPICobservations(Sauvageotetal.2001)con- TheSExtractorsoftwarepackage(Bertin&Arnouts1996)
firmed the presenceof a two-componentstructure.Thismoti- wasusedtoidentifyallsourcesinthefieldaswellastoclassify
vated new optical observations in order to better characterize themandmeasuretheirmagnitudes.Thelatterwerecorrected
the merger scenario of this complex cluster. In this paper we forgalacticabsorptionusingE(B−V)=0.027asderivedfrom
analyze the spectroscopic and photometric galaxy catalogues Schlegeletal.(1998),yieldingA =0.09mag,A =0.07mag
V R
of the central ∼1.8×1.2 Mpc2 region of the cluster, with new and A = 0.05 mag. They were also transformed to the AB
I
data ofmulti-objectspectroscopy(239newspectra)andVRI- systemgivenbythefollowingrelations:V =V−0.01,R =
AB AB
band imaging, obtained with EFOSC2 at the 3.6 m ESO R+0.19,I =I+0.49.
AB
C.Ferrarietal.:DynamicalstateandstarformationpropertiesofthemerginggalaxyclusterAbell3921 21
2.2.Spectroscopy
Inthispaperwepresenttheresultsoftheanalysisofnewspec-
troscopicdataobtainedthroughtwosessionsofobservationsat
theESO3.6mtelescope(2nightsinSeptember2001,2nights
inOctober2002).We usedtheESOFaintObjectandCamera
(EFOSC2) with grism#03 and a punching head of 1.35(cid:4)(cid:4), ob-
taining a spectral resolution of FWHM ∼ 7.5 Å over the
wavelength range 3050–6100 Å. For each frame we made at
leasttwoscienceexposuresinordertoeliminatecosmicrays,
withanintegratedexposuretimeof5400sforbrighterobjects
(R <18)andof7200sforfainterones(R <19).Wemade
AB AB
a standardspectroscopicreductionusingourautomatedpack-
ageformulti-objectspectroscopybasedonthe task“apall”in
IRAF1. Spectra were wavelengthcalibratedusing the helium-
argonlamp spectrataken aftereach scienceexposure.We de-
terminedredshiftsusingthecross-correlationtechnique(Tonry
&Davis1981)implementedinthetask“xcsao”oftheRVSAO
package.Spectraoflate-typestarswereusedasradialvelocity
Fig.1.R-bandmagnitudedistributionsofthegalaxiesofourspectro-
standards.
scopicsample(207objects–dottedline),ofallthegalaxieswithgood
In Table A (available at the CDS) we list the results of
velocitydetermination(122–dashedline),and,amongthem,ofthose
our spectroscopic observations in the following way: Col. 1:
belongingtoA3921(104–shadedarea).
identification number of each target galaxy; Cols. 2 and 3:
rightascensionanddeclination(J2000.0)ofthetargetgalaxy;
Cols. 4–6:theV, Rand I bandmagnitudesin theABsystem;
Cols. 7 and 8: best estimate of the heliocentric redshift (ex-
pressed as cz) and associated error from the cross-correlation
technique (those values have been set to “–2” if the object is
a starandto“–1”ifwe havenoredshiftinformation);Col. 9:
runofobservations;Col.10:aqualityflagfortheredshiftde-
termination:1=good determination(R parameterof Tonry &
Davis ≥3), 2=uncertain determination, 3=no determination;
Cols.11and12:theequivalentwidths(negativeorpositivein
the case of emission or absorption features respectively) for
[OII]λ3727 and Hδ (λ = 4101 Å) lines, measured only for
theclustermembers;Col.13:thespectralclassificationofthe
galaxymembers.
