The dynamical status of the galaxy cluster Abell 115

17 Pages·2007·0.83 MB·English

A&A469,861–872(2007) Astronomy
DOI:10.1051/0004-6361:20077407 &
(cid:1)c ESO2007 Astrophysics
(cid:1)
The dynamical status of the galaxy cluster Abell 115
R.Barrena1,W.Boschin2,3,M.Girardi3,4,andM.Spolaor3,5
1 InstitutodeAstrofísicadeCanarias,C/VíaLácteas/n,38200LaLaguna,Tenerife,Spain
e-mail:[email protected]
2 FundaciónGalileoGalilei-INAF,C/AlvarezdeAbreu70,38700SantaCruzdeLaPalma,CanaryIslands,Spain
3 DipartimentodiAstronomiaoftheUniversitàdegliStudidiTrieste,viaTiepolo11,34131Trieste,Italy
4 INAF–OsservatorioAstronomicodiTrieste,viaTiepolo11,34131Trieste,Italy
5 CentreforAstrophysics&Supercomputing,SwinburneUniversity,Hawthorn,VIC3122,Australia
Received5March2007/Accepted19April2007
ABSTRACT
Aims.Wepresent the results of a new spectroscopic and photometric survey of the hot, binary X-ray cluster A115 at z = 0.193,
containingaradiorelic.
Methods.OuranalysisisbasedonnewspectroscopicdataobtainedattheTelescopioNazionaleGalileofor115galaxiesandonnew
photometricdataobtainedattheIsaacNewtonTelescopeinalargefield.Wecombinegalaxyvelocityandpositioninformationto
select85galaxiesrecognizedasclustermembers,determineglobaldynamicalpropertiesanddetectsubstructures.
Results.WefindthatA115appearsasawellisolatedpeakintheredshiftspace,withagloballine-of-sight(LOS)velocitydispersion
σ =1362+126 kms−1.Ouranalysisconfirmsthepresenceoftwostructuresofcluster-typewellrecognizableintheplaneofthesky
v −108
andshowsthattheydifferof∼2000kms−1 intheLOSvelocity.Thenorthern,highvelocitysubcluster(A115N)islikelycentredon
thesecondbrightestcluster galaxy(BCM−A,coincident withradiosource3C28) andthenorthernX-raypeak. Thesouthern, low
velocitysubcluster(A115S)islikelycentredonthefirstbrightestclustergalaxy(BCM−B)andthesouthernX-raypeak.Weestimate
thatA115SisslightlydynamicallymoreimportantthanA115Nhavingσv =900−1100kms−1 vs.σv =750−850kms−1.Moreover,
wefindevidencefortwosmallgroupsatlowvelocities.Weestimateaglobalclustervirialmassof2.2−3.5×1015 h−701 M(cid:3).
Conclusions.Our results agree with a pre-merging scenario where A115N and A115S are colliding with a LOS impact velocity
∆v ∼ 1600kms−1.Themostlikelysolutiontothetwo-bodyproblemsuggeststhatthemergingaxisliesat∼20degreesfromthe
rf
planeoftheskyandthatthecoreswillcrossafter∼0.1Gyr.Theradiorelicwithitslargestdimensionperpendiculartothemerging
axisislikelyconnectedtothismerger.
Keywords.galaxies:clusters:general–galaxies:clusters:individual:Abell115–galaxies:distancesandredshifts–
cosmology:observations
1. Introduction thatappeartobeassociatedwithveryrichclustersthathaveun-
dergonerecentmergersandthusit hasbeensuggestedbyvari-
The evolutionof clustersof galaxiesas seen in numericalsim- ousauthorsthatclusterhalos/relicsarerelatedtorecentmerger
ulations is characterized by the asymmetric accretion of mass
activity(e.g.,Tribble1993;Burnsetal.1994;Feretti1999).
clumps from surrounding filaments (e.g. Diaferio et al. 2001).
Nearbyclustersarecharacterizedbyavarietyofmorphologies, The synchrotronradio emission of halosand relics demon-
indicativeofdifferentdynamicalproperties,elongateddistribu- strates the existence of large scale cluster magnetic fields, of
tion,tracedbyseveralgalaxyclumps(e.g.,Barrenaetal.2002; the order of 0.1−1 µG, and of widespread relativistic particles
Boschin et al. 2006; Sauvageot et al. 2005; Yuan et al. 2005). of energy density 10−14−10−13 ergcm−3. The difficulty in ex-
Thepresenceofsubstructureisindicativeofaclusterinanearly plaining halos/relics arises from the combination of their large
phase of the process of dynamical relaxation or of secondary size and the short synchrotronlifetime of relativistic electrons.
infall of clumps into already virialized clusters (see Girardi & Theexpecteddiffusionvelocityoftheelectronpopulationison
Biviano2002,forareview). the order of the Alfvén speed (∼100 km s−1) making it diffi-
An interesting aspect of these investigationsis the possible cult for the electronsto diffuse overa megaparsec-scaleregion
connectionofclustermergerswiththepresenceofextended,dif- within their radiative lifetime. Therefore, one needs a mecha-
fuseradiosources.Thesesourcesarelarge(upto∼1h−1Mpc), nismbywhichtherelativisticelectronpopulationcanbetrans-
70
amorphous cluster sources of uncertain origin and generally ported over large distances in a short time, or a mechanism by
steep radio spectra (Hanisch 1982; see also and Giovannini & which the localelectronpopulationisreacceleratedandthe lo-
Feretti 2002, fora review).Theyare classified as halos,if they cal magnetic fields are amplified over an extended region. The
arelocatedintheclustercentre,orrelics,iftheyappearinthepe- cluster-cluster merger can potentially supply both mechanisms
ripheralregionsofthecluster.Halosandrelicsareraresources (e.g.,Giovanninietal.1993;Burnsetal.1994;Röttgeringetal.
1994; see also Feretti et al. 2002; Sarazin 2002, for reviews).
(cid:2) Figures1,2andTable1areonlyavailableinelectronicformat However, the question is still debated since the diffuse radio
http://www.aanda.org sources are quite uncommon and only recently we can study
Article published by EDP Sciences and available at http://www.aanda.orgor http://dx.doi.org/10.1051/0004-6361:20077407
862 R.Barrenaetal.:ThedynamicalstatusofA115
these phenomena on the basis of a sufficient statistics (a few The diffuse radio source belongs to A115N, and extends
dozens of clusters up to z ∼ 0.3, e.g., Giovannini et al. 1999; from this sub-cluster to the periphery. According to its non-
seealsoGiovannini&Feretti2002;Feretti2005). central location and its elongated structure, it is classified as a
Growing evidence of the connection between diffuse ra- clusterrelic.However,elongatedrelicsaregenerallyattheclus-
ter periphery, and with the largest dimension roughly perpen-
dio emission and cluster merging is based on X-ray data (e.g.,
dicular to the cluster radial orientation. So this source is quite
Böhringer & Schuecker 2002; Buote 2002). Studies based on
unusual although several considerations confirm that it should
a large number of clusters have found a significant relation
beaclusterrelic(Govonietal.2001b).
between the radio and the X-ray surface brightness (Govoni
et al. 2001a,b) and between the presence of radio-halos/relics Todatefewspectroscopicdataarereportedintheliterature.
and irregular and bimodal X-ray surface brightness distribu- Beersetal.(1983)measuredtheredshiftfor19clustermembers
tion (Schuecker et al. 2001). New unprecedented insights into (seealsoZabludoffetal.1990foralaterre-reductionwithanew
mergingprocessesinradioclustersareofferedbyChandraand template).InDecember2003weobtainedspectraof115galax-
XMM-Newtonobservations(e.g.,Markevitch&Vikhlinin2001; iesintheclusterregion,withthepurposeofconstrainingitsdy-
Fujitaetal.2004). namicalstatus.
