Table Of ContentAstronomy&Astrophysicsmanuscriptno.paper (cid:13)cESO2015
January13,2015
Witnessing a merging bullet being stripped
in the galaxy cluster, RXCJ2359.3-6042
GayoungChon1 andHansBöhringer1
Max-Planck-InstitutfürextraterrestrischePhysik,Giessenbachstrasse,GarchingD-85748Germany
e-mail:[email protected]
Receivedxx10,2014;accepted3112,2014
5
1
0 ABSTRACT
2
Wereportthediscoveryofthemergingcluster,RXCJ2359.3-6042,fromtheREFLEXIIclustersurveyandpresentourresultsfromall
n
threedetectorscombinedintheimagingandspectralanalysisoftheXMM-Newtondata.AlsoknownasAbell4067,thisisaunique
a system, where a compact bullet penetrates an extended, low density cluster at redshift z=0.0992 clearly seen from our follow-up
J
XMM-Newtonobservation.Thebulletgoesrightthroughthecentralregionoftheclusterwithoutbeingdisruptedandwecanclearly
0 seehowthebulletcomponentisstrippedofitslayersoutsidethecore.Thereisanindicationofashockheatedregionintheeastof
1 theclusterwithahighertemperature.Thebulktemperatureoftheclusterisabout3.12(±0.13)keVimplyingalowermasssystem.
Spearheading thebulletisacool corecentredbyamassiveearlytypegalaxy.Thetemperaturesandmetallicitiesofafewregions
] inthecluster derivedfromthespectral analysissupportsour conjecture basedonthesurfacebrightness imagethatamuchcolder
A
compactcomponent at1.55(±0.10)keVwithlargemetallicity(0.75Z )penetratesthemaincluster,wherethecoreoftheinfalling
⊙
G component survivedthemergerandleftstrippedgasbehindatthecentreofthemaincluster.Wealsogiveanestimateofthetotal
masswithinr ,whichisabout2×1014M fromthedeprojectedsphericalβmodellingoftheclusteringoodagreementwithother
. 500 ⊙
h massestimatesfromtheM–T andM-σ relations.
X v
p
- Keywords. X-rays:galaxies:clusters–galaxies:clusters:individual(RXCJ2359.3-6042)
o
r
t
s1. Introduction tobeless violentandprobablyasa resultweclearlysee a trail
a
[ of debris behind the bullet which is not observed in the Bullet
Merging galaxy clusters have been found to be interesting Cluster. This can provide interesting information on the strip-
1laboratories for the study of a variety of astrophysical pro- pingprocessactingontheinfallingcomponent.Themassofthe
vcesses(Ferettietal.2002;Markevitch&Vikhlinin2007).They clusterandthe associatedICM temperaturearealso lowerthan
1are also the major process in which galaxy clusters grow and in the Bullet Cluster making temperaturemeasurementseasier.
7havethereforealargeinfluenceontheglobalpropertiesofthese Thesecircumstancesmakethisobjectuniqueasalaboratoryfor
3
systems.Observationallyweseea widerangeofdifferentmor- thestudyofthestrippingprocessasexplainedbelow.
2
0phologies in cluster mergers, which is due to different merger ThroughoutthepaperweadoptaflatΛ-cosmologicalmodel
1.gteerosminectlruiedsinagndditffheerveanrtiemtyasosfrmatoiorsp.hologiesofthemergingclus- wthiethcaΩlcmu=la0t.i3onasndofaaHlludbibstlaenctodnesptaenntdoenftHq0u=a7n0titkiems.s−W1eMupsce−d1Nfor
0 H
Basedonthecompleteflux-limitedclustersurveyREFLEX of 1.41×1020cm−2 for the spectral analysis. All quoted uncer-
5
II in the southern sky (Böhringeretal. 2013) comprising more taintiesrefertoonesigmaerror.