A total of 239 new spectra has been obtained, among
which56arestars, 100aregalaxieswith averygoodredshift
determination and 83 are spectra with R < 3. In the follow-
inganalysis,wealsoconsidertheredshiftsmeasuredinthere-
gionofA3921byotherauthors(ENACScollaboration,Katgert
et al. 1996; Mazure et al. 1996). We compare the values of Fig.2.Completenessofthespectroscopicsampleasafunctionofthe
the redshifts of 11 galaxies obtained from the common sam- R-bandmagnitudeinthecentral18×12.5arcmin2fieldforallgalaxies
pletothosepublished(35)andweobtainameandifferenceof observed (dashed line) and for those that led to a Quality Flag=1
−12.7±75.8kms−1,whichshowsagoodconsistencybetween (solidline).
the two datasets. The final sample includes our 239 redshifts
and24availableredshiftsfromtheENACScatalogue(Katgert
etal.1996;Mazureetal.1996),amongwhich22withquality
andcomparedto the distributionof galaxieswith a goodred-
flag=1followingourcriterion.Itcoversafieldof∼25.5(cid:4)×25(cid:4), shift determination, i.e. quality flag=1 (122 galaxies), and
slightlylargerthanthesizeofourownspectroscopicfollow-up
among them, those belonging to A3921 (104 galaxies, see
(∼18(cid:4)×12.5(cid:4)).
Sect.3).
The R-band magnitude distribution of the whole spectro-
The ratio of the number of galaxies with measured red-
scopicsampleofgalaxiesispresentedinFig.1(207galaxies),
shifttothetotalnumberofgalaxiesdetectedwithinthecentral
1 IRAF is distributed by the National Optical Astronomy field of 18×12.5arcmin2 coveredby ourlast observationsis
Observatories,whichareoperatedbytheAssociationofUniversities plotted in Fig. 2 as a function of the R-band magnitude. The
for Research in Astronomy, Inc., under cooperative agreement with spectroscopic catalogue is complete to better than 50% up to
theNationalScienceFoundation. RAB =18.5(R∗AB+2.1).
22 C.Ferrarietal.:DynamicalstateandstarformationpropertiesofthemerginggalaxyclusterAbell3921
thegalaxycatalogueperformedonfivesuccessivescalesfrom
which the significant structures are recombined into the final
map(followingtheEq.(C7)ofFaddaetal. 1998).Thesesig-
nificant structures are obtained by thresholding each wavelet
plane at a level of three times the variance of the coefficients
ofeachplaneexceptforthetwosmallestscalesforwhichthe
thresholdisincreasedtofourandfivetimesthevarianceinor-
dertoreducefalsedetectionsduetotheverylowmeandensity
ofthePoissonprocessatthesescales(0.01forachosengridof
128×128pixel2).
Such density maps are presented in Fig. 5 for three in-
put catalogues. In the left panel, all galaxies with R < 19
AB
(R∗ +2.6)areused,leadingtoamaprevealingtwodominant
AB
clumpsinthecentralpartofthefield(hereafterA3921-Ainthe
centreandA3921-BtotheNW)inthecentral∼34×34arcmin2.
Inordertoavoidpossibleprojectioneffects,we isolategalax-
ieslikelytobeearlytypesatthesameredshiftonthebasisof
theircolourproperties.Indeed,despitethebimodalstructureof
A3921 one can notice in the two colour magnitude diagrams
Fig.3. Apparent radialvelocity(cz)histogram ofthegalaxiesinthe
centralfieldoftheclusterA3921,withabinningof500kms−1. ofFig.7a welldefinedredsequence,thecharacteristiclinear
structuredefinedbythebulkofearly-typegalaxiesinacluster.
The determination of the slope, intercept and width of these
Amongthe263objectsofourspectroscopicsamplewewill “red sequences” is performed using the technique described
consider in the followingonlythe 122galaxieswith redshifts in Appendix A. The central panel of Fig. 5 shows the result-
determinedwithaqualityflag=1(i.e.ournew100highquality ing red sequence density map (keeping only galaxies at ±1σ
spectraplus22publishedpreviously). around the red sequence), where one can notice that several
small-scale clumps in the cluster core disappear as well as in
its surroundings, leaving the larger-scale structure of the two
3. Clustermembership
mainclumpsunaffected.