Theplanofthispaperisthefollowing.InSect.2wedescribe
Opticaldata are a powerfulway to investigatethe presence
ournewphotometricandspectroscopicobservations.InSect.3
and the dynamics of cluster mergers (e.g., Girardi & Biviano
weprovideourresultsaboutclustermembership,globalproper-
2002), too. The spatial and kinematical analysis of member
ties, andsubstructure.InSect. 4we drawourconclusionabout
galaxies allow us to detect and measure the amount of sub-
clusterstructureandestimateclustermass.InSect.5wediscuss
structure, to identify and analyse possible pre-mergingclumps
ourresultsandpositaplausiblescenarioforthedynamicalstatus
or merger remnants.This opticalinformationis really comple-
ofA115.WesummarizeourresultsinSect.6.
mentary to X-ray informationsince galaxies and ICM react on Throughout this paper, we use H = 70 h kms−1Mpc−1
differenttimescalesduringamerger(seenumericalsimulations 0 70
by Roettiger et al. 1997). Unfortunately,to date optical data is iandoaptfledatccoosmsmoolologgyy,1w(cid:4) citohrrΩesmpon=ds0to.3∼a1n9d2Ωh−Λ1k=pc0at.7t.heIncltuhse-
lackingorpoorlyexploitedandsparseliteratureconcernssome 70
terredshift.
few individual clusters. In this context we have carried on an
intensiveprogramtostudytheclusterscontainingextended,dif-
fuse radio emission (Boschin et al. 2004; Boschin et al. 2006;
2. Data
Girardietal. 2006; Barrenaet al. 2007). In particular,we have
conducteda studyofAbell115(hereafterA115)havinganex- 2.1.Spectroscopy
tendedarc-shaperelic(Ferettietal.1984;Giovanninietal.1987;
Multi-object spectroscopic observations of A115 were carried
Govonietal.2001b).
out at the TNG telescope in December 2003 during the pro-
A115isarichcluster(Abellrichnessclass=3;Abelletal. gram of proposal AOT8/CAT–G6. We used DOLORES/MOS
1989) known in the literature to be characterized by a double
with the LR–B Grism 1, yielding a dispersion of 187 Å/mm,
X-ray peak (A115N and A115S; Forman et al. 1981) and by
andtheLoralCCDof2048×2048pixels(pixelsizeof15µm).
thepresenceofastrongcoolingflowinitsnortherncomponent
This combinationof grating and detector results in dispersions
(White et al. 1997). White et al. (1997) give the followingval-
uesfortheX-rayluminosityandtemperature:L =14.7×1044, of 2.8Å/pix. We have taken five MOS masks for a total of
9.1×1044 ergs−1 (in their cosmology)and T X= 6.6, 5.7 keV 152 slits. We acquired three exposures of 1800 s for each
X masks. Wavelength calibration was performed using helium-
forthenorthernandsoutherncomponents,respectively.Slightly
smaller temperatures are also found in the literature: T = argon lamps. Reduction of spectroscopic data was carried out
4.9+0.7and5.2keVforthenortherncomponent,andT =5.X2+1.4 withIRAF1package.
−0.6 X −1.0 Radial velocities were determined using the cross-
and 4.8 keV for the southern component (from Shibata et al.
correlationtechnique(Tonry&Davis1979)implementedinthe
1999; and Gutierrez & Krawczynski 2005, respectively). Both
RVSAO package (developed at the Smithsonian Astrophysical
ASCAandChandradatashowthatthetemperatureoftheICMis
ObservatoryTelescopeDataCenter).Eachspectrumwascorre-
highlynonuniformacrosstheclustersuggestingthatthemerger
latedagainstsixtemplatesforavarietyofgalaxyspectraltypes:
is well underway, but not disturbing the cool subcluster cores
E, S0, Sa, Sb, Sc, Ir (Kennicutt1992). Thetemplateproducing
(Shibataetal.1999;Gutierrez&Krawczynski2005).Fromthe
the highest value of R, i.e., the parameter given by RVSAO
opticalpointofview,Beersetal.(1983)mappedthegalaxydis-
and related to the signal-to-noise of the correlation peak, was
tributionfindingthreemajorclumpsofgalaxies(A,B,andCin
chosen.Moreover,allspectraandtheirbestcorrelationfunctions
Fig. 4a of theirpaper).NorthernandsouthernclumpsA andB
were examined visually to verify the redshift determination.
correspond to the peaks of the X-ray surface brightness distri-
In some cases (IDs 85, 112 and 115, see Table 1) we took the
bution.However,no X-rayemission hasbeenfoundassociated
EMSAOredshiftasareliableestimateoftheredshift.
withthethirdeasternclump.
Forelevengalaxiesweobtainedtworedshiftdeterminations,
The northern subcluster contains the very strong radio
which are of similar quality. This allow us to obtain a more
galaxy 3C28 (Riley & Pooley 1975; Feretti et al. 1984;
rigorous estimate for the redshift errors since the nominal er-
Giovannini et al. 1987). Its host galaxy is the brightest galaxy
rors as given by the cross-correlation are known to be smaller
of the northern subcluster and is classified as “elliptical” by
than the true errors (e.g., Malumuth et al. 1992; Bardelli et al.
Schombert1987. The strong X-ray and radio emissions of this
1994;Ellingson& Yee1994).We fitthefirstdeterminationvs.
clustermemberseemtobeexplainedbythepresenceofalarge
amountofhotgaswhichislikelytobeaccretingontothegalaxy. 1 IRAF is distributed by the National Optical Astronomy
Thisscenarioagreeswiththeobservationinthespectrumofthis Observatories, which are operated by the Association of Universities
galaxyoftheHα-line andotheremissionlines, likelytracinga forResearchinAstronomy,Inc.,undercooperativeagreementwiththe
coolingflowsystem(Crawfordetal.1999). NationalScienceFoundation.
R.Barrenaetal.:ThedynamicalstatusofA115 863
thesecondonebyusingastraightlineandconsideringerrorsin astrometricsolution(rms∼0.5(cid:4)(cid:4))acrossthefullframe.Thepho-
both coordinates(e.g.,Press etal. 1992). The fitted line agrees tometriccalibrationwasperformedusingLandoltstandardfields
withtheonetoonerelation,but,whenusingthenominalcross- achievedduringtheobservation.
correlation errors, the small value of χ2-probability indicates a We finallyidentifiedgalaxiesinour B andR imagesand
H H
poor fit, suggesting the errors are underestimated. Only when measuredtheirmagnitudeswiththeSExtractorpackage(Bertin
nominalerrorsaremultipliedbya∼1.3factortheobservedscat- &Arnouts1996)andAUTOMAGprocedure.Infewcases,(e.g.,
ter can be explained. Therefore, hereafter we assume that true close companion galaxies, galaxies close to defects of CCD),
errorsarelargerthannominalcross-correlationerrorsbya fac- the standard SExtractor photometric procedure failed. In these
tor 1.3.For the elevengalaxieswe used the averageof the two cases we computedmagnitudesby hand. This method consists
redshiftdeterminationsandthecorrespondingerror. in assuming a galaxy profile of a typical elliptical and scale it
Ourspectroscopicsurveyconsistsof115galaxiestakenina to the maximumobservedvalue.The integrationof thisprofile
fieldof∼15(cid:4)×20(cid:4). give us an estimate of the magnitude. The idea of this method
Wealsodeterminedtheequivalentwidths(EWhereafter)of issimilartothePSFphotometry,butassumingagalaxyprofile,
the absorption line Hδ and the emission line [OII], in order to moreappropriateinthiscase.
