Sub-Structures in the Halo of the Milky Way

The latest results of the QUEST survey for RR Lyrae stars are described. This survey, which is designed to find and characterize sub-structures in the halo of our Galaxy, has covered about 700 square degrees of the sky and has detected 693 RR Lyrae stars,

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002 yaM 81 1v4745060/hp-ortsa:viXraToappearin“XILatin-AmericanRegionalIAUMeeting(2006)”RevMexAA(SC)

SUB-STRUCTURESINTHEHALOOFTHEMILKYWAY

A.KatherinaVivas1,RobertZinn2,YolimarSubero1andJes´usHern´andez1

RESUMEN

Sedescribenlosu´ltimosresultadosdelab´usquedaQUESTdeestrellasRRLyrae.Estab´usqueda,lacualfue

dise nadaparaencontrarycaracterizarsub-estructurasenelhalodenuestraGalaxia,hacubiertoaproximada-mente700gradoscuadradosdelcieloydetectado693estrellasRRLyrae,lamayor´ http://doc.guandang.netdistribuci´onespacialdelasestrellasRRLyraehareveladovariosgruposintere-santesenelhalo.Algunosdeellosparecenestarasociadosconladestrucci´ondegalaxiasesferoidalesenanas(lascolasdemareadeSagitario,lacorrienteestelardeVirgo,elanillodeMonoceros)oc´umulosglobulares(Palomar5),mientrasqueotrosgrupostienentodav´ aunorigendesconocido.

ABSTRACT

ThelatestresultsoftheQUESTsurveyforRRLyraestarsaredescribed.Thissurvey,whichisdesignedto ndandcharacterizesub-structuresinthehaloofourGalaxy,hascoveredabout700squaredegreesoftheskyandhasdetected693RRLyraestars,mostofwhicharenewdiscoveries.ThespatialdistributionoftheRRLyraestarsrevealsseveralinterestinggroupsinthehalo.Someofthemappeartoberelatedtopreviousdetectionsofthedestructionofdwarfspheroidalgalaxies(theSagittariustidalstreams,theVirgostellarstream,theMonocerosring)orglobularclusters(Palomar5),whileothersstillhaveanunknownorigin.

KeyWords:GALAXY:STRUCTURE—GALAXY:HALO—STARS:VARIABLES:OTHERS

1.INTRODUCTION

Overthepastfewyears,severaldetectionsofde-brisfromdisruptedgalaxieshavebeenmadebysur-veysofthehalooftheMilkyWay(Newbergetal.2002;Majewskietal.2003;Yannyetal.2003;Vivas&Zinn2006,amongothers).SomeofthemaredescribedelsewhereinthisVolume(seecontri-butionsbyS.Du auandH.Rocha-Pinto).Thede-brisisusuallyobservedasanexcessinthenumberdensityofdi erentstellartracers(e.g.Mgiants,F-typemain-sequencestars,horizontalbranchstars),and/orasgroupsofstarswithsimilarkinematicproperties.TheseresultsindicatethatthedensitydistributionofthestarsintheGalactichaloisnotsmoothlyvarying,aswasoncebelieved.Ancientmergerswithsmallsatellitegalaxiesseemtohaveleftasignatureofintricatestreams,aspredictedbysometheoreticalmodels(Bullock,Kravtsov&Weinberg2001;Bullock&Johnston2005).Somesmallsub-structuresmayhavebeenproducedinsteadbythedestructionofglobularclusters(Odenkirchenetal.2003;Grillmair&Dionatos2006).To ndthosetidalstreamsandcharacterizetheirstellarpopu-lationsareimportantfortheunderstandingof

the

The latest results of the QUEST survey for RR Lyrae stars are described. This survey, which is designed to find and characterize sub-structures in the halo of our Galaxy, has covered about 700 square degrees of the sky and has detected 693 RR Lyrae stars,

2VIVASET

AL.

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Right Ascension (deg)

Fig.1.RegioncoveredbytheQUESTsurveyforRRLyraestarsinequatorialcoordinates.

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Galactic Longitude

Fig.2.RegioncoveredbytheQUESTsurveyingalacticcoordinates.

foundinVivasetal.(2004).

2.1.ObservedRegion

Todate,wehaveobservedandanalyzed～700squaredegreesofthesky,intwostripsofconstantdeclinationnearthecelestialequator.TheregionobservedisshowninFigures1and2inequatorialandgalacticcoordinatesrespectively.SinceourmaingoalistostudytheGalactichalo,weskippedare-gionnearthegalacticplane(aroundRA～7hatthisdeclination).Nonetheless,thesurveycoversawiderangeingalacticlatitude.Thedatacorrespondingtothestripcenteredonδ= 1 hasalreadybeenpublished(Vivasetal.2004).

2.2.KnownProblems

Thecustom-madeautomatedsoftwareforthede-tectionofobjectsintheQUESTdatahasnocapa-bilitiesfordeblendingcloseobjects.Wehavedis-coveredthatblendedobjectsmayappearvariablebecauseofseeingvariationsandthatseveralofthestarsinthe rstQUESTcatalogarenotrealRRLyraevariablesbutblendedobjects.Mostoftheseblendsoccurredintheregionsofthelowestgalac-ticlatitudewherecrowdingismoresevere,whichisalsowherethefewestnumberofobservationshadbeenmade.Withrelativelyfewepochsavailableforeachstar,therewasagreaterchancethatafakevariablemightmimicthelightcurveofaRRLyraestarandthereforebeincludedintheQUESTcata-log.Vivas&Zinn(2006)haveeliminatedallknownblendsfromthecatalog.Athighgalacticlatitudes,only1%ofthevariableswerecasesofblendedim-ages.

Anothersourceofcontaminationinthe rstcata-logwasfoundafterspectroscopicfollow-up.Wehavefoundafewcaseswherethespectraoftypecstarsindicatemuchcoolerthanexpectede ectivetemper-atures.Typecstarsarealwayslocatedinthebluesideoftheinstabilitystrip.TheseunusuallycooltypecvariablesareprobablyinsteadvariablebluestragglersorWUMaeclipsingvariables.Again,thistypeofcontaminatingobjectappearsmostlyatlowgalacticlatitudes(seeVivasetal.2004),andonlyfortypecstarswhicharelesscommonthanthetypesab.

3.SPATIALDISTRIBUTIONOFRRLYRAESIN

THEHALOFigures3showsthespatialdistributionoftheQUESTRRLyraestarsintheregionswhereasingledeclinationstripwassurveyed.Thus,theobservedregionisa2.2degwidestripofthesky.TheradialaxisintheseplotscorrespondtoextinctioncorrectedVmagnitudes,whichisequivalenttodistancefromtheSunforRRLyraestars.ThebrightandfaintlimitsofthesurveyenablethedetectionofRRLyraestarsfrom～4and～60kpcfromtheSun.ThespatialdistributionoftheRRLyraestars(seealsoFigure4)showsthatthehalodoesnothaveasmoothdistribution.Severalsub-structuresarepresent.

Inparticular,theSagittarius(Sgr)tidalstreamsareseeninFigure3.TheplotintherightshowsSgrstreamparticlesfromthenumericalsimulationsbyLaw,Johnston,&Majewski(2005)inexactlythesameregionoftheskyasourobservations.Averystr …… 此处隐藏：9659字，全部文档内容请下载后查看。喜欢就下载吧 ……