1
than900galaxyclusters,weconstructedavolume-limitedsub-
:
vsample(Böhringeretal.,inprep.),inwhichwestudy108of157
Xiclustersin XMM-Newtonfollow-upobservationsfora system- 2. XMM-Newtonobservations
atic study of galaxy cluster structure statistics (Böhringeretal. Thedigitisedsky-survey(DSS)opticalimageisshownwiththe
r
a2010;Chon&Böhringer2012). XMM-Newton X-ray contoursin Fig. 1, which gives a first im-
In this survey we found a very interesting merging cluster, pression of the geometric configuration of RXCJ2359.3-6042.
RXCJ2359.3-6042, also known as Abell 4067, at a redshift of On top of the extended X-ray emission from the cluster we
0.0992with a specialmergerconfigurationthat hasneverbeen see a smaller, much stronger emission from a source, which
observedinthisformbefore.Inthissystem wewitness theen- coincides with an optical galaxy, near the centre of the image
counteroftwo verydissimilar systems, a smaller compactsys- with an extended tail towards the south-west. This galaxy is at
temwithacoolcomponent,whichpenetratesalarger,moredif- z=0.100(Teagueetal.1990).
fuseclusterwithlowcentraldensityinabullet-likefashion.The We used the XMM-Newton observation of RXCJ2359.3-
factthatacompactbulletmergeswithanextendedclusterwith- 6042withObs.ID0677180601asapartoftheREFLEXIIclus-
outa dense coreresemblesthe case of 1E 0657-558,knownas tersurveyprojecttoinvestigatefurther.Aftertheflarecleaning
theBulletCluster(Markevitchetal.(2004)).Unlikethiscluster, bothMOShave11ksexposurewhilepnisleftwith8ks.Stan-
however,therelativevelocitiesandtheMachnumberassociated darddataanalysiswas performedwith SAS v13.0.1.Our basic
with RXCJ2359.3-6042are much lower, and so the interaction X-rayimagingdatareductionfollowscloselytheproceduresout-
processesarequitedifferent.Inparticular,theinteractionseems linedinChonetal.(2012),andwebrieflysummarisethemhere.
Articlenumber,page1of5
A&Aproofs:manuscriptno.paper
thevaluesofallbackgroundcomponentswerefixedtothepre-
determined values from the background region except for the
normalisations,whicharescaledappropriatelyaccordingtothe
areas.Thereareknowncross-calibrationissuesamongtheEPIC
cameras(Readetal.2014);however,thisislessofaproblemfor
coolerclusters(seee.g.Schellenbergeretal.(2014)).Hence,in
the final fitting the temperature, metallicity, and normalisation
oftheclusterwerekeptfree,andallthreedetectorswerelinked
apartfromthenormalisation.
3. Interpretationofthedata
3.1.Imageanalysis
Figure2 shows a combinedflux image of three XMM-Newton
detectorsof the cluster in the [0.5-2]keV band, smoothedwith
4”kernel.
Fig. 1. RXCJ2359.3-6042 shown in the DSS optical image overlaid
with the XMM-Newton X-ray contours in the [0.5-2]keV band from
thesamedataasinFig.2.Thecontoursareobtainedfromabackground
subtractedimagewherethefirstcontourtraces3×10−5countss−1pix−1.
The flux of the contours increases by steps of 1.5 subsequently. The
compactcomponent withhighX-rayemissioniscentredatthegalaxy
at23h59m04.2sinR.A.and-60d36m34sinDec.
TheXMM-Newtonobservationforallthreedetectorswereflare-
cleanedandout-of-time(oot)eventswerestatisticallysubtracted
fromthepndata.Pointsourceswereremoved,andthemodelled
backgroundwas subtractedbased on the blank sky observation
providedbyRead&Ponman(2003). The cleanedimagesfrom
allthreedetectorswerecombinedwithanexposurecorrection.
ForthespectralanalysisweusedFilterWheelClosed(FWC)
data to remove the contribution from the particle background.