classify post-starburst and starburst galaxies. We estimated the We transformed all magnitudes into the Johnson-Cousins
minimum measurable EW of each spectrum as the width of a system (Johnson & Morgan 1953; Cousins 1976). We used
linespanning2.8Å (ourdispersion)inwavelength,with anin- B = BH + 0.13 and R = RH, as derived from the Harris fil-
tensitythreetimesthermsnoiseintheadjacentcontinuum.This tercharacterization(http://www.ast.cam.ac.uk/∼wfcsur/
estimationyieldsuppermeasurablelimitsof∼4.5ÅinEW. technical/photom/colours/)and assuming a B−V ∼ 1.0
for E-type galaxies (Poggianti 1997). As a final step, we
Weuseaconservativeapproachleadingtoasparsespectral
classification(∼50%ofthesample,seeTable1).Wefollowthe estimated and corrected the galactic extinction AB ∼ 0.25,
A ∼ 0.15 from Burstein & Heiles (1982) reddening maps.
classification by Dressler et al. (1999; see also Poggianti et al. R
We estimatedthatourphotometricsampleiscompletedownto
1999). We define “e”-type galaxies those showing active star
B = 19.5(21.0)andR = 22.0(23.0)forS/N = 5(3)withinthe
formationas indicated by the presence of an [OII] and, in par-
observedfield.
ticular,“e(b)”galaxieswhentheequivalentwidthofEW([OII])
is≤−40Å(likelystarburstgalaxies);“e(a)”-typegalaxiesthose We assigned B and R magnitudes to the whole spectro-
scopic sample.We measuredredshiftforgalaxiesdowntoR ∼
havingEW(Hδ) ≥ 4Å;“e(c)”-typegalaxiesthosehavingmod-
20.5 mag, but we are complete to 60% down to R = 18 mag
erate emission lines and EW(Hδ) < 4 Å (likely spiral galax- (withinaregionof13(cid:4)×20(cid:4) aroundRA = 00h56m02s.0,Dec =
ies). We define “k+a” and “a+k”-type galaxies those having +26◦23(cid:4)00(cid:4)(cid:4)(J2000.0)).
4 ≤ EW(Hδ) ≤ 8 Å and EW(Hδ) > 8 Å, respectively, and
Table1liststhe velocitycatalogue(seealso Figs.1and2):
no emission lines (the so called “post-starbust”). Moreover,
identification numberof each galaxy,ID (Col. 1);ID code fol-
out of galaxies having the cross-correlation coefficient R >∼ 7
lowingtheIAUnomenclature(Col.2);rightascensionanddec-
– corresponding to S/N >∼ 10 as obtained using A773 data lination, α and δ (J2000,Col. 3); B and R magnitudes(Cols. 4
(Barrenaetal.2007)–wedefine“k”-typegalaxiesthosehaving and5,respectively);heliocentricradialvelocities,v = cz(cid:3) with
EW(Hδ) < 3 Å andnoemission lines(likely passivegalaxies). errors,∆v(Cols.6and7,respectively);spectralclassificationSC
We classify 62 galaxies finding 38 “passive” galaxies, 24 “ac- (Col.8).
tive”galaxies(i.e.,14“k+a”/“a+k”and10“e”galaxies).
3. Analysisandresults
2.2.Photometry
3.1.Memberselectionandglobalproperties
As far as photometry is concerned, our observations were car-
ried out with the Wide Field Camera (WFC), mounted at the To select cluster members out of the 115 galaxies having red-
primefocusofthe2.5mINTtelescope(locatedatRoquedelos shifts,wefollowthetwostepsprocedure.First,weperformthe
Muchachos observatory,La Palma, Spain). We observed A115 adaptive-kernel method (hereafter DEDICA, Pisani 1993 and
in18December2004inphotometricconditionswithaseeingof 1996; see also Fadda et al. 1996; Girardi et al. 1996; Girardi
about2(cid:4)(cid:4). & Mezzetti 2001). We find the significant peaks in the veloc-
The WFC consists of a 4 chip mosaic coveringa 30(cid:4) ×30(cid:4) itydistribution>99%c.l.ThisproceduredetectsA115asaone-
field of view, with only a 20% marginally vignetted area. We peakstructureatz∼0.1937populatedby88galaxiesconsidered
took10exposuresof720sin B and360sinR Harrisfilters as candidatecluster members(see Fig. 3). Out of non-member
H H
(atotalof7200sand3600sineachband)developingadither- galaxies, 12 and 15 are foreground and background galaxies,
ingpatternoftenpositions.Thisobservingmodeallowedusto respectively.
build a master “supersky” image that was used to correct our The projected clustercentric distance vs. the rest-frame ve-
imagesforvignettingandfringingpatterns(Gullixson1992).In locity of the 88 galaxies is shown in Fig. 4 where we also
addition,theditheringhelpedustocleancosmicraysandavoid show the three brightest galaxies in our sample, each of
gaps between CCD chips. The complete reductionprocess (in- them corresponding to a different galaxy density clumps as
cludingflatfielding,biassubtractionandbad-columnselimina- found by Beers et al. (1983), i.e. IDs 54, 21(=3C28) and 104
tion) yielded a final co-addedimage where the variationof the (hereafter BCM−B, BCM−A, and BCM−C) corresponding to
skywaslowerthan0.4%inthewholeframe.Anothereffectas- B (A115S), A (A115N), and C clumps, respectively. The po-
sociatedwiththewidefieldframesisthedistortionofthefield. sition of BCM−B is very close to that of the southern X-ray
Inordertomatchthephotometryofseveralfilters(inourcase, peak(RA = 00h55m58s.81,Dec = +26◦19(cid:4)58(cid:4).(cid:4)9(J2000.0)asre-
onlyB andR ),agoodastrometricsolutiontakingintoaccount coveredfromX-rayChandradata,seeFig.2).BCM−A=3C28
H H
thesedistortionsisneeded.UsingIRAFtasksandtakingasref- is coincident with the northern X-ray peak and is very close
erencetheUSNOB1.0catalogwewereabletofindanaccurate to the position of the X-ray centroid when masking the X-ray
864 R.Barrenaetal.:ThedynamicalstatusofA115
Table2.Resultsofthekinematicalanalysis.
Sample N (cid:8)v(cid:9) σa BCMgalaxies
g v
kms−1 kms−1
Wholesystem 85 57817±148 1362+126 A,B,C,D
WholesystemSample2 80 57956±132 1175−+19508 A,B,C
WGAP1 8 54448±212 529+1−2746 D
WGAP2 6 55912±78 162−+4349
DS−A 11 59199±263 816−+51528 A
DS−C 3 56138±1001 − −103 C
KMM1 10 54819±299 873+225 D
KMM2 44 57833±155 101−71+11409 B,C
KMM3 31 58500±203 1109−+71799 A
CORE−A 6 59457±364 750+3−31712 A
CORE−B 6 57493±434 894−+125930 B
Passivegals 34 57325±252 144−75+92747 bC,D
Activegals 14 58950±411 1462−+175024 A
Balmerabs.gals 9 58988±668 1817−+484304
Emissionlinesgals 5 58558±527 898+3−69394 A
−218
Fig.3. Redshiftgalaxydistribution.Thesolidlinehistogramrefersto a We use the biweigth and the gapper estimators by Beers et al.
galaxiesassignedtotheclusteraccordingtotheDEDICAreconstruc- (1990) forsampleswith N ≥ 15and with N < 15galaxies, re-
g g
tionmethod. spectively(seealsoGirardietal.1993).