Figure 3 shows the radial velocity (cz) distribution of this Inordertogoonestepfurtherinavoidingprojectioneffects
datasetinbinsof500kms−1.Thebulkoftheclusterisconcen- wetakeadvantageofourspectroscopicfollowup.Eventhough
tratedbetween25400and30400kms−1.Fourobjectshavecz it is complete down to a level of 50% at R = 19, most of
AB
higherthan75000,onegalaxyappearstobeintheforeground, theclumpsseenvisuallyhavebeenspectroscopicallysampled
while 13 are located between 31000 and 50000 kms−1 and allowing us to discard possible externalgroups.For example,
most of them (12) are also spatially concentrated in the re- asmentionedintheprevioussection,thebackgroundredshift-
gion around the two brightest objects of the West side of peakgroupmayleadtoanover-densityunrelatedtothecluster
the cluster, BG2 and BG3. If we consider only the 8 galax- in the iso-density map. Therefore,from the sample of red se-
ies around 40000 kms−1, they correspond to a peak located quencegalaxiesweadditionallyexcludegalaxiesknownfrom
at CBI = 40378 kms−1 (z¯ = 0.135) with a velocity disper- spectroscopynottobeclustermembers.Theresultisshownin
sion SBI = 427 kms−1, indicating that they could be a back- the right panelof Fig. 5, which is quite similar to the map in
groundgroup. thecentralpanelexceptthatthelocationofclumpBisshifted
In order to eliminate the galaxies not belonging to the towards the South and its central peak is shifted close to the
cluster, we apply the standard iterative 3σ clipping (Yahil & positionoftheBG2.Ifwecantrustatahighconfidencelevel
Vidal1977).104clustermemberswithczbetween25400and thatthesub-structuresstillvisiblearepartofA3921,itisstill
30400kms−1 areselected.Notethatthebackgroundpeakbe- notclear whetheror not theyare projectioneffects within the
ing located ∼10000 kms−1 behind the mean velocity of the cluster.InFig.6therelativevelocitydistributionsofthesevar-
clusterisexcludedasasubgroupofthecluster.Figure4shows ioussub-structuresarepresentedusingtheavailableredshifts.
thelocationofthe104identifiedmemberssuperimposedona AtthelevelofR = 19theseclumpsactuallyhavefullspec-
AB
fractionoftheR-bandimage. troscopic coverage except for the eastern group with 5 con-
firmedclustermembersoutof7galaxiesbelongingtothered
sequence.Thestrongclusteringinredshiftspaceforallthesub-
4. Colourpropertiesandspatialmorphology
structuresconfirmstheaccuracyofthepicturereflectedbythis
ofA3921
densitymap.
In order to investigate the projected spatial morphology of InFig.8thesamemapsarepresentedforthreemagnitude
A3921 several density maps of the galaxy distribution have cuts. The globalbi-modalstructureof the cluster remainsun-
beenbuiltonthebasisofamulti-scaleapproach.Theadopted changedwithluminosity.Thisisparticularlytrueforthecentral
algorithm is a 2D generalization of the algorithm presented partsofclumpsAandB.However,notethattheeasterngroup,
in Faddaetal. (1998). Itinvolvesa waveletdecompositionof morecompactthanclumpB,isstrongeratfainterlevels.
C.Ferrarietal.:DynamicalstateandstarformationpropertiesofthemerginggalaxyclusterAbell3921 23
N
E
BG2
BG3
BG1
BG4
Fig.4.GalaxiesidentifiedasmembersofA3921(squares),superimposedonafractionoftheR-bandimage(27(cid:4)×20(cid:4)).