TheFWCspectrumwasscaledaccordingtotheratioofthecor-
ner eventsbetweenthe observationand FWC data. We also re-
Fig.2. XMM-NewtonfluximageofRXCJ2359.3-6042.Allthreede-
movedtheooteventsoftheFWC-pndatabeforescaling,which
tectorswerecombinedaftertheexposurecorrectioninthe[0.5-2.0]keV
mostly affects the very low energy range∗. The region outside band.Ascaleofthreearcminisshownwithabar.
r † of the cluster within the field of view was taken as a
500
background from which the contributions from the X-ray cos-
Themainclustershowsaveryshallow,extendedX-raysur-
mic backgroundtogether with the residualparticle background
face brightnessdistribution. It appears as a dynamically young
were determined.We consideredan unabsorbedthermalmodel
system which has not virialised, and has not formed a dense
for the Local Hot Bubble, unresolved point sources described
core.Alternatively,onecanattributetheshallowsurfacebright-
by an absorbed power law, and a cool absorbed thermal halo.
nessdistributiontothe disturbancecausedbythemergeritself,
Theunabsorbedthermalcomponentisbestfittedwith0.09keV
butinanycasethemainclustercouldnothavebeenaveryregu-
while the absorbedthermalmodelwith 0.29keV. In both cases
larclusterwithadensecorebeforethemerger,aswewillargue
themetallicityissettounityandtheredshifttozero.Thepower
below. Embeddedin this low surface brightnesscluster we see
lawdescribingunresolvedpointsourcesisbestfittedwithanin-
asmall,brightcompactcomponentwithatrailofpatchyX-ray
dexof1.41.Theresidualparticlebackgroundwasmodelledwith
emissionbehind.Weextractedaradialsurfacebrightnessprofile
a power law, where the best fit indices for MOS1, MOS2 and
ofthiscomponent,showninFig.3asasolidline.Thisprofileis
pnare0.1,0.41,and0.46.Thespectralregionswerethenfitted
calculatedonlyfromtheleadingedge,whichisalmostidentical
witha thermalcomponentrepresentingthe clusteremissionto-
tothatoftheentireregionofthecompactcomponent.Thiscon-
gether with the backgroundcomponentsdescribedaboveusing
firmsthattheextendedemissiondoesnotresultfromthedebris
theAPECmodelinXSPEC.Giventhelimitedphotonstatistics
trailingfrom the compactcomponent.The centreof the profile
istakenatthemaximumoftheX-rayemission,whichcoincides
∗Newly added FWC oot pn data are found at
with the optical galaxy described in Sect. 2. Clearly it shows
http://xmm2.esac.esa.int/external/xmm_sw_cal/background/filter_closed/pn/FF/pn_FF_2014_v1.shtml
†Theradiuswithinwhichthemeanclustermassdensityis500times extended emission, which can be compared to the radial pro-
thecriticaldensityoftheUniverse.Tocalculater forRXCJ2359.3- files of typicalpoint sourcesin the same observationshown as
500
6042weusedtherecipesasdescribedinBöhringeretal.(2013). dashedanddash-dottedlines.Themostobviousinterpretationof
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Chon&Böhringer:DiscoveryofamergingbulletinRXCJ2359.3-6042
Fig. 3. Profiles of the cool and extended component penetrating the
cluster (solid) in the [0.5-2.0]keV band in comparison to two point
sourcesinthesameobservation.Onepointsourceislocatedatasimilar
off-axisangle(dash-dotted)andtheotherpointsourceisatabouttwice
theoff-axisangle(dashed)asthecoolcomponent.
Fig.4. Definedregionsforthespectralanalysis.Thecentral0.7×r
Fig.2isthenthatacompactcoolcoreclusterhasfallenintothe 500
is marked by a green circle centred at the yellow cross along with
diffuse, extended main system, and the trail of gas seen in the
three boxes that divide the main cluster. The centre of the cluster is
middle of the picture is a mixture of the gas stripped from the at 23h58m52.3s in R.A. and -60d37m59s in Dec. The cool component
infallingbulletandmaincluster.Inspectionoftheopticalimage (CC)ismarked byaredcirclecentred atthehighest X-rayemission.
inFig.1showsthatthebrightdominantgalaxyisstillatthecen- Thesuspectedshockedregionsareenclosedbytwogreenboxes.