b NoticethatBCM-Bresembles characteristicsofapassivegalaxy,
butitdoesnotappearsinourclassificationsinceitsspectrahasR
slightlybelowthethresholdvalueof7.
bin that shifts along the distance from the cluster centre. The
procedureis iterated until the numberof cluster memberscon-
verges to a stable value. Following Fadda et al. (1996) we use
a gapof1000kms−1 –in thecluster rest-frame–anda binof
0.6h−1Mpc,orlargeenoughtoinclude15galaxies.Inthecase
70
oftheA115complex,whichisnotanindividualsystem,wefix
alternativelyBCM−Aand–Basclustercentres.Figure4shows
the results. Theveryhighvelocitygalaxy(ID 91)is a clear in-
terloper far more than 3000 km s−1 from other galaxies. The
othertwohighvelocitygalaxies(IDs96and100),thatareclose
enough in 2D and far from the high velocity BCM−A galaxy
(see Fig. 1), are rejected in both our analyses. The conclusion
aboutthelowvelocitytailislessclearalsobecausefewofthese
galaxieslieintheeasternregionoftheclusterandmightbeas-
Fig.4. Projected clustercentric distance vs. rest-frame velocity v =
(v−(cid:8)v(cid:9))/(1+z)forthe88galaxiesinthemainpeak(seeFig.3)shrofw- sociated to the C clump (or to the D clump, see Sect. 3.3).We
decide to define 85 likely cluster members rejecting the three
inggalaxiesdetectedasinterlopersbyour“shiftinggapper”procedure
(stars and crosses) when using BCM−A and –B as alternative cluster highest velocity galaxies (basic sample). In some analyses we
centres(topandbottompanels,respectively).Werejectthethreegalax- also consideranalternativesampleof 80galaxies– Sample2–
ieswithhighestvelocities(stars)tobuildourbasicsampleof85galax- also rejecting the low velocity galaxiesindicated in Fig. 4 (the
ies and all galaxies indicated by crosses and stars in the top panel to crossesinthemiddlepanel).
buildourSample2.Thethreebrightestclustermemberseachonecor-
By applying the bi-weight estimator to cluster members
respondingtoBeersetal.(1983)clumps(BCM−A,–Band–C)anda
fourthbrightgalaxy(BCM−D)areindicatedbysquares. (Beers et al. 1990), we compute a mean cluster redshift of
(cid:8)z(cid:9) = 0.1929±0.0005,i.e. (cid:8)v(cid:9) = 57817±148kms−1. We es-
timate the LOS velocity dispersion, σv, by using the bi-weight
emission of the point source (RA = 00h55m53s.2, Dec = estimatorandapplyingthecosmologicalcorrectionandthestan-
+26◦24(cid:4)59(cid:4)(cid:4) (J2000.0) by Govoni et al. 2001b). We also con- dardcorrectionforvelocityerrors(Daneseetal.1980).Weob-
siderthebrightgalaxyID81(hereafterBCM−D,seeSect.3.3) tain σv = 1362+−112068 kms−1, whereerrorsareestimated through
abootstraptechnique.ConsistentresultsarefoundforSample2
locatedinthemiddleoftheabovethreegalaxies.
(seeTable2).
All the galaxiesassigned to the A115 peak are analysed in
a second step by applying the “shifting gapper” technique by Hereafter,forpracticalreasons,weconsiderascentreofthe
Faddaetal.(1996),whichusesthecombinationofpositionand whole A115 complex the position of the bi-weight centre ob-
velocityinformation.Thisprocedurerejectsgalaxiesthataretoo tained using bi-weight mean estimators for RA and Dec sepa-
farinvelocityfromthemainbodyofgalaxiesandwithinafixed rately(RA=00h56m01s.31,Dec=+26◦22(cid:4)26(cid:4).(cid:4)6(J2000.0)).
R.Barrenaetal.:ThedynamicalstatusofA115 865
Fig.6. Spatial distribution on the sky of spectroscopically confirmed
Fig.5.Toppanel:velocitydistributionofthe85fiducialclustermem-
bers. Arrows correspond to the bright galaxies BCM−A, –B, –C cluster members and the relative isodensity contour map. The three
and −D. Bottom panel: stripe density plot where the arrows indicate brightest galaxies BCM−A –B, and –C corresponding to Beers et al.
(1983)groups, afourth bright galaxy (BCM−D)and theX-raypeaks
thepositionsofthesignificantgaps.Thegapatthelowervelocityhasa
normalizedsize=2.41,theother=2.25. are indicated by large squares, too. The plot is centred on the cluster
centredefinedasthebi-weightcentre(seetext).
3.2.Velocitystructure
3.3.2Dgalaxydistribution
We analysethe velocitydistributionto lookfor possibledevia-
tions from Gaussianity that could provideimportantsignatures When applying the DEDICA method to the 2D distribution of
ofcomplexdynamics.Forthefollowingteststhenullhypothesis A115galaxymemberswe findthreesignificantpeaks.Thepo-
isthatthevelocitydistributionisasingleGaussian. sitionofthehighestpeakisclosetothelocationoftheBCM−B
We estimate three shape estimators, i.e. the kurtosis, the and of the southern peak of X-ray emission (XS in Fig.6).
skewness,andthescaledtailindex(see,e.g.,Beersetal.1991). Another peak is close to the location of the BCM−A, of the
Thevalueoftheskewness(−0.462)showsevidencethattheve- northern peak of X-ray emission (XN) and of the X-ray cen-
locity distribution differs from a Gaussian at the 95−99% c.l. troidwhenmaskingtheX-rayemissionofthepointsource(X,
(see Table 2 of Bird & Beers 1993). Moreover, the W-test Govoni et al. 2001b). When dividing the sample in bright and
(Shapiro & Wilk 1965) marginally rejects the null hypothesis faintgalaxies–usingthemedianmagnitudevalueR = 18.32–
ofaGaussianparentdistributionatthe92%c.l. wefindthatthe2Ddistributionsofthetwosamplesaredifferent
Thenweinvestigatethepresenceofgapsinthedistribution. atthe98.5%c.l.accordingtothetwo-dimensionalKolmogorov-
Aweightedgapinthespaceoftheorderedvelocitiesisdefined Smirnov test (hereafter2DKS–test; see Fasano & Franceschini
as the difference between two contiguous velocities, weighted 1987, as implemented by Press et al. 1992). In fact, the faint
bythelocationofthesevelocitieswithrespecttothemiddleof galaxies sample shows both peaks A and B, while the bright
thedata.Weobtainvaluesforthesegapsrelativetotheiraverage galaxiessampleonlyshowsthepeakB.
size,preciselythemidmeanoftheweighted-gapdistribution.We Our spectroscopic data do not cover the entire cluster field
lookfornormalizedgapslargerthan2.25sinceinrandomdraws and suffer from magnitude incompleteness. To overcomethese
ofaGaussiandistributiontheyariseatmostinabout3%ofthe limits we recover our photometric catalogue selecting likely
cases,independentofthesamplesize(Wainer&Schacht1978; members on the basis of the colour−magnituderelation (here-
see also Beersetal. 1991). Two significantgapsin theordered afterCMR),whichindicatestheearly-typegalaxylocus.Tode-
velocitydatasetaredetectedindividuatingtwogroupsinthelow termine CMR we fix the slope according to López-Cruz et al.
