The Virgo stellar over-density Mapping the infall of the Sag(2)

Sgrgalaxydebris.Inordertoexploreapossibleassocia-tionoftheVSSandVODwiththeSgrtidalstream,we

shallcomparetheirderivedspatialandkinematicprop-ertieswiththoseobtainedfromup-to-datetheoreticalmodels.

OurGalaxymodelconsistsofaMiyamoto-Nagai(1975)disk,aHernquist(1990)bulgeandaNavarro,Frenk&White(1996)darkmatterhalo(hereafter,NFWhalo).Thegravitationalpotentialofeachofthosecom-ponentsincylindricalcoordinatesis:

ΦGMd(r)=

R2+(a+

√

d

r+c

,(2)Φh(r)=GMh

rs) rvir

2rs

×

(3)

∞

m(u)

(1+u)

2

r2(1+u+

zs

)ln(1+r/rs)

ln(1+

rvir

rs+rvir

r/r.

s

FollowingJohnstonetal.(1999)we xthediskandbulgeparametersasM5kpc,bd=1.0×1011M⊙,M=0.26kpcandc=0.7kpc.b=3.4×1010M⊙,a=6.TheMilkyWayhaloparametersatz=0weretakenfromKlypin,12Zhao&Somerville(2002)beingMrh=1.0×10M⊙,concentrationvir=258kpc,ratthepresents=21.5kpc,whichleadstoaepochofc=rAccordingtotheresultsofPe narrubiaetal.vir/r(2006),s=12.theevolutionofthehostGalaxypotentialisnotre ectedinthepresentpropertiesoftidalstreams,sothatweuseastaticMilkyWaypotentialforsimplicity.

Theactualdisruptionofsatellitesismodelledby‘live’(i.e.self-consistent,self-gravitating)N-bodyrealiza-tionsofaKing(1960)model,withadimensionlesscentralpotentialW≡log0=4,orconcentrationparameterc10(rt/rk) 0.84,whererkandrtaretheKingandtidal5radii,respectively.OursatellitemodelshaveN=10particles.Theinitialand nal(i.epresent)massesareMs(t0)=109M⊙andMs(tTheinitialKingandtidalf)=5×108M⊙,respec-tively.radiusarerk(t0)=0.58kpcandrt(t0)=4.01kpc.

Theequationofmotionforeachsatelliteparticleis:

d2ri

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

TheoriginofremnantsinVirgo

3

whereΦsistheself-gravitationalpotentialofthesatellitegalaxyandi=1,...,N.Weusesuperbox(Fellhaueretal.2000)tocalculateΦaleap-frogschemesateachtime-stepandsolveEq.6throughwithaconstanttime-stepof t=0.65Myr,whichisabout1/100ththesatellite’sdynamicaltime.

NotethattheGalacticpotentialisstaticand,therefore,Eq.6doesnotimplementtheresponseoftheMilkyWaytothepresenceofSgr.Wealsoneglectthee ectsofdynamicalfrictionontheSgr’sorbit(assuggestedbyLawetal.2005).

WehavereproducedtheN-bodymodelspresentedinLawetal.(2005).Thesemodelsreproducethefollowingobservationalconstraints8:

1.Theheliocentricpositionandradial velocityoftheSgrdwarfare(D,l,b)=(24kpc,5.6, 14.2 )andvr=171km/s,respectively.2.TheorbitalplaneofSgrhasaninclinationwithrespecttotheMilkyWaydiskofi 76 .3.TheaveragedheliocentricdistanceforSgrleadingdebrisisD～50kpc.Items1.and2.aboveshowthatwehaveobservationalmeasurementsfor wetal.(2005)surveyedtheun-knowncoordinate(namely,thetangentialvelocitycom-ponent,vtan)sothattheresultingN-bodymodelsrepro-ducedalltheobservationalconstraintslisted.

Amongotherfreeparameters,Lawetal.(2005)alsoexploredwhichhaloaxis-ratio(qh)wouldproducethebest- ttingmodeltotheavailableobservationaldata.Theyfoundthat,whereasthegeometryandkinematicsofthetrailingarmarescarcelysensitivetotheadoptedqh,radialvelocitymeasurementsoftheleadingarmwerebestmatchedbyprolate(qh>1)halomodels,inagree-mentwithHelmi(2004).However,Johnston,LawandMajewski(2005)showedthatSgrmodelsinprolateha-loscannotreproducetheobservedprecessionrateintheyoungestpiecesoftheSgrstream.Furthermore,theywereabletoconstrainthehaloaxis-ratiotobeqh=0.83 0.92,excludingmodelswithqtheseresults,wehaveperformedh>1ata3σlevel.InviewofN-bodysimulationswherethehaloaxis-ratiowas xedeithertoqh=0.8ortoqh=1.4,excludingforthesakeofbrevitysphericalhalos,whichneitherreproducethera-dialvelocityvariationalongthestreamnoritsprecessionrate.

InacoordinatesystemwheretheSunislocatedat(X,Y,Z)=(8,0,0)kpc,withavelocityof(U,V,W)=( 10, 220,7)km/s(Binney&Merri eld1998),theSgrvelocitiesthatbest tthekinematicandspatialdistri-butionsofstreamdebrisare(237, 35,220)km/sforqh=0.8(oblatehalo)and(244, 39,249)km/sforqh=1.4(prolatehalo).

Withthoseinputs,wehaveevolvedtheN-bodysatel-litemodelfrom4Gyrinthepasttothepresent.

8

observationalItisalsoimportantthatdataofthetoVODremarkinthethat ttingweofhavethesenotsimulations,inputanytheSgraretidalonlystream.

basedinthe ttingofanindependentsetofdataof3.THEORIGINOFTHEVIRGOOVER-DENSITY

InthisSectionweanalysethelikelihoodofvarioussce-nariosthatmayshedlightonthenatureoftheVOD.

3.1.Thesignatureofatri-axialGalacticstellarhalo

Analternativeexplanationforthepresenceoftheun-expectedMSfeatureintheCMDreportedinseveralstudies(andso-calledtheVOD:Newbergetal.2002;Du auetal.2006;Juricetal.2006;seeSec.1)istopostulatetheexistenceofanon-axisymmetryGalacticmodelcomponent,suchasatriaxialhalo(Newberg&Yanny2005;Xuetal.2006).WehaveexploredthispossibilitybyinspectingasampleofdeepCMDs(withlimitingmagnitudeRwiththeWideFieldCameralim～24(WFC).5)ofGalacticattheprime eldsfocustakenofIsaacNewton2.5-metertelescope(LaPalma,Spain)dur-ingdi erentobservingrunsdevotedtothesearchoftidaldebrisassociatedtoknownGalacticsatellitesorglobularclusters(Mart´ nez-Delgadoetal.2004b).Table1liststhepositionofour eldswithpositiveandnegativede-tectionsoftheMSfeatureintheCMD.Thesenegativedetectionscanbeinterpretedasthelackoftidaldebrisorthatitssurfacebrightnessisextremelylowtobede-tectedinthesmall eldofviewoftheWFC(35′Fig.1showsthepositionofthesenegativedetection×35′)9.(opencircles)over-plottedinthedensitymapofVODderivedbyJuri´cetal(2006)fromSDSSobservations.Themostinterestingresultisanegativedetection～20 Northofthecenterofourtarget eld(seeTable1)thatclearlyshowsadecreaseofstreamstarcountsinasmallareaofthesky.Althoughoursparsepencilbeamsurveycannotconstrainthespatialextentofthisstellarpopu-lationinthesky,thed …… 此处隐藏：5918字，全部文档内容请下载后查看。喜欢就下载吧 ……