The Virgo stellar over-density Mapping the infall of the Sag

The recently discovered Virgo stellar over-density, which expands over \~1000deg^2 perpendicularly to the Galactic disk plane (7< Z <15 kpc, R~7 kpc), is the largest clump of tidal debris ever detected in the outer halo and is likely related with the accre

DraftversionFebruary5,2008

APreprinttypesetusingLTEXstyleemulateapjv.10/09/06

THEVIRGOSTELLAROVER-DENSITY:MAPPINGTHEINFALLOFTHESAGITTARIUSTIDALSTREAM

ONTOTHEMILKYWAYDISK

arrubia3,4,MarioJuric´5,6,EmilioJ.Alfaro2,ZeljkoIvezic´5DavidMart´ nez-Delgado1,2,JorgePen

DraftversionFebruary5,2008

arXiv:astro-ph/0609104v2 31 Jan 2007

ABSTRACT

TherecentlydiscoveredVirgostellarover-density,whichexpandsover～1000deg2perpendicularlytotheGalacticdiskplane(7<Z<15kpc,R～7kpc),isthelargestclumpoftidaldebriseverdetectedintheouterhaloandislikelyrelatedwiththeaccretionofanearbydwarfgalaxybytheMilkyWay.WecarryoutN-bodysimulationsoftheSagittariusstreamtoshowthatthisgiantstellarover-densityisacon rmationoftheoreticalmodelpredictionsfortheleadingtailoftheSagittariusstreamtocrosstheMilkyWayplaneintheSolarneighborhood.Radialvelocitymeasurementsareneededtocon rmthisassociationandtofurtherconstraintheshapeoftheMilkyWaydarkmatterhalothroughanewgenerationoftheoreticalmodels.Iftheidenti cationofVirgoover-densityandtheSagittariusleadingarmiscorrect,wepredicthighlynegativeradialvelocitiesforthestarsofVirgoover-density.ThedetectionofthisnewportionoftheSagittariustidalstreamwouldrepresentanexcellenttargetfortheon-goingandfuturekinematicsurveysandfordarkmatterdirectdetectionexperimentsintheproximityoftheSun.

Subjectheadings:galaxies:individual(Sagittarius)—galaxies:interactions—Galaxy:structure—

Galaxy:halo

1.INTRODUCTION

Inthelastdecade,variouslargescaleCCDsurveys(SloanDigitalSkySurvey,2MASS7,QUEST)haveforthe rsttimefoundstrongevidenceforthepresenceofasigni cantamountofsubstructureinthehalooftheMilkyWayinformoflongtidalstreamsorstellarclumps(Newbergetal.2002;Majewskietal.2003).Thesehavebeeninterpretedastheremnantsoflatestmergersoccur-ringasapartofthehierarchicalbuild-upofourGalaxy.Inaddition,N-bodysimulationsofthesemergerevents(Helmi2004;Mart´ nez-Delgadoetal.2004a;Law,John-ston&Majewski2005;Pe narrubiaetal.2006)havebeenextensivelyusedto, rstly,associatethetidalde-briswiththeirprogenitorgalaxies,secondly,determinethedynamicalhistoryoftheprogenitorsatellitesand,thirdly,constrainthedistributionofdarkmatterintheMilkyWay.TheultimategoalofthesestudiesistoinfertheformationprocessoftheMilkyWayfromtheprop-ertiesofthepresentfossilrecords.

