A&A479,335–346(2008) Astronomy DOI:10.1051/0004-6361:20077723 & (cid:1)c ESO2008 Astrophysics The galaxy luminosity function of the Abell 496 cluster (cid:1) and its spatial variations G.Boué1,C.Adami2,F.Durret1,3,G.A.Mamon1,4,andV.Cayatte5 1 Institutd’AstrophysiquedeParis(UMR7095:CNRS&UniversitéPierreetMarieCurie),98bisBdArago,75014Paris,France e-mail:boue@iap.fr 2 LAM,TraverseduSiphon,13012Marseille,France 3 ObservatoiredeParis,LERMA,61Av.del’Observatoire,75014Paris,France 4 ObservatoiredeParis,GEPI(UMR8111:CNRS&UniversitéDenisDiderot),61Av.del’Observatoire,75014Paris,France 5 ObservatoiredeParis,sectionMeudon,LUTH,CNRS-UMR8102,UniversitéParis7,5Pl.Janssen,92195Meudon,France Received26April2007/Accepted27November2007 ABSTRACT Context.Thefaintendslopesofgalaxyluminosityfunctions(LFs)inclustersofgalaxieshavebeenobservedinsomecasestovary withclustercentricdistanceandshouldbeinfluencedbythephysicalprocesses(mergers,tides)affectingclustergalaxies.However, thereisawidedisagreementonthevaluesofthefaintendLFslopes,rangingfrom−1to−2.3inthemagnituderange−18 21), ourstarcountsdecrease,whiletheBesançonmodelcountskeep 17 18 19 20 21 22 23 increasing.Thiscouldbeduetoasteeperfallofthestellarden- sity distribution in the direction of Abell 496, in comparison r’ with what is in the Besançon model. Alternatively, we may be Fig.2. Comparison of star counts obtained with the Besançon model wronglyclassifyingstarsasgalaxiesatr(cid:3) > 21,andatr(cid:3) = 22 (Robinetal.2003)(orangeshadedregion)andfromtheAbell496im- we may be overestimating the galaxy counts in the Abell 496 age(blacksolidhistogram)usingourstar-galaxyseparation. field.Ifweadoptthestar countsfromtheBesançonmodel,we wouldendupwith114,156and343fewergalaxiesatr(cid:3) =21.5, 22.0and22.5,respectively.However,iftherewereasmanystars all saturated stars, spikes and CCD edges. Galaxies and stars as predicted by the Besançon model, then, for 22 < r(cid:3) < 22.5 brighter than r(cid:3) = 18 occupy less than 2% of the pixels of our (seeFig.1),thedistributionofstellarhalflightradiiwouldrise image. to a maximum(nearr = 2.3pixels),falltoa minimum(near The final catalogue will be electronically available at the 50 2.6pixels),thenriseagain(tothelimitof2.8pixels).Itisdiffi- following address: http://cencosw.oamp.fr/and in a few culttounderstandwhatwouldcausethisfinalrise.Therefore,in monthstheimageswillbeavailableatthesameaddress. whatfollows,weadoptourownestimationofthestarcounts. We also corrected the magnitudes for Galactic extinc- 2.2.CFHTLScomparisonfielddata tion based on the Schlegel et al. (1998) maps. We finally computed the useful covered area (cf. Table 1), by masking We used the CFHTLS Deep (D1, D2, D3 and D4, i.e. 4 MegaCamfields) and Wide (W1, W2 andW3, 59 MegaCam 2 WemodifiedSkyMakertohandleSérsicprofilesratherthanjustthe fields)ascomparisonfielddata.BecausetheLFsarecomputed deVaucouleursprofile. by subtraction of the average of the 4 Deep fields (DFs) from 338 G.Bouéetal.:GalaxyluminosityfunctionofAbell496 Abell 496 analytical approximation 0.14 deep 1 u* 0.6 g’ 0.12 r’ i’ 0.4 0.1 ’-i’) 0.2 mag) 0.08 r r’)-( 0 σ ( 0.06 - ’ g ( 0.04 -0.2 0.02 -0.4 0 15 16 17 18 19 20 21 22 23 -0.6 mag 0 0.5 1 1.5 2 r’-i’ Fig.4.Magnitudeuncertaintiesestimatedfromoverlappingareasinthe 19CFHTLSW1fields(dottedanddashedcurves)withtheiranalytical Fig.3.Colour-colourdiagramfor17 −14) that distancetoasliceof45MpcattheredshiftofAbell496.Onthe ishigherthanorcomparabletothatofthecentralregion.Ifthis