N
B E
x xx xx
x xx xx
xx x xxxx x xxxx x
A
Fig.5.Projectedgalaxydensitymaps(withR ≤19)ona34×34arcmin2fieldcentredonA3921.Leftpanel:allgalaxies;centralpanel:the
AB
redsequencegalaxies,andrightpanel:sameascentralpanelbutremovinggalaxiesknownnottobeclustermembersfromspectroscopy.The
whitecrossesshowthepositionsofthefourbrightestgalaxiesindicatedinFig.4.
Theresultsgivenabovearebasedontheonlyredsequence Tosummarize,A3921ischaracterizedbya)abimodalmor-
galaxies.InFig.9thegalaxiesbluerthantheredsequenceare phology; b) the presence of several substructures inside each
presented,superimposedontheredsequenceiso-densitymap. of the two main clumps; c) an offset of the Brightest Cluster
Thesebluegalaxiesappearmuchlessclusteredthantheredder Galaxy(BCG)fromthemaindensitypeakofclumpA,andd)
onesoverthewholefieldandshowonlylittlecorrelationwith an excess of blue galaxiesaround the clump B. These results
the red density peaks except in the case of clump B showing suggest that this system is out of dynamical equilibrium and
an excess of blue galaxies relative to the other clumps and in thatitisprobablycomposedofamainclusterinteractingwith
particular to the center of clump A. This asymmetry is also at least two groups,one to the North-East(clumpB) and one
presentin the distributionof emissionline galaxiesaswill be totheEast,thelatterbeingsignificantlylessluminousthanthe
shownanddiscussedinSect.7.3.1. former.Inthefollowingsectionwewillanalyzethedynamical
24 C.Ferrarietal.:DynamicalstateandstarformationpropertiesofthemerginggalaxyclusterAbell3921
xx
xx
xxxx x
Fig.6.Velocitydistributionsofthemainstructuresoftheprojectedgalaxydensitymap(sameastherightpanelofFig.5).Thedottedlines
inthehistogramsindicatetherelativevelocitiesoftheneighbouringbrightestgalaxy.AtthelevelofR = 19alltheselectedclumpshave
AB
a100%coverageexcepttheeasterngroupwith70%completeness.
propertiesofA3921inordertounderstandwhichphaseofthe deviations from Gaussianity could provide important indica-
mergingprocesswearewitnessing. tionsofongoingdynamicalprocesses.Inthefollowing,weare
therefore interested in measuring a possible departure of the
observedclustervelocitydistributionfromaGaussian.
5. Clusterkinematics
A velocity distribution with slight tails can indicate the
5.1.Velocitydistributionofthewholecluster
presenceoftwoormoreoverlappingGaussiancomponentsin
the whole velocity histogram, while an asymmetric distribu-
Using the biweight estimators for location and scale
tioncanbetheresultofthecontemporarypresenceofsubclus-
(Beers et al. 1990), we find a mean apparent velocity of
C =28047+76kms−1, corresponding to a mean redshift of ters with differentnumbersof galaxies(Ashman et al. 1994).
BI −77
z¯(cid:8)0.0936,andavelocitydispersionofS = 831+100 kms−1 Weusetwokindsofshapeestimators:thetraditionalthirdand
BI −76
(at1σsignificancelevel,seeTable2).Figure10showsthehis- fourthmoments,i.e.skewnessandkurtosis,andtheasymmetry
andtailindices(Bird&Beers1993).InTable2wereportthe
togramofthecosmologicallyandrelativisticallycorrectedve-
locityoffsetsfromthemeanclusterredshift(∆v=c(z−z¯)/(1+ corresponding values and significance levels, estimated from
z¯))ofthe104clustermembers.Contrarytotheprojectedmor- Table2inBird&Beers(1993)underthenullhypothesisofa
Gaussiandistribution.
phologyontheskywedonotfindanybi-modalstructureinthe
velocity distribution. In dissipationless systems, gravitational ThevaluesobtainedforSkewnessandAIshowthattheve-
interactions of cluster galaxies over a relaxation time gener- locityhistogramisquitesymmetric(significancelevel>10%).
