A New Giant Stellar Structure in the Outer Halo of M31

The Sloan Digital Sky Survey has revealed an overdensity of luminous red giant stars ~ 3 degrees (40 projected kpc) to the northeast of M31, which we have called Andromeda NE. The line-of-sight distance to Andromeda NE is within approximately 50 kpc of M31

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aANewGiantStellarStructureIntheOuterHaloofM31DanielB.Zucker1,AlexeiY.Kniazev1,EricF.Bell1,DavidMart´ nez-Delgado1,EvaK.Grebel1,2,Hans-WalterRix1,ConstanceM.Rockosi3,JonA.Holtzman4,ReneA.M.Walterbos4,ˇZeljkoIvezi´c5,J.Brinkmann6,HowardBrewington6,MichaelHarvanek6,S.J.Kleinman6,JurekKrzesinski6,http://doc.guandang.netmb7,DanLong6,PeterR.Newman6,AtsukoNitta6,StephanieA.Snedden6ABSTRACTTheSloanDigitalSkySurveyhasrevealedanoverdensityofluminousredgiantstars～3 (40projectedkpc)tothenortheastofM31,whichwehavecalledAndromedaNE.Theline-of-sightdistancetoAndromedaNEiswithin～50kpcofM31;AndromedaNEisnotaphysicallyunrelatedprojection.AndromedaNEhasag-bandabsolutemagnitudeof～ 11.6andcentralsurfacebrightnessof～29magarcsec 2,makingitnearlytwoordersofmagnitudemoredi usethananyknownLocalGroupdwarfgalaxyatthatluminosity.Basedonitsdistanceandmorphology,AndromedaNEislikelyundergoingtidaldisruption.AndromedaNE’sredgiantbranchcolorisunlikethatofM31’spresent-dayouterdiskorthestellarstreamreportedbyIbataetal.(2001),arguingagainstadirectlinkbetweenAndromedaNEandthesestructures.However,AndromedaNEhasaredgiantbranchcolorsimilartothatoftheG1clump;itispossiblethatthesestructuresarebothmaterialtorno ofM31’sdiskinthedistantpast,orthatthesearebothpartofoneancientstellarstream.Subjectheadings:galaxies:general—galaxies:evolution—galaxies:stellar

content—galaxies:dwarf—galaxies:inpidual(M31)—LocalGroup

The Sloan Digital Sky Survey has revealed an overdensity of luminous red giant stars ~ 3 degrees (40 projected kpc) to the northeast of M31, which we have called Andromeda NE. The line-of-sight distance to Andromeda NE is within approximately 50 kpc of M31

1.Introduction

Thestudyofdwarfgalaxieso ersspecialinsightintothephysicsofgalaxyformationandevolution.Therearemanyfewerdwarfgalaxiesthanpredictedbyadirectscalingofthedarkmatterhalomassfunction(e.g.,White&Frenk1991).Anumberoftheoreticalexplanationsforthishavebeenexplored:feedbackfromsupernovaeleadingtodisruptionofdwarfgalaxies(e.g.,Dekel&Silk1986)orstrongreductionoftheirluminous/darkmatterratio(e.g.,Hayashietal.2003),reducedsubstructureinwarmdarkmatterhaloes(e.g.,Mooreetal.2000),enhancedsatellitedisruptionandreionization(e.g.,Bullock,Kravtsov,&Weinberg2001),orsuppressionofdwarfgalaxyformationthroughphotoionization(e.g.,Somerville2002).Inthiscontext,thedwarfgalaxycontentoftheLocalGroupplaysaspecialrole.Theluminosityfunctioncanbeprobeddowntoveryfaintlimits,wherethediscrepancieswiththemodelsarelargest(Mooreetal.1999).Furthermore,thedetailedstarformationhistoriesofthedwarfscanbeestimatedthroughanalysisofthecolorsandmagnitudesoftheirresolvedstellarpopulations.Coupledwiththeexquisitelydeepsurfacebrightnesslimitsachievablethroughanalysisofresolvedstellarpopulations,onecanexplorethephysicalprocesseswhichshapedwarfgalaxyevolutiondirectly,suchasrampressurestripping(e.g.,Grebel,Gallagher,&Harbeck2003)ortidaldisruptionbythegiantgalaxiesintheLocalGroup(e.g.,Ibata,Gilmore&Irwin1994).

Nevertheless,constructingacompletecensusoffaintstellarstructures(dwarfgalaxies,disrupteddwarfs,stellarstreams)hasprovedchallenging,owingtothetwinconstraintsofimagingdepthandlargeareacoverage(see,e.g.,Armandro ,Jacoby,&Davies1999;Fergusonetal.2002).Inthisletter,wereportthediscoveryofafaint,extendedstellarfeatureintheouterhaloofM31,usingaspecialscanfromtheSloanDigitalSkySurveywhichreachesdepthssu cienttodetectstarsatthetipoftheredgiantbranch(TRGB).Weusethesedatatoestimatethefeature’sphysicalpropertiesandtotrytodetermineitstruenature.WeassumeadistancetoM31of760kpc(vandenBergh1999).

2.ObservationsandDataAnalysis

TheSloanDigitalSkySurvey(SDSS)(Yorketal.2000)isanimagingandspectroscopicsurveythatwilleventuallycoverabout1/4ofthesky.Drift-scanimagingfromtheSDSS

2.5mtelescopeinthe veSDSSbandpasses(u,g,r,i,z)(Fukugitaetal.1996;Gunnetal.1998;Hoggetal.2001)isprocessedthroughdatareductionpipelinestomeasurephotometricandastrometricproperties(Luptonetal.2002;Stoughtonetal.2002;Smithetal.2002;Pieretal.2003)andtoidentifytargetsforspectroscopy.M31wasobservedon5October2002,duringtimewhenprimarysurveyareaswereinaccessible.Thedatawereobtainedintwo

The Sloan Digital Sky Survey has revealed an overdensity of luminous red giant stars ~ 3 degrees (40 projected kpc) to the northeast of M31, which we have called Andromeda NE. The line-of-sight distance to Andromeda NE is within approximately 50 kpc of M31

overlappingstripsorientedalongthemajoraxisofM31,coveringanareaof～18 ×2 .5.Conditionswerephotometric,buttheseeingvariedbetween1′′ 2′′FWHMinr-band.TherawdatawereprocessedandcatalogsconstructedusingthesamepipelineasDataRelease1(DR1)(Abazajianetal.2003).Weselectedonlyobjectswithinthecoordinaterangeassignedtoeachstrip,thuseliminatingduplicateobjectdetections.Inthefollowing,allreferencestodereddeningandconversionfromSDSSmagnitudestoV,Imagnitudes(forliteraturecomparison)utilizeSchlegel,Finkbeiner,&Davis(1998)andSmithetal.(2002),respectively.

ThespatialdensityofSDSS-detectedstarswith21<i≤22isshowninFig.1.ThecentralregionsofM31,itsdisk,anditsbrightsatellitesM32andNGC205aretoocrowdedforthestandardSDSSphotometricpipeline.Theslighthorizontal“striping”istheresultofseeingvariations;underbetterseeingconditions,morestarsweredetected.Thise http://doc.guandang.netplexstellarsubstructureisevidentintheouterdiskandhaloofM31(Fig.1a).EssentiallyallofthemorphologicalfeaturesvisibleinFigures2and3ofFergu …… 此处隐藏：19000字，全部文档内容请下载后查看。喜欢就下载吧 ……