velocitytailofthevelocitydistribution,ofeightandsixgalaxies (2004, see their Fig. 3) and apply the two-sigma-clipping fit-
(seeWGAP1andWGAP2inTable2andFig.5). ting procedure to the cluster members obtaining B − R =
We usethe resultsofthe gapanalysisto determinethe first 3.611−0.069 × R (see Fig. 7). Out of our photometric cata-
guess when using the Kaye’s mixture model (KMM) to find a logue we consider galaxies (objects with SExtractor stellar in-
possible group partition of the velocity distribution (as imple- dex ≤ 0.9) lying within 0.25 mag of the CMR. To avoid
mented by Ashman et al. 1994). The KMM algorithm fits an contaminationbyfieldgalaxieswedonotshowresultsforgalax-
user-specifiednumberofGaussiandistributionstoadatasetand ies fainter than 21 mag (in R-band). The contour map for 369
assesses the improvementof that fit over a single Gaussian. In likelyclustermembershavingR≤21showsagainthetwopeaks
addition, it provides the maximum-likelihood estimate of the in correspondenceofBCM−A andBCM−Bandalsoa peakin
unknown n-mode Gaussians and an assignment of objects into correspondence of BCM−C. Finally, a less dense peak lies in
groups.Wedonotfindanytwo-orthree-groupspartitionwhich themiddleoftheabovethreepeaksandcorrespondstothelumi-
isasignificantbetterdescriptorofthevelocitydistributionwith nousgalaxyBCM−Dataverylowvelocity(seeFig.8).Similar
respecttoasingleGaussian. results are found analysing the 268 galaxies with R ≤ 20. The
866 R.Barrenaetal.:ThedynamicalstatusofA115
Fig.7. B−R vs. R diagram for galaxies with available spectroscopy
is shown by small dots and crosses (cluster and field members, re-
spectively).LargesoliddotsindicateluminousgalaxiesBCM−A,–B, Fig.9.Spatialdistributionontheskyof85clustermembers.Openand
–C and –D. The solid line gives the best-fit colour−magnitude rela- solidcirclesindicatelowandhighvelocitygalaxies,thecrossthegalaxy
tion as determined on member galaxies; the dashed lines are drawn withmedianvelocity.Thelargerthesymbol,thesmalleristheradialve-
at ±0.25 mag from the CMR. Large open symbols indicates classi- locity.Thesolidandfaintlinesindicatethepositionangleofthecluster
fied galaxies: circles (“k”), squares (“k+a”), rombs (“a+k”), triangles gradientandrelativeerrors,respectively.Thefaintbigsquaresindicate
(“e(c)”),andstars(“e(a)”/“e(b)”). the14galaxiesbelongingtoWGAP1andWGAP2(seeSect.3.2).The
plotiscentredontheclustercentre.
The cluster velocity field may be influenced by the pres-
ence of internal substructures. To investigate the velocity field
oftheA115complexwedividegalaxiesinalowandahighve-
locity samples by using the median cluster velocity and check
thedifferencebetweenthetwodistributionsofgalaxypositions.
Figure 9 shows that low and high velocity galaxies are segre-
gated roughly in the E–W direction. The two distributions are
different at the 99.5% c.l. according to the 2DKS-test. In or-
der to estimate the direction of the velocity gradient we per-
form a multiple linear regression fit to the observed velocities
withrespecttothegalaxypositionsintheplaneofthesky(see
also den Hartog & Katgert 1996; Girardi et al. 1996). We find
apositionangleonthecelestialsphereofPA = 269+19 degrees
−18
(measured counter-clock-wise from north), i.e. higher velocity
galaxies lie in the western region of the cluster (see Fig. 9).
To assess the significance of this velocity gradientwe perform
1000MonteCarlosimulationsbyrandomlyshufflingthegalaxy
velocitiesandforeachsimulationwedeterminethecoefficientof
Fig.8. Spatial distribution on the sky and relative isodensity contour
mapof369likelyclustermembers(accordingtothecolour−magnitude multipledetermination(RC2,seee.g.,NAGFortranWorkstation
relation) withR ≤ 21,obtainedwiththeDEDICAmethod. Thethree Handbook1986).Wedefinethesignificanceofthevelocitygra-
brightest galaxies BCM−A –B, and –C are indicated by large circles dientasthefractionoftimesinwhichtheRC2 ofthesimulated
andtheX-raypeaksareindicatedbylargesquares.Itisalsoshownthe data is smaller than the observed RC2. We find that the veloc-
position of the luminous galaxy BCM−D. The plot is centred on the ity gradientismarginallysignificantatthe 90%c.l. Similar re-
clustercentre. sultsareobtainedfortheSample2(PA = 267+18 degreesatthe
−17
91%c.l.).
We combine galaxy velocity and position information to
analysisof 136galaxieswith R ≤ 19showsasverysignificant compute the ∆-statistics devised by Dressler & Schectman
onlythesouthernpeakclosetoBCM−Binagreementwiththe (1988). This test is sensitive to spatially compact subsystems
resultscomingfromthespectroscopicsample(seeabove). that have either an average velocity that differs from the clus-
ter mean, or a velocity dispersion that differs from the global
one, or both. We find ∆ = 121 for the value of the parameter
3.4.Position-velocitycorrelations
which gives the cumulative deviation. This value is an indica-
Theexistenceofcorrelationsbetweenpositionsandvelocitiesof tionofsubstructure,significantatthe96%c.l.,asassessedcom-
clustergalaxiesisafootprintofrealsubstructures.Hereweuse puting 1000 Monte Carlo simulations, randomly shuffling the
differentapproachestoanalysethestructureofA115combining galaxyvelocities.Figure10showsthedistributionontheskyof
velocityandpositioninformation. all galaxies, each marked by a circle: the larger the circle, the
R.Barrenaetal.:ThedynamicalstatusofA115 867
Fig.10.Spatialdistributionofclustermembers,eachmarkedbyacir-
cle: thelarger thecircle,thelarger isthedeviation δi of thelocal pa- Fig.11. The distribution of δi deviations of the Dressler-Schectman
rametersfromtheglobalclusterparameters,i.e.thereismoreevidence analysisforthe85membergalaxies.Thesolidlinerepresentstheob-
forsubstructure(accordingtotheDressler&Schectmantest,seetext). servations,thedashedlinethedistributionforthegalaxiesofsimulated
Theboldfacecirclesindicatethosewithδi ≥2.2(seetext).Theplotis clusters,normalizedtotheobservednumber.
centredontheclustercentre.
larger the deviation δ of the local parameters from the global
i
clusterparameters,i.e.thehighertheevidenceforsubstructure.
This figure provides information on the positions of substruc-
tures:oneintheeasternregioncorrespondingtotheclumpCand
oneinthenorthernregioncorrespondingtotheclumpA.Similar
resultsobtainedforSample2(∆=110andac.l.of95%),butthe
easternsubstructureisnolongersoobviousintherelativeplot.