ate a Gaussian distribution of their radial velocities; possible Values observed for kurtosis and TI indicate a heavy-tailed
C.Ferrarietal.:DynamicalstateandstarformationpropertiesofthemerginggalaxyclusterAbell3921 25
Fig.7.(V−R) vs.R (left)and(R−I) vs.I (right)colour–magnitudediagrams.Allgalaxieswithin34×34arcmin2 areshown.Big
AB AB AB AB
symbolscorrespondtoconfirmedclustermemberswhereasdotscorrespondtogalaxieswithoutspectroscopicinformation.Trianglescorrespond
toemissionlinegalaxies,squarestok+atype,circlestogalaxiespresentinganH−K inversionfollowingtheclassificationdescribedinthe
text.Thesolidlineisthebestlinearfittotheredsequenceofthecluster,whilethedottedlineisat±1σ .
RS
R < 18 R < 19 R < 20 N
AB AB AB
E
x xx xx
x xx xx
xx x xxxx x xxxx x
Fig.8.Projectedgalaxydensitymaps(usingredsequencegalaxiesandexcludinggalaxiesknownnottobeclustermembersfromspectroscopy,
asintherightpanelofFig.5)ona34×34arcmin2fieldcentredonA3921andforthreemagnitudecuts.Thewhitecrossesindicatethepositions
ofthefourbrightestgalaxiesindicatedinFig.4.
distribution,rejectingtheGaussianhypothesisforthefirst(i.e. ourdataset(Beersetal.1990)atcz = 27381.9kms−1 (shown
kurtosis)at∼10%significancelevel,andforthesecondateven inFig.10)andcharacterizedbyanormalizedsizeof2.47and
betterthan1%.Whilelight-taileddistributionsindicatemulti- a significance of 3%. On the contrary,among the 6 tests per-
modality,heavilypopulatedtailscouldbeduetocontamination formedbyROSTATthatdonotrejecttheGaussianhypothesis,
by non-cluster galaxies. We think that the analysis of Sect. 3 theDIPtest(Hartigan&Hartigan1985)acceptstheunimodal
excludesthispossibility,butweapplyanadditionaltestinor- hypothesis at better than 99% level, in agreement with the
dertodefinitivelyrejectthepresenceofoutliersinourvelocity conclusions obtained with the symmetry tests (i.e. Skewness
sampleof104galaxies.Extensivedataavailableforlow-zclus- andAI).
tersshowthatmost(≥95%)ofthegalaxiesinthecentralregion Classical tests of Gaussianity therefore give controversial
oftheclustershaveradialvelocitieswithin±3500kms−1ofthe resultsforthekinematicalpropertiesofA3921.
meanclusterredshift(Postmanetal.1998).Wethusverifythat
allthe104galaxiesofourfinalsamplehave|∆v|<3500kms−1
(with∆vdefinedatthebeginningofthissection).Inanycase, 5.2.Analysisofapossiblepartitioninredshiftspace
theshapeparametersdonotshowstrongevidenceofsubcom- andkinematicalindicatorsofsubclustering
ponentsinvelocityspace.