planeofthesky,welimitedoursearchtoaboxof17×17deg2 is not caused by field contamination (see Sect. 6.3), then one (40Mpcattheclusterredshift).Thissizeistypicalofthelargest wouldconcludethatthefaint(Mr(cid:3) > −14)galaxiesdonottrace cosmicbubbles(Hoyle&Vogeley2004). thecluster,whichpresentsnosurfacedensityenhancement. Figure 10shows thatthe 6dFGSgalaxiesin the neighbour- Although they are both contained within the virial radius hood of Abell 496 are more concentrated along a strip in the of the cluster, the two external regions present LFs differing southeast/northwestdirection.Thisisconfirmedbythedistribu- from one another. The Southern region (red) is still quite pop- tionofgroupsandclustersthatwefoundinNED(whichismuch ulated comparedto the greenregion,which is sometimesquite lesshomogeneous)usingthesamesearchcriteria.Notethatthe poor with LFs often not significantly positive. Since there are 6dFGS-DR3iscompletetoK ≤12.65(Jonesetal.2008)down noclustersorgroupsknownnearby(assearchedwithNED,see togalacticlatitude|b|>10◦,asndinourzone,wehaveb<−24◦. Fig.10),thissuggeststheremaybematterintheSouthernregion ThegalaxiesdisplayedinFig.10areclearlydistributedalonga infalling from the surroundingcosmologicalweb, as discussed large-scalefilament,whichappearstobeatleast30Mpclong. inthenextsection. We tested the prominence of this filament in the following manner.Wesearchedfortherectangleoflength15◦ (36Mpcat 5. LargescalefilamentintheAbell496 thedistanceofAbell496)andwidth2◦encompassingthelargest amountofgalaxies,imposingthatAbell496liesalongthelong neighbourhood axisoftherectangle,within4◦ fromtheclosestedgealongthat Abell496has beenshownto be a noticeablyquiescentand re- axis.Wefoundthatthefurthestedgealongthelongaxisisatpo- laxed cluster (e.g. Durret et al. 2000). The only sign of sub- sitionangle(PA)137◦ anti-clockwisefromnorth.Wethenbuilt structurefoundwasanenhancedconcentrationofemissionline 1000randomsamplesofasmany(487)galaxiesintheframeof galaxiesinthenorthwest.FromFig.8,wealsoseethatthesouth- Fig. 10 as observed, and checked for the most populated rect- ernpartofthesurroundingareaaroundtheclustershowsasig- angle,definedasabove,covering360PAsinstepsof1◦.While nificantgalaxypopulation,withanLFthatresemblesthatofthe thefilamentintheobserveddatasethas221galaxieswithinthe centralcluster. rectangle,noneofthe1000randomdatasetseverreachedmore We searched the Six degree Field Galaxy Survey than81galaxies.Therefore,thefilamentatpositionangle137◦ (6dFGS-DR3) database (Jones et al. 2004, 2005, 2008) for ishighlysignificant.Thisfilamentshouldconstituteapreferen- nearbygalaxiesin thelarge-scaleneighbourhoodofAbell496, tialavenueforinfallingmaterialintoAbell496,aswellasback- ina±0.005redshiftslicearoundthemeanvalueof0.033.This splashingmaterialfromthecluster.However,thisfilamentdoes slice corresponds to ±1.5 times the velocity dispersion of a notfullyexplaintheexcessofgalaxiesinthesouthernregionof 342 G.Bouéetal.:GalaxyluminosityfunctionofAbell496 Abell496sinceitisinclinedrelativelytothenorth-southdirec- (e.g.,Pracyetal.2005).Finally,tidaleffectsfromtheclusterpo- tion. tential are the same,to firstorder,ongiantanddwarfgalaxies. Given the trend that low luminosity ellipticals are less concen- tratedthanmoreluminousones(Grahametal.2001),thelatter 6. Discussionandconclusions willsurvivebetterthestrongtidesneartheclustercentre,which shouldmaketheLFshallower,notsteeper.Theonlypossibleex- 6.1.ComparisonwithpreviousanalysesofAbell496 planationforasteeperfaint-endslopeinclusterswouldbethat TheLFofAbell496waspreviouslymeasuredbyMolinarietal. galaxiesofmoderatelylowluminosityaretidallyfragmentedby (1998), who analysed the cluster in 4 small fields, one includ- theclusterpotentialorbycloseencounters. ingtheclustercentre,andbyDurretetal.