To obtain further information, we resort to the technique
developed by Biviano et al. (2002), who used the individual
δ-valuesoftheDressler&Schectmanmethod.Thecriticalpoint
i
is to determinethe valueofδ thatoptimallyindicatesgalaxies
i
belongingtosubstructure.Tothisaimweconsidertheδ-values
i
of all1000MonteCarlo simulationsalreadyused to determine
the significance of the substructure (see above). The resulting
distributionofδ iscomparedtotheobservedonefindingadif-
i
ferenceofP>99.99%c.l.accordingtotheKS-test.The“simu-
lated”distributionisnormalizedtoproducetheobservednumber
ofgalaxiesandcomparedtotheobserveddistributioninFig.11:
thelattershowsatailatlargevalues.Thetailwithδ>∼2ispop-
ulatedbygalaxiesthatpresumablyareinsubstructures.Forthe Fig.12.Spatialdistributionontheskyofthe85membergalaxies.Solid
circles,opencirclesandtrianglesindicateKMM1,KMM2,andKMM3.
selectionofgalaxieswithinsubstructureswechoosethevalueof
δ =2.2,sincethegalaxieswithδ >δ arewellseparatedin The large faint squares indicate the position of the brightest cluster
lim i lim membersBCM−A,–B,–Cand–D.Theplotiscentredonthecluster
thesky(seeFig.10)andassignthreeandelevengalaxiestothe
centre.
clumpsCandA,respectively(hereafterDS–CandDS–A).The
northernstructureispopulatedbyhighvelocitygalaxies,while
thepoorstatisticspreventustoobtainfirmconclusionsaboutthe
easternstructure(seeTable2). three-groupspartitionatthe97%c.l.accordingtothelikelihood
TheGaussianmodelforthe2Dgalaxydistributionispoorly ratiotest(hereafterKMM1,KMM2,KMM3groupsfromlowto
supportedby theoreticaland/orempiricalargumentsand, how- highmeanvelocities).Theresultsforthethreegroupsareshown
ever,ourgalaxycatalogueisnotcompletedowntoamagnitude in Table 2 and Fig. 12. KMM1 group is sparse in the sky, but
limit. However,since the3D diagnosticsisin generalthe most well distinct in velocity from the other two groups. Several of
sensitiveindicatorsofthepresenceofsubstructure(e.g.,Pinkney itsgalaxieswerealreadydetectedbytheweightedgapanalysis
etal.1996),weapplythe3DversionoftheKMMsoftwareusing asbelongingtoWGAP1andWGAP2(seeSect.3.2).Moreover,
simultaneouslygalaxyvelocityandpositions.Weusethegalaxy KMM1containsBCM−D.KMM2andKMM3groupsarewell
assignment given by Dressler-Schectman method to determine distinctinthesky.KMM2containsbothBCM−BandBCM−C,
the first guess when fitting three groups. The algorithm fits a whileKMM3containsBCM−A.
868 R.Barrenaetal.:ThedynamicalstatusofA115
3.5.KinematicsofA115NandA115S
The spatial agreement between the two brightest cluster mem-
bers(BCM−Aand–B)andthepeaksofX-rayemissionaswell
asthehighdensityofgalaxiesaroundBCM−Aand–Bprompts
us to analyse the profiles of the mean velocity and the veloc-
itydispersionofgalaxysystemssurroundingthesetwogalaxies
(see Figs. 13and14,respectively).Thisallowsanindependent
analysis ofthe individualgalaxyclumps. Althoughan increase
in the velocity-dispersion profile in the cluster central regions
might be due to dynamical friction and galaxy merging (e.g.,
Menci & Fusco-Femiano 1996; Girardi et al. 1998; Biviano &
Katgert 2004), in the case of A115 it mightbe simply induced
bythecontaminationofthegalaxiesofasecondaryclump(e.g.,
Girardietal.1996;Girardietal.2005).Thelatterhypothesiscan
beinvestigatedbylookingatthebehaviourofthemeanvelocity
profile.
Sincefromtheabovesectionsweknowthepresenceofone
orlikelyafewlowvelocitygroups,weanalysetheSample2to
avoid,atleastpartially,thepossiblecontamination.Wealsocon-
sidertheresultsobtainedrejectingallgalaxiesbelongingtolow Fig.13. Kinematical profiles of the northern subcluster A115N ob-
velocityWGAP1andWGAP2.Figures13and14showvelocity- tainedassumingBCM−Aascentre.Theverticallineindicatesthere-
dispersion and mean-velocity profiles, as well as regions not gionlikelynotcontaminatedfromothergalaxyclumps(seetext).Top
likelytobecontaminatedbyothergalaxysystemsandthusreli- panel: rest-frame velocity vs. projected distance from the subcluster
ableforkinematicalanalysis.Detailedresultsofthisanalysisare centre (BCM−A):crosses indicate thegalaxiesbelonging to WGAP1
andWGAP2.Differential(bigcircles)andintegral(smallpoints)mean
includedinTable2wherethe“uncontaminated”galaxysystems
arenamedasCORE−AandCORE−B. velocity and LOS velocity-dispersion profiles are shown in middle
and bottom panels, respectively. For the differential profiles we plot
Figure13showshowtheintegralvelocity-dispersionslightly
the values for seven annuli from the centre of the subcluster, each
increases with the distance fromthe BCM−A. Simultaneously, of 0.25 h−1Mpc. Forthe integral profiles, themean and dispersion at
themeanvelocityshowsacontinuousdeclinefromhighvalues a given (p70rojected) radius from the subcluster–centre is estimated by
∼59000 km s−1 suggesting a strong contaminationof galaxies consideringallgalaxieswithinthatradius–thefirstvaluecomputedon
from structures connected with BCM−B, −C and –D, all hav- thefivegalaxiesclosesttothecentre.Theerrorbandsatthe68%c.l.are
ing lower mean velocities. In a conservative view we consider alsoshown.Inthebottompanel,thehorizontallinerepresentstherange
the likely uncontaminated region within 0.25 h−1Mpc, where ofX-raytemperaturesasgivenintheliteratureforA115N(seeSect.1)
we find σv (cid:10) 750 km s−1. Figure 14 shows a70n enough ro- gtraasnsafnodrmgaeldaxiinesσ,vi.ea.ssβumin=g1th(seedeetnexsitt)y.-energy equipartition between
bustmeanvelocityandasharpincreaseoftheintegralvelocity- spec
dispersionwiththedistancefromtheBCM−Buptoapeakvalue
∼1450 km s−1 at ∼0.3 h−1Mpc. We interpret these features as
70
thecontaminationofthestructuresconnectedwithBCM−A,–C
and–D withhigherandlowermeanvelocities.Theircombina-
tiondoesnotaffectthemeanvelocitybutstronglyincreasesthe
velocity dispersion. In fact, the peak value of the velocity dis-
persion goes down from 1450 to 1200 km s−1 when rejecting
lowvelocitygalaxiesofWGAP1andWGAP2.Wethenconsider
thelikelyuncontaminatedregionwithin0.25h−1Mpc,wherewe
70
findσv (cid:10)900kms−1.
4. 3DstructureandvirialmassofA115
On the basis of the above section we conclude that A115 is
formed by two subclusters well distinct in the sky and centred
aroundBCM−Aand–B,hereafterA115NandA115S,withthe
additionofseverallowvelocitygalaxies,aboveallintheeastern
cluster region,likely organisedin small groups(see Fig. 9 and
thePAofthevelocitygradient).
Asforthelowvelocitygroupswedetecttwo2Dgalaxycon-
centrationsaroundthetwobright,lowvelocitygalaxiesBCM−C Fig.14.ThesameasinFig.13,butreferringtothesouthernsubcluster
and–D.Moreover,wefindtwopossiblegroupsinthelowveloc- A115S centredaround BCM−B.Thedashed lineinthe bottompanel
itytailofthevelocitydistribution(WGAP1containingBCM−D gives the integral velocity dispersion profile when rejecting the low
velocity galaxies of WGAP1 and WGAP2. In the bottom panel, the
andWGAP2).TheDressler–Schectmantestdetectsa substruc-
horizontal linerepresentstherangeofX-raytemperaturesasgiven in
tBuCreMa−roDunfidndthsesoBmCeMs−uCpp,otortoi.nTthheepfarienstenXc-eraoyfeamgirsosiuopnasrhoouwnnd dtheensliittyer-aetnuerregyfoerqAu1ip1a5rStit(iosenebSeetwcte.e1n) tgraasnsafnodrmgeadlaixnieσs,via.es.suβming=th1e
spec
there (see Gutierrez & Krawczynski 2005 and Fig. 2). That (seetext).
we find evidence of a few groups rather than of an individual
R.Barrenaetal.:ThedynamicalstatusofA115 869
massive cluster agrees with the absence of a third X-ray lumi- To compare our results with the estimate recovered from
nouspeak. a X-ray surface brightness deprojection analysis (White et al.