Because of the uncertain results obtained in the previous
7ofthe13normalitytestsperformedbyROSTATrejectthe section we apply the KMM mixture-modeling algorithm of
Gaussianhypothesisatbetterthan10%significancelevel(see McLachlan & Basford (1988) which, using a maximum-
Table 3). Moreover, we find one significant weighted gap in likelihoodtechnique,assigns each galaxyto a possible parent
26 C.Ferrarietal.:DynamicalstateandstarformationpropertiesofthemerginggalaxyclusterAbell3921
(Quintana et al. 1996; Muriel et al. 2002). Such integrated
N
measurementsare thereforeperformedwith the 104members
E of A3921 up to a distance of ∼2 h −1 Mpc from the cluster
75
centre(taken to be at the position ofBG1), and are presented
in the left panels of Fig. 11. The mean velocity has a very
constant value (∼28000 kms−1) from the centre of the clus-
tertoitsouteredges.Followingtheclassificationoftheradial
profiles of the velocity dispersions of den Hartog & Katgert
(1996),A3921presentsan“inverted”shape:thecumulativeve-
locity dispersion shows an initial increase with radius up to
∼4 arcmin away from the cluster centre and then decreases
nearly to its first bin value (∼900 kms−1). Finally, the veloc-
itydispersionbecomesflatintheexternalregionsoftheclus-
ter(≥1h −1 Mpc).Thisresultsuggeststhatthisfinalvalueis
75
representative of the total kinetic energy of the cluster mem-
bers(Faddaetal.1996).A similarshapewasdetectedbyden
Hartog&Katgert(1996)andNikogossyanetal.(1999)forthe
galaxyclusterA194,andtheseauthorsinterpretedsuchapro-
Fig.9. Projected galaxy density map (34×34 arcmin2) of the red-
fileasoriginatinginanearlyrelaxedregion.Therefore,neither
sequence galaxies (see Fig. 6). The symbols represent the galaxies
theintegratedradialprofilesofthevelocitydispersionnorthose
withR <20andbluerthantheredsequencegalaxies.
AB
ofthemeanvelocityrevealthepresenceofsignificantvelocity
gradients that could be produced by the presence of internal
population,andevaluatestheimprovementinfittingamultiple- sub-structures. These results are confirmed by the right pan-
componentmodeloverasingle-one.Firstofall,wetrytoallo- elsofFig.11,wheremeanvelocitiesandvelocitydispersions
catethe104galaxiesofourdatasetintotwopossiblesubclus- are estimated in rings containing the same number of galax-
ters,respectivelywithmeanvelocitylowerandhigherthanthe ies(13).Theshapesofthesedifferentialprofilesagreewiththe
gapposition.Severaltestsarethenperformedbychangingboth integrated ones, again presenting an inverted shape. Its mini-
the estimated mean velocities and the estimated mixing pro- mumvelocitydispersioncorrespondstotheringcontainingthe
portionfor each group,butwe always obtainthe same result: group of galaxies located around BG2. Within the errors, the
the KMM algorithmtries to fit a 2-grouppartition fromthese mean velocities are nearly constant around 28000 kms−1, as
guesses, obtaining intermediate confidence levels that cannot fortheintegratedprofile.
rejectthenullhypothesisofunimodaldistribution.Thisresult
actuallyagreesbothwiththeDIPtestandwithwhatisobserved
5.4.VelocitydistributionsofA3921-AandA3921-B
fromtheshapeparametersofthevelocitydistribution,theab-
senceoflight-populatedtailsandofasignificantasymmetryin
Asshownabove,thedynamicalandkinematicalpropertiesof
thehistogramexcludingthepossiblepresenceofseveralover-
the whole cluster do not reveal strong signatures of merging.
lappingsubunitspopulateddifferently.
More indications on the dynamical state of the cluster could
Classical statistical tests for sub-clustering are then ap-
comefromthe analysisof the velocitydistributionof the two
pliedtothe104clustermembersinordertolookfortheexis-
main subclusters detected separately on the iso-densitymaps.
tenceofcorrelatedsubstructuresin velocityandspatialdistri-
Forthis,ourvelocitysampleisdividedintotwodatasets,con-
bution(Dressler&Shectman1988;Bird1994;West&Bothun
taining the confirmed cluster members in the two circles dis-
1990). The results, obtained by the bootstrap technique and
playedinFig.14,chosenasthelargestnotintersecting.Thera-
1000MonteCarlomodels,arepresentedinTable4.