(2002),whomeasured Now,ifthefieldcountsintheclusterfieldareunderestimated thLFina42(cid:3)×28(cid:3) fieldinthe I band.Thefaint-endslopesof (because the reference fields are slightly underdense), the re- −1.69±0.04inrand−1.49±0.04inifoundbyMolinarietal. sultant net cluster counts will be highly contaminated by field areconsistentwithourslopesof−1.62±0.05and−1.52±0.05 counts. Writing the field counts as dN/dm = dex[β(m− m )] 0 in thesebands.Moreover,theirfaint-endslopeof−1.34±0.04 andtheclusterfaint-endluminosityfunctionasΦ(L) ∝ Lα,itis ingisprobablyconsistentwithourslope(−1.53±0.08),given easy to show that if the field dominates the net cluster counts, thattheirestimatederrorneglectscosmicvariance,which,aswe one will end up measuring α = (−β/0.4)−1. For example, if showinFig.5,ismanytimesgreaterthanthePoissonvariance. β = 3/5 (Euclidean counts), one would measure α = −5/2, Durret et al. (2002)find a slope of −1.79±0.01 in I, whereas while if β (cid:6) 0.36(asin the DFsin the magnituderangewhere wefindhereaslopeof−1.52±0.05.Givenouranalysisofcos- wearemeasuringthefaint-endslopeoftheLF),oneshouldfind micvariance,weestimatetheerrorfromcosmicvarianceonthe α=−1.9. slopeofDurretetal.(2002)toberoughly0.08.Ifthiserrores- Weareconfidentthatwesufferlittlefromabackgroundcon- timate is correct, then the difference in faint-end slopes would taminationofourLFs.Indeed,thefaint-endslopesthatwehave bestatisticallysignificant.Thisdifferencemaybecausedbythe computedfortheglobalLF(Table2)andforthecentralregion differentregionsprobedinthetwostudiesandthedifferentref- ofAbell496(Table2)areallconsiderablyandsignificantlyshal- erencefieldsused. lower than α = −1.9(exceptin the less sensitive u∗ band).We alsotookgreatcarenottounderestimatethecountsinourfield count process by the use of very homogeneousfield data cov- 6.2.Comparisonofradialtrendswithotherclusters eringalargeenoughfieldofviewinordertotreatproperlythe Our g(cid:3), r(cid:3) and i(cid:3) LFs exhibit slopes that increase significantly cosmic variance (as described above). Finally the selection in from the centre outwards (Table 2). This agrees with the trend colourthatwehaveappliedeliminates(bothintheclusterfields for flatter LFs aroundcluster cDs foundby Lobo et al. (1997), andinthefieldsusedforstatisticalsubtraction)veryredobjects with the steeper slope in the outer envelope of Coma found that are veryunlikely to belong to the cluster, so the field sub- by Beijersbergen et al. (2002), and with the greater dwarf-to- tractionisquiteconservativeandthereforesecure. giant ratio found by Driver et al. (1998a). Note, however, that The MegaCam colour redshift diagrams derived by Ilbert all three trends are either qualitative or of marginal statistical et al. (2006)from the VIMOS VLT Deep Survey (VVDS) fol- significance.Althoughourradialtrendisoppositetothatfound lowupoftheCFHTLSDFshowthatgalaxyspectraareshiftedto by DeProprisetal.(2003)in 2dFGRS clusters, their shallower theredtosuchanextentthat,intheredshiftrange0.4

The galaxy luminosity function of the Abell 496 cluster and its spatial variations

Similar The galaxy luminosity function of the Abell 496 cluster and its spatial variations