Muchoftheobservationalandtheoreticale orthasfocusedspeciallyonthetwolargesttidalstreamsdis-coveredsofar:thetidalstreamoftheSagittarius(Sgr)dwarfgalaxy,whichwrapsaroundtheGalaxyinahighlyinclinedorbitwithrespecttotheMilkyWaydisk(Ibataetal.2001;Mart´ nez-Delgadoetal.2001;Majewskietal.2003)andtheMonocerostidalstream(Yannyetal.2003;Ibataetal.2003),alow-latitudetidalstreamwhoseprogenitor,possiblythecontroversialCanisMajor

InstitutodeAstrof´ sicadeCanarias,LaLaguna,SpainInstitutodeAstrof´ sicadeAndaluc´ a(CSIC),Granada,Spain3Max-PlanckInstitutefuerAstronomie,Heidelberg,Germany4DepartmentofPhysicsandAstronomy,UniversityofVictoria,VictoriaBC,V8P1A1,Canada

5DepartmentofAstrophysicalSciences,Princeton,NJ08544USA

6InstituteforAdvancedStudy,SchoolofNaturalSciences,Ein-steinDrive,Princeton,NJ085407TwoMicronAllSkySurvey

12

dwarfgalaxy(Martinetal.2004;Bellazzinietal.2004;Mart´ nez-Delgadoetal.2005),movesonaprograde,nearlycircularorbit(Pe narrubiaetal.2005),accordingtotherecentpropermotionsmeasuredbyDinescuetal.(2005).Additionally,Rocha-Pintoetal.(2004)reportedthediscoveryofanewpossibletidalstreamintheTrian-gulum/Andromedaconstellations,althoughsubsequenttheoreticalsimulationsidenti edthistidaldebrisasapartofamoredistant,metalpoorwrapoftheMono-cerostidalstreamprogenitor(Pe narrubiaetal.2005;Mart´ nez-Delgadoetal.2007,inpreparation).

Inadditiontothesetwolargetidalstreams,theanalysisoflargesamplesofstellartracersofhalosub-structure(RRLyrae,F-typeturno stars)haverevealedthepresenceofnumerousstellarover-densitiespossiblyassociatedwithunknownmergereventsintheouterhalo.IntheVirgoconstellation,severalgroupshavereportednumerousdetectionsofstellarover-densitiesusingdi erenttracers:

(i)Themostconspicuousoneisadi useconcentrationofmetalpoor,oldstarssituatedat～20kpcfromtheSuninthedirectionoftheVirgoconstellation, rstdetectedbymeansoftheidenti cationofitsassociatedmain-sequenceturno (MSTO)inacolor-magnitudediagram(CMD)ofaSDSSslice(namedS297+63-20.0;Newbergetal.2002).Additionally,Vivas&Zinn(2003)reportedaclumpof21RRLyraestarslocatedat～20kpcfromthesuninthesameregionofthesky(seealsoIvezicetal.2005andVivas&Zinn2006).Thisover-density(namedthe12h.4clump)extendsoveranareaoftheskyof15 ×1 (correspondingtotoaspatialsizeof5.3kpc×0.3kpcatD 20kpc)anddisplayaline-of-sightdepthofonly 1.5kpc(Vivas2002),whichisconsiderablysmallerthanthatofasimilarRRLyraeover-densitydetectedattheapocenteroftheSgrstream(～4kpc;Vivas2002).Owingtoitspossibletidalorigin,theydenoteditastheVirgostellarstream(VSS).Mart´ nez-Delgadoetal.(2004a)suggestedthatthe

The recently discovered Virgo stellar over-density, which expands over \~1000deg^2 perpendicularly to the Galactic disk plane (7< Z <15 kpc, R~7 kpc), is the largest clump of tidal debris ever detected in the outer halo and is likely related with the accre

2Mart´ nez-Delgadoetal.

VSSmightberelatedtothecomplexdebrisstructureoftheSgrstreampredictedbythetheoreticalmodelsinthisregionofthesky.However,spectroscopicfollow-upofasampleoftheseRRLyraestarsbyDu auetal.(2006)showedafairlynarrowvelocitydistributionthatpointedtoaprogenitorgalaxylessmassivethantheSgrdwarfand,therefore,ingSDSSphotometricdata,theseauthorsestimatedthattheVSScoversroughlyanareaof106deg2inthesky.

(ii)Morerecently,Juri´cetal.(2006)detecte …… 此处隐藏：5684字，全部文档内容请下载后查看。喜欢就下载吧 ……