diusofthetwocirclesis(cid:8)0.34Mpcandtheyarecentredonthe
Neither∆norαparametersfindevidenceofsubstructures
densitypeakspreviouslydetected(Sect.4).WeplotinFig.13
withahighsignificancelevel,whilethe(cid:6) testhasaninterme-
theratioofthenumberofgalaxieswithverygoodredshiftde-
diate andinconclusivevalue.Figure12showsgraphicallythe
terminations(QF=1)tothetotalnumberofobjectsdetectedin
resultsobtainedwiththe∆test.
the two circles of Fig. 14 as a functionof R-bandmagnitude;
Inconclusion,whilenobi-modalityisshowninthevelocity
thespectroscopicsamplingofthesetworegions,andinpartic-
distribution,thevarioustestsofGaussianityandsubclustering
ularofA3921-A,appearstobequitegood,withcompleteness
donotreachaconsensus,butdonotshow“extreme”departures
levels≥50%forR ≤19.5((cid:8)R∗ +3.1).
fromGaussianity. AB AB
Figure 15 shows the velocity distributions of the two
datasetsandinTable2wequotetheirvelocitymeansanddis-
5.3.Radialprofileofthevelocitydispersion persions.Bothdatasetsshowameanvelocityveryclosetoeach
otherandtothewholeclustervalue(thevelocityoffset2∆vbe-
The analysis of the mean velocity and particularly of the
tween the mean velocities of A3921-A and A3921-B is only
radial profile of the velocity dispersion provides a useful
tool for investigating the dynamics of galaxy clusters, as
they can reveal signs of subclustering and ongoing merging 2 Asusualcosmologicallyandrelativisticallycorrected.
C.Ferrarietal.:DynamicalstateandstarformationpropertiesofthemerginggalaxyclusterAbell3921 27
Table2.PropertiesoftheczdistributionandsignificancelevelsofshapeestimatorsforvarioussubsamplesofA3921.C andS arethemean
BI BI
velocityandthevelocitydispersionofthedifferentdistributions(biweightestimatorsforlocationandscale,Beersetal.1990).
Subsample N C S Skewness AI Kurtosis TI
gal BI BI
[kms−1] [kms−1] % % % %
Wholesample 104 28047+76 831+100 ≥10 >20 ≤10 ≤1
−77 −76
A3921-A 41 28017+145 1008+156 >10 >20 >20 >20
−173 −106
A3921-B 20 27920+88 451+215 <10 <5 >20 >10
−86 −80
Table3.1-DstatisticaltestsperformedintheROSTATpackagethat
exclude the hypothesis of a single Gaussian distribution for the dif-
ferentvelocitydatasetsconsideredinthepaper.InCols.1and2we
reportthenameandthevalueofthestatistics,whileCol.3indicates
theirsignificancelevels.
Wholecluster
Statisticaltest Value Significance
A 0.735 <1%
B2 3.567 8.8%
I 1.099 <5%
KS 0.901 5.0%
V 1.621 2.5%
W2 0.163 1.6%
U2 0.159 1.2%
A2 0.907 2.1%
A3921-A
Statisticaltest Value Significance
none – –
A3921-B
Fig.10. Top: stripe density plot of the radial velocity offsets of the
StatisticalTest Value Significance
104A3921membersfromz¯(correctedforcosmologicalandrelativis-
A 0.684 <5%
tic effects). The position of the gap detected in the cluster velocity
U 4.822 ∼1%
distribution is indicated by an arrow. Bottom: velocity histogram of
B1 –0.657 7.8%
the confirmed cluster galaxies in bins of 200 kms−1. The Gaussian
B2 4.738 2.1%
best-fittothevelocitydistributionissuperimposed.
B1andB2 6.123 4.7%
Table4.3-Dsubstructureindicatorsforthesampleof104objectswith
qualityflag=1inthevelocityrange25400÷30400kms−1.
