Galaxy Population in a Cluster of Galaxies around the Radio Galaxy 3C 324 at $z=1.2$

摘要

Wediscussthepropertiesofgalaxiesaroundtheradiogalaxy3C324at$$z=1.2$$basedon$$BVRIK^prime$$multi-bandimagingdata.Wehaveappliedaphotometric-redshifttechniquetoobjectsinthe3C324field,andidentified35objectsasplausibleclustermembers.Wehavefoundthatredandluminousmembersareconcentratedinasmallregionenclosedbyacircleof40$$^prime$$radius(0.33Mpcat$$z=1.2$$for$$Omega_0=0.3,lambda_0=0.7,H_0=70,mathrm{km},mathrm{s}^{-1},mathrm{Mpc}^{-1}$$cosmology)fromthe3C324galaxy.The3C324clusterisprobablymuchmorecompactinsizecomparedwiththelocalclusters.Weconstructeda$$K^prime$$-bandluminosityfunctionoftheclustermembersandfittedaSchechterfunction,andfoundthecharacteristicmagnitudetobe$$K^{astprime}_mathrm{AB}=20.2pm0.6$$.Thisvalueisconsistentwiththeextrapolationofthepurepassiveevolutionseenfor$$zlt1$$clusters.Wehaveidentifiedeightbrightgalaxieswhichformaredcolora€“magnitudesequence.Theslopeofthesequenceisconsistentwiththepassiveevolutionmodeldownto$$K^{prime}_mathrm{AB}lt22$$;wealsofoundthatthereisnoclearagevariationinthesebrightredgalaxies.However,sevenoutoftheseeightgalaxiesexhibitasignificantexcessintherestUVlightwithrespecttothepassiveevolutionmodel.Thismaysuggestthatthemassiveearly-typegalaxiesinthishigh-redshiftclusterarestillformingstarstosomeextent.Wehaveconfirmedatruncationofthecolora€“magnitudesequenceat$$K^{prime}_mathrm{AB}sim22$$;faintpassively-evolvinggalaxiesmaynotyetbepresentinthisclusterat$$zsim1.2$$.Theoverallcolordistributionoftheclustermembers,selectedbythephotometricredshifttechnique,isfoundtobeverybroad.WederivedthefractionofbluegalaxiesinthisclusterfollowingadefinitionofButcherandOemler(1984,AAA038.160.068),andobtained$$f_mathrm{B}=0.39pm0.28$$,whichishigherthanthatfor$$zlt1$$clusters.Thisindicatesthatthestar-formationactivityofthisclusteris,ontheaverage,higherthanthatoflowerredshiftcounterparts.