treoftheinfallingcompactcluster.Withthe18galaxyredshifts
available in the NED‡ within the r of RXCJ2359.3-6042we
500
inferthe approximatedirectionofthe infall. Fora cluster mass thecontactdiscontinuitytobequitehot.Anobviousdifference
of2×1014M ,aswillbeestimatedinSect.4,theexpectedinfall totheBulletClusteristhatwehaveaveryshallowopeningan-
⊙
velocityisabout1310kms−1.Theline-of-sightrelativevelocity gleoftheconeofthecontactdiscontinuitywhichisexpectedto
difference between the infalling bright core with respect to the followtheshapeofthepossibleMachcone.Thisimpliesavery
cluster redshiftis about430kms−1. This implies that the infall mild shock. The possible shock wave still needs to be located
directionisclosetotheplaneoftheskywithinabout20degrees. and needs to be better quantified together with the cold front
Thisconfigurationallowsarelativelysimpleinterpretationofthe withbetterdatainthefuture.
mergingprocesslimitingtheambiguityofprojectioneffects.
Theaforementionedfeaturesrevealaparticularcasethatthe
3.2.Spectralanalysis
“bullet”hasgonethroughthecentreofthecluster,andstillhas
preservedits core.Inthiscasewe expectthemetallicityofthis
Inthissectionwewillstudythetemperatureandmetallicitydis-
coreregionto be large,whichindicatesthatitwas undisturbed
tributioninthecluster,whichwillhelptoverifyourspeculations
for long time (Böhringeretal. 2004). From this we can further
aboutthehistoryofthemergerlaidoutintheprevioussection.
inferthatmainclusterdidnothaveadensecoreitself,otherwise
Figure 4 defines the regions from which we extracted the
thecollisionofthetwocoreswouldhavedisruptedtheinfalling
spectra.TheclusterbulkpropertiesaredeterminedfromtheX-
core.Wecanfurtherdeducethatifthemainclusterdidnothave
rayemissioninsidethelargecircleenclosing0.7×r excluding
acoolcore,thepatchyemissionthatweobserveasatrailofthe 500
thecircularregioncentredattheinfallingcore,whichwasanal-
“bullet” must be the debris of the outer layers of the infalling
ysed separately. Given the extent of the cool component (CC)
system.
found in the surface brightness profile, the CC region was de-
Furtherinspectionoftheimageshowsasharperedgeofthe
fined by a radius of one arcmin. The main part of the cluster
clusterontheeastsidethanonthewesterncounterpart,andfaint
isdividedintothreeboxes;Boxes1and3 areconsideredtobe
emission outsidethe eastedge.Analogousto the bulletcluster,
lessaffectedbytheinfallingcomponentthanBox2.Wealsoex-
weexpectthesharpedge(approximatelyboundedbythewest-
cluded the CC from Box 2 when extracting the spectrum. The
ern sides of the green rectangular regions in Fig. 4) to be the
suspected shock-heated regions are marked by two green rect-
contactdiscontinuityofthebulletmaterial.Inthiscasewewould
angles.Thecorrespondingbestfittemperaturesandmetalabun-
expecttoseeashockfurthertotheeastofthisedge,butbecause
dancesfortheseregionsaregiveninTable1.
ofthelowsurfacebrightnessandpoorphotonstatisticswecan-
Thetemperaturemeasuredwithin0.7×r is3.12keVwith
notdetectasurfacebrightnessjumpthatwouldrevealtheloca- 500
a metallicity of 0.17Z without the CC. The bright, compact
tionofthisshock.Nevertheless,asanotherconsequenceofsuch ⊙
CC is foundto be atthe lowest temperaturein the system with
ashock,wewouldexpecttheshockheatedregiontotheeastof
thelargestmetalabundance.Thisnicelyconfirmsourconjecture
‡The NASA/IPAC Extragalactic Database (NED) is operated by aboutthisregionbeinganoldcoolcore.Thenextcoldestcom-
the Jet Propulsion Laboratory, California Institute of Technology, un- ponentisfoundinBox2,whichweassumedtobethedebrisof
dercontractwiththeNationalAeronauticsandSpaceAdministration. the infalling system mixed with the main cluster. Boxes 1 and
Articlenumber,page3of5
A&Aproofs:manuscriptno.paper
region T(keV) Z⊙ cool core clusters (see Prattetal. (2010)). It is less ex-
CC 1.55±0.10 0.75±0.19 treme than that of Coma with its central entropy value
cluster(-CC) 3.12±0.13 0.17±0.04 of 535 keVcm−2 (Fusco-Femianoetal. 2013), and that of
Box1 3.54±0.26 0.18±0.09 the low surface brightness cluster A76 with a value of
Box2(-CC) 3.08±0.12 0.27±0.08 400keVcm−2 (Otaetal.2013),butitshowsthattheproperties
Box3 3.51±0.22 0.18±0.07 ofthemainclusterisfarfromthestagewhereitcouldhavede-
shock(N+S) 4.5±0.59 0.35±0.22 velopedacoolcore,inagreementwithourconclusionabove.