89+155 kms−1);clumpAischaracterizedbythehighestveloc- Indicator Value Significance
ity−d17is7persion. ∆ 109.284 0.680
(cid:6) 0.991×10+27kg 0.353
Most(8)ofthe13normalitytestscontainedintheROSTAT α 0.129h −1Mpc 0.914
75
package accept the hypothesis of Gaussianity for the veloc-
ity distributionoftheA3921-Bcomponent,andactuallynone
of them rejects the null hypothesisfor the radial velocities of 6. Dynamicalstudyofthesystem
theclumpA3921-A(Table3).Shapeparameters(Table2)ac-
cept the Gaussian hypothesis at more than 10% significance 6.1.Massestimateofthetwomainclumps
level, with the exception of the skewness and of the kurtosis,
Determiningtherespectivemasses ofA3921-AandA3921-B
which indicates the possible presence of an asymmetric ve-
requires idealized assumptions which cannot be strictly valid
locity distribution (significance level lower than 10%) and of
for an interacting system. However, as seen in Sect. 5.4, the
heavily populated tails (significance level lower than 5%) in
dynamicsofthecentralregionsofthetwoclumpsappeartobe
theA3921-Bvelocityhistogram. relativelyunaffectedbythemergingevent.
Summarizing, Gaussianity tests suggest that the merging We have therefore assumed that each subcluster is virial-
eventhasnotstronglyaffectedtheinternaldynamicsofthetwo ized.Asinthecaseofvelocitydispersions,thevirialradiusof
sub-clusters. eachclumpwasestimatedselectingonlythosegalaxieswithin
28 C.Ferrarietal.:DynamicalstateandstarformationpropertiesofthemerginggalaxyclusterAbell3921
Fig.11. Profileof thevelocitymeans (top) and dispersions(bottom) inthecluster: left:asacumulativefunction of galaxiescentred onthe
positionofBG1–right:differentialvaluesasafunctionofradiusinringswith13galaxies;thelastpointisfarfromtheothersduetothelower
degreeofcompletenessreachedbyspectroscopicdataintheouterregionsofthecluster(beyond∼12arcminfromthecenter).
a projecteddistance of ∼0.34 h −1 Mpc from its center (see InEq.(2)U isthepotentialenergyofthesystemandr isthe
75 h
Fig.14).Inordertohavebetterstatisticsandatthesametime meanharmonicradius,definedas:
 
tgoromunindimcoiznetabmiainsaetsiodnu,ewteoisnpcelcutdreadl ianlclothmepgleatleanxeisessabnedlonbgacinkg- rh = N(N−1)(cid:4)N−1 (cid:4)N 1 −1 (3)
to the red sequence (thus assuming that early-type galaxies 2 i<j j=i+1rij
tracethemassprofile;seeKatgertetal.2004),exceptforthose
where r is the separation between the ith and jth galaxies,
withameasuredredshiftandidentifiedasoutliersonthebasis ij
andN isthetotalnumberofobjectsinthesystem).
ofourpreviousanalysis.
We have to apply the above relations to projected separa-
Themasswascalculatedwiththeclassicalvirialequation:
tions, so that we can estimate the projected mean harmonic
r σ2
M = vir vir (1) radius RH and obtain the corresponding projected virial ra-
vir G dius Rvir. Assuming spherical symmetry√, rvir = (π/2)Rvir (see
where σ is the three-dimensionalvelocity dispersion of the Limber & Mathews 1960) and σ = 3σ , where σ is the
vir vir r r
system,andr isthevirialradius: radialvelocitydispersionofthesystem.
vir
 
rvir ≡−GMUv2ir = N2(cid:4)Ni<−j1j(cid:4)=Ni+1r1ij−1 = N2−N1rh. (2) htoarrm(TsoheeneiaCcbraoarvdlbeiuemrsg:eatenhtoaadllt.eis1rn9ba9at6sive).edWiosneththheaesvope-acaiarpwlpleilsdieerdiensbgtoiwmthiasmteoeerstohtifomtdhase-,