As for A115N, we find comparable kinematical prop- 1997) we assume that each subcluster is described by a King-
erties in the analysis of DS–A and CORE−A, i.e. (cid:8)v(cid:9) >∼ like massdistribution(see above)or,alternatively,aNFW pro-
59000 km s−1 and σv ∼ 800 km s−1. Instead KMM3 is filewherethemass-dependentconcentrationparameteristaken
contaminated by the presence of low velocity galaxies, e.g. from Navarro et al. (1997) and rescaled by the factor 1 + z
IDs 10 and 19 close to BCM−A. As for A115S, we find (cid:8)v(cid:9) ∼ (Bullocketal.2001;Dolagetal.2004).WeobtainMN(<Rout,N =
57500 km s−1 and σv ∼ 900−1000 km s−1 (see KMM2 0.235 h−701Mpc) = (4.2−9.9) ×1013 h−701 M(cid:3)and MS(<Rout,S =
and CORE−B). The velocity dispersion of the two subclus- 0.157 h−1Mpc) = (2.5−5.5) ×1013 h−1 M(cid:3). The first esti-
70 70
ters are roughly comparable to their average X-ray tempera- mate is somewhat smaller and the second one is in agreement
ture as listed in the literature and transformed in σv assuming withthosefoundbyWhiteetal.(1997,seetheirTable3where
tβhspeecde=ns1it2y(-seeneerFgiygse.q1u3ipaanrtditi1o4n).bTethweetewnogsausbacnludstgearsladxiiffese,rib.ey. 5M.0gra×v,1N0(1<3Rho−u1t,NM)(cid:3)=in1o4u.0rc×o1s0m1o3lohg−7y01).M(cid:3)and Mgrav,S(<Rout,S) =
∼2000kms−1 intheLOSvelocity,i.e.almostthreetimesmore As for t7h0e mass of the whole system, the contribution of
thanwhatfoundbyBeersetal.(1983)withonly19clustermem- thelowvelocitygroupsisofminorimportancesincetheylikely
bers.InagreementwithBeersetal.(1983),wefindthatA115S havelowvelocitydispersionandthevirialmassscaleswithσ3.
v
isdynamicallysomewhatmoreimportantthanA115N,whilethe E.g., we estimate that M ∼ 2.4 ×1014 h−1 M(cid:3) for a σv ∼
70
contraryis foundby X-raydata (Jones& Forman1999; White 500kms−1 group(seeWGAP1inTable2).Consideringthepos-
etal.1997;Gutierrez&Krawczynski2005;butseeShibataetal. siblepresenceof,atmost,twooftheseσv ∼500kms−1 groups,
1999).AsalreadysuggestedbyBeersetal.(1983)thepresence a reliable mass estimate of the whole system is then M =
of3C28mightaffectstheX-rayresultsoverestimatingtheX-ray 2.2−3.5×1015 h−1 M(cid:3),inagreementwithrichclustersreported
luminosityandthetemperatureofA115N. 70
in the literature (e.g., Girardi et al. 1998; Girardi & Mezzetti
Although A115 is likely in a phase of interaction (see the
2001).AsmallermassestimateisgivenbyGovonietal.(2001b;
followingsection),thetwomaingalaxysubclustersarestillwell M(<R=2.3 h−1Mpc) = 0.6−1.2×1015 h−1 M(cid:3) centredonthe
detectable and roughly well coincident with the X-ray peaks. 70 70
X-ray centroidin A115N),but notice thatit refersto a smaller
Thus we assume that each subcluster is in dynamical equilib-
clusterregion,i.e.likelyexcludingalargepartofA115S.
rium to compute virial quantities. Hereafter we assume σv =
750−850kms−1 and900−1000kms−1 forA115NandA115S,
respectively. 5. Mergingscenario
Following the prescriptions of Girardi & Mezzetti (2001),
Since A115N and A115S are well detectable and optical and
we assume for the radius of the quasi-virialized region
R2.v6ir−,N2.9=h−01.M17pc×fσorv/AH1(1z5)N=and2.A2−1125.5S,hr−7e01spMecptcivealnyd–Rseveir,tShe=ir Xin-graayttdhaetacluinsdteicraptreioarvtoermyesrigminilga.rHloocwaetivoenr,wAe11a5reNlaiknedlyAl1o1o5kS-
70 subclustersarealreadystartingtointeract,assuggestedbysev-
eq. 1 after introducing the scaling with H(z) (see also Eq. (8)
eral pieces of evidence:the slight displacementbetween peaks
of Carlberg et al. 1997 for R ). Therefore, our spectroscopic
200 ofgasdistributionandofgalaxydistribution(seeourFig.8);the
cataloguesamplesmostofthevirializedregion.
presence of an hotregion likely due to the interaction(Shibata
Onecancomputethemassusingthevirialtheorem(Limber
etal.1999;Gutierrez&Krawczynski2005).Verynoticeably,the
& Mathews 1960; see also, e.g., Girardi et al. 1998) under the
assumptionthatmassfollowsgalaxydistribution: M = M − largest dimension of the radio relic is somewhat perpendicular
SPT,whereM = 3π/2·σ2R /Gisthestandardvirialmsvairss, totheaxisconnectingA115NandA115Sinagreementwithbe-
svir v PV ingoriginatedbyshockwavesconnectedtotheongoingmerger
R a projected radius (equal two times the harmonic radius),
PV (e.g.,Ensslin&Brüggen2002).
and SPT is the surface pressure term correction (The & White
When the mergingscenariois assumed to explainthe pres-
1986). The value of R dependson the size of the considered
PV ence of the hot region located between A115N and A115S, a
region, so that the computed mass increases (but not linearly)
relativecollidingvelocityisnecessarytoheatuptheICMtem-
with the increasing considered region. Since the two subclus-
peraturekT from∼4 to ∼9 keV (see Gutierrez& Krawczynski
tersareinteractingandwedonotcoverthewholevirializedre-
2005).Assumingthatthetwosubclustersaretocauseahead-on
gion we use an alternative estimate which was shown be good
collisionandthattheirkineticenergiesarecompletelyconverted
whenR iscomputedwithinR (seeEq.(13)ofGirardietal.
PV vir to thermal energy, the necessary value of the colliding veloc-
1998).Thisalternativeestimateisbasedontheknowledgeofthe
ity is v = (3k∆T/µm )1/2 (see Shibata et al. 1999). We find
galaxydistributionand, in particular,a galaxyKing-likedistri- coll p
butionwithparameterstypicalofnearby/medium-redshiftclus- vcoll ∼ 1600km s−1 in goodagreementwiththe observedrela-
ters: a core radius R = 1/20 × R and a slope-parameter tiveLOSvelocityintherestframeVr = 1646kms−1 asrecov-
β = 0.8, i.e. thecvoroelume galaxy vdirensity at large radii as eredfromCORE−AandCORE−Bintheclusterrestframe,i.e.
r−fi3tβfit = r−2.4 (Girardi & Mezzetti 2001). For the whole virial- ((cid:8)v(cid:9)CORE−A(cid:8)v(cid:9)CORE−B)/(1+z)).