Table1. Best fit temperature and metallicity of the spectral areas de-
finedinFig.4with1σerrors.
4.1.Dynamicalmassestimate
An estimate of the dynamicalmass of a cluster can be inferred
frommeasurementsofspectroscopicredshiftsofmembergalax-
3containrelativelyhotgasincomparison.Inparticular,wefind
ies.Inthepresenceofalargenumberofredshiftsofahundredor
theregionmarkedingreenrectanglesontheeasternfront,which
moreadetailedanalysisoftheredshiftdistributionisappropriate
we assume to be shock-heated, to be the hottest region in the
asoutlinedbyBeersetal.(1990),forexample.Forsmallerdata
cluster.Thisisanalogoustotheshockheatedregioninthebul-
setswithatleasttenredshiftsascalingrelationofvelocitydis-
letcluster,buteverythingisscaledtolowertemperature.Hence,
persionandclustermassasprovidedforexamplebyBivianoet
thetemperatureandmetallicitydistributionssupportourconjec-
al.(2006)isconsideredareasonableandrobustapproach.This
tureaboutthemergingprocesslaid outin theprevioussection.
calibration of the scaling relation is based on the study of 64
Currently,wearelimitedbythephotonstatisticsandcanneither
galaxyclustersincosmologicalN-bodysimulations.Theycon-
pin-pointtheexactlocationoftheshockedregionnorthecontact
siderthatmassestimatesarereliableforasamplethatinvolves
discontinuitywiththespectralanalysis.
at least ten galaxy redshifts within the virial radius. There are
19 spectroscopic redshifts available in the NED database for
4. Modellingthemainclustercomponent RXCJ2359.3-6042withinr500,withwhichwecalculatedtheve-
locity dispersion based on Harrison (1974) and Bivianoetal.
To obtain more insight into the properties of the main cluster (2006). We applied a three-sigmaclipping,which removesone
component,weuseasphericalapproximationtotheshapeofthe galaxy, to obtain the final dispersion. We followed the recipes
cluster,excisingacircularareaaroundtheCCandpointsources. givenin Bivianoetal. (2006) to calculatea mass estimate con-
We fit a β-model with a constant background to the surface sidering their Eq. 2 and Fig. 4, and obtain a mass estimate
brightness distribution of the cluster whose centre, marked by of 2.2×1014M for the one-dimensional velocity dispersion of
⊙
ayellowcrossinFig.4,isdeterminedbythecentreshiftmethod 605±87kms−1 andh=0.7.Thequotederroronthevelocitydis-
describedinChonetal.(2012).Thebestfitvalueofβisfound personresultsfrom1000bootstraprealisations,whichweresub-
to be 0.64±0.062, and that of the core radius to be 3.04±0.32 ject to the same procedureas were the data, which results in a
arcmin, both with 1σ error. A variation of ∼10% in the back- 43%errorforthemassestimateincludingtheuncertaintyofthe
groundcorrespondstoachangeintheestimatesoftwoparame- scalingrelationgiveninBivianoetal.(2006).Sincethedynam-
terssimilartotheerrors.Withthesevaluesandtheclusterbulk icalmassestimateisbasedonthedispersionofradialvelocities,
temperaturefoundintheprevioussectionwecanestimatetheto- fairagreementsbetweendifferentmassestimatesofthissystem
talmassofthemainclusterassumingasphericalsymmetryand impliesthatthedynamicaldisturbancecannotbelargeinthera-
isothermalityof the ICM, which allows an analyticaldeprojec- dial direction. It provides another signature that the merger is
tionoftheobservedsurfacebrightness.Wethengetanestimate closetotheplaneofthesky,inagreementwiththeangulartra-
ofthe totalmassof2.1(±0.37)×1014M⊙ forthe cluster temper- jectoryofthebulletcomponentthatwecalculatedearlier.