ized region we obtain RPV,N = 1.6−1.8 h−701Mpc and RPV,S =
1.9−2.2h−1Mpc.AsfortheSPT correction,weassumea 20% 5.1.Bimodalmodel
70
computedcombiningdata on manyclustersone (e.g.,Carlberg
HereweinvestigatetherelativedynamicsofA115NandA115S
et al. 1997; Girardi et al. 1998). This leads to virial masses
MMNS((<<RRvviirr,,SN==22..62−−22..95hh−7−70101MMppcc))==183.0.8−−1118.6.9××11001144hh−7−70101MM(cid:3)(cid:3)afnodr uiNnsetiewnggtroadnliifffaoenrrmegnraatlviasinmtayli(ynet.itgch.ea,pBfprearemorasecwehtoeasrlk.w1oh9fi8clo2hc)a.arlTelyhbeflavsaeatdlsupoeansceoatnfimtehneeearrengldy-
thetwosubclusters.
evant observable quantities for the two-clumps system are: the
2 βspec = σ2v/(kT/µmp) with µ = 0.58 the mean molecular weight relative LOS velocity in the rest frame, Vr = 1646 km s−1 (as
andm theprotonmass. recovered from CORE−A and CORE−B); the projected linear
p
870 R.Barrenaetal.:ThedynamicalstatusofA115
distance between the two clumps, D = 0.89 h−1Mpc (as re-
70
coveredfrom BCM−A and –B cluster rest frame);the mass of
the system obtained by adding the masses of the two subclus-
terseachwithinitsvirialradius,logM = 15.4107+0.0734(see
sys −0.0734
Sect.4).
First, we consider the Newtonian criterion for gravita-
tional binding stated in terms of the observables as V2D ≤
r
2GM sin2αcosα, where α is the projection angle between
sys
the plane of the sky and the line connectingthe centres of two
clumps.Thefaintcurvein Fig.15 separatesthe boundandun-
boundregionsaccordingtotheNewtoniancriterion(aboveand
below the curve, respectively). Considering the value of M ,
sys
the A115N+S system is bound between 20◦ and 84◦; the cor-
responding probability, computed considering the solid angles
(cid:1)
(i.e., 84ocos αdα),is65%.Wealsoconsidertheimplemented
20o
criterionVr2D ≤ 2GMsin2αvcosαd,whichintroducesdifferent
anglesαd andαv forprojectionofdistanceandvelocity,notas-
sumingstrictlyradialmotionbetweentheclumps(Hughesetal.
1995).Weobtainabindingprobabilityof60%.
Fig.15.Systemmassvs.projectionangleforboundandunboundsolu-
Then, we apply the analytical two-body model intro-
tionsofthetwo-bodymodelappliedtoA115NandA115Ssubclusters
duced by Beers et al. (1982) and Thompson (1982; see also
(solidanddottedcurves,respectively,seetext).Thethincurveseparates
Lubinetal.1998,forarecentapplication).Thismodelassumes the bound and unbound regions according to the Newtonian criterion
radialorbitsfortheclumpswith noshear ornetrotationofthe (aboveandbelowthecurve,respectively).Thehorizontallinesgivethe
system.Furthermore,theclumpsareassumedtostarttheirevo- observationalvaluesofthemasssystemanditsuncertainties.
lution at time t = 0 with separation d = 0, and are moving
0 0
apart or coming together for the first time in their history; i.e.
weareassumingthatweareseeingtheclusterpriortomerging acircularKeplerianorbitwiththeorbitalplaneperpendicularto
(atthetimet = 11.106Gyrattheclusterredshift).Thebimodal theLOS,werefertothoseauthors.
model solution gives the total system mass M as a function As for the low velocity groups, we consider a group corre-
sys
ofα(e.g.,Gregory&Thompson1984).Figure15comparesthe sponding to WGAP1 likely centred around BCM−D and pos-
bimodal-modelsolutionswiththeobservedmassofthesystem, sibly interacting with the A115N+S complex. The values of
whichisthemostuncertainobservationalparameter.Thepresent the relevant observable quantities for the two-clumps system
bound outgoing solutions (i.e. expanding), BO, are clearly in- are then: the relative LOS velocity in the rest frame, Vr =
consistentwith the observedmass. The possiblesolutionsspan 3376kms−1 (asrecoveredfromWGAP1andthemeanvelocity
these cases: the bound and present incoming solution (i.e. col- ofCORE−AandCORE−Bclusterrestframe);theprojectedlin-
lapsing),BIaandBIb,andtheunbound-outgoingsolution,UO. eardistancebetweenthetwoclumps,D = 0.89h−1Mpc (asre-
70
For the incoming case there are two solutions because of the coveredfromBCM−DandthemeanpositionbetweenBCM−A
ambiguityintheprojectionangleα.Wecomputetheprobabili- and –B); the mass of the system obtained by adding the mass
tiesassociatedtoeachsolutionassumingthattheregionofMsys of a σv ∼ 500 km s−1 group to A115N+S, i.e. logMsys =
valuesbetweenuncertaintiesareequallyprobableforindividual 15.4427+0.0625. According to the Newtonian criterion for grav-
solutions:P ∼83.5%,P ∼16.5%,P ∼6×10−4%. itational−0b.i0n62d5ing we find a binding probability of 38%. The
BIa BIb UO
Betweenthetwopossibleboundsolutions,α∼21and76de- bimodal model indicates that the group is infalling onto the
grees,thesecondoneisratherunlikelysinceitmeansadistance A115N+S complex with a merging axis intermediate between
of ∼3.7 h−1Mpc between A115N and A115S, i.e. well larger theLOSandtheplaneofsky(i.e.,α∼40−60degrees).
70
thanthevirialradii,whilethesecondsolutionmeansadistance
of ∼1.0 h−1Mpc in agreement with a certain degree of inter-
action. W7h0en assuming α = 21 degrees the present colliding 5.2.Activegalaxies
velocityisverylarge(∼4400kms−1)andtheclustercoreswill Ithasbeensuggestedthatcluster-clustercollisionsmay trigger
crossafter0.08Gyr. starformationinclustergalaxies(Bekki1999;Moss&Whittle
The characterization of the dynamics of A115 using these 2000;Girardi&Biviano2002,andreferencestherein).Caldwell
modelsisaffectedbyseverallimitations.Forinstance,possible &Rose(1997)noticedthatpost-starburstgalaxiesarefrequently
underestimatesofthemasses–e.g.,ifthesubclustersextendout- foundinclusterswithevidenceofpastcollisionevents.Herewe
side the virialradius – lead to bindingprobabilitieslargerthan analysepossiblesegregationsbetweenpassiveandactivegalaxy
thosecomputedabove.Themodelsdonottakethemassdistri- populations.
butioninthesubclustersintoaccountwhentheseparationofthe Out of 85 cluster members we classify 48 galaxies finding
subclustersiscomparablewiththeirsize(i.e.atsmallα)anddo 34 “passive”galaxies(“k” inTable 1)and14“active”galaxies
not consider the possible effect of small, low velocity groups. (i.e., 9 “k+a”/“a+k” and 5 “e” galaxies, respectively),see also
Moreover,the two-bodymodelbreaksdownin aregimewhere Fig.7.
A115NandA115Ssubclustersarealreadystronglyinteracting. Thevelocitydistributionofpassivegalaxiesdiffersfromthat
Finally,thetwo-bodymodeldoesnotconsiderthepossibility ofactivegalaxiesatthe99.47%c.l.accordingtotheKS-test,ac-
of anoff-axismergerassuggestedbytheX-raysurfacebright- tive galaxieshavinga largermean velocity(see Table 2). With
ness distribution(Gutierrez& Krawczynski2005). For an ana- present data this difference seems due to both Balmer absorp-
lytical treatmentwhich assumes that A115Nand A115Sare in tion lines (“k+a”/“a+k”) and emission lines “e” galaxies. This