atureof3.54keV.WeconsiderthetemperaturesinBoxes1and
3of3.54keVabetterrepresentationofthebulktemperatureof
theundisturbedmaincluster,comparedtothetemperaturemea- 5. Summaryandoutlook
surementin Box 2orthe totalclusterregion,sincethe mixture
ofmainclustergaswithcoldergasstrippedawayfromthebullet WepresentedtheanalysisoftheXMM-Newtondataofamerg-
willbiasthesingletemperaturefitlow.Nevertheless,tobracket ingsystem,RXCJ2359.3-6042,whichshowsamainclusterwith
theuncertaintiesinthetemperatureestimate,wealsoestimated alowX-raysurfacebrightnessbeingpenetratedbyacoolercom-
themassoftheclusterfortwotemperaturelimits,3and3.8keV. pactcomponentwhosecoreisstillintact.Ourconjectureabout
For this temperaturerange we find thatthe estimated mass lies themergingconfigurationissupportedbythespectraltempera-
between1.8and2.2×1014M . turesandmetalabundancesfoundindifferentregionsoftheclus-
⊙
It is also interesting to comparethis mass estimate to other ter.Wefindthatthemassofthemainclusterwithouttheinfalling
massprobes.Withtheestimatedclustertemperaturewecanuse componentis about 2×1014M⊙, which is based on three differ-
theM–T scalingrelation,forexamplebyArnaudetal.(2005), entmassestimatesforabulktemperatureof3.5keV.Withinthe
X
toobtaintheclustermass.Usingtheoverdensityδof500weget currentlimitofphotonstatisticswecantentativelyinferthatthe
amassestimateof2(±0.25)×1014M fortheclustertemperature infallingcomponentmusthave induceda mild shock giventhe
⊙
of 3.54keV. For 3.12keV we get a roughly 5.5% lower mass, wideopeningangle.Adeeperobservationwillofferachanceto
henceinbothcaseswehaveagoodagreementwithourhydro- quantifythe processes ongoing in this special object in greater
staticmassestimate.Wewillconsideranotherprobeofmassin detail.Inparticular,itmayallowustoidentifytheexactlocation
thenextsectionwithopticalspectroscopicdata. of the contact discontinuity,to obtain more quantitative details
From the β-model fit to the cool core excised main abouttheshock,andtoreconstructsomeofthepropertiesofthe
cluster image, we determine a central electron density of infallingsystemsuchasalimittothetotalgasmass.
about 2.5×10−3cm−3. This yields a central entropy value
Acknowledgements. WethanktheXMM-Newtonusersupportteam,especially
of 190±14 keVcm−2, which is well in the regime of non- IgnaciodelaCalleandMariaSantos-Lleofortheirsupportwiththecalibration
Articlenumber,page4of5
Chon&Böhringer:DiscoveryofamergingbulletinRXCJ2359.3-6042
issues.Wethanktherefereeforthecomments.Thisworkisbasedonobserva-
tions obtained withXMM-Newton, anESAscience missionwithinstruments
andcontributionsdirectlyfundedbyESAMemberStatesandtheUSA(NASA).
ThisresearchhasmadeuseoftheNASA/IPACExtragalacticDatabase(NED)
whichisoperatedbytheJetPropulsionLaboratory,CaliforniaInstituteofTech-
nology,undercontractwiththeNationalAeronauticsandSpaceAdministration.
WeacknowledgesupportfromtheDfGTransregioProgramTR33,theMunich
Excellence Cluster “Structure and Evolution of the Universe” and Deutsches
ZentrumfürLuft-undRaumfahrt(DLR)withtheprogram50OR1403.
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