The Virgo stellar over-density Mapping the infall of the Sag(3)

InFig.3weplottwoequal-areaprojectionsofstreamparticlesfromthemodelswithoblateqh=0.8andpro-lateqh=1.4halos(topandbottom,respectively).Intheseplots,theNorthGalacticpoleisinthecenter,con-centriccirclesarelinesofconstantGalacticlatitudebandtheradiallinesdenotelocationsofconstantGalacticlon-gitudel.TheGalacticcenteristotheleft.WerestrictthetheoreticalsampletothedistancerangeofJuri´cetal.(2006)databyincludingonlyparticlesatb>0and5kpc<d<15kpc.Toestimatethetwo-dimensionalprojecteddensitydistribution,weconvolvetheparticleswitha2DGaussianσ=2.9degkernel.Theresultingdensityisshownasvaryingshadesofgray(blackandwhiteregionsdenotingthehighestlowestdensity,respec-tively)andcanbedirectlycomparedwiththeobservedmorphologyoftheVODdisplayedinFig1(thatwasmadewiththesamescale).

Thesemapsshowaconspicuous,di uselumpofSgrstreamstarsthatextendsoverafewhundredsofsquaredegrees,whoseshapeandextentresemblesthoseofthestellarover-densitydisplayedinFig.1.Thepositionoftheover-densitycenter(de nedasthelocationofmax-imumdensity)intheskydependsontheshapeofthehalo,andisconsistentwithsimulationsofLawetal.(2005),Fig.13.Theoblatemodel(qwiththeSDSSdata,h=0.8)producesabetteragreementwiththeover-densitybeingcenteredat(l,b)=(277,63).Thiscorre-spondstoasmallo set,～11deg,withrespecttotheob-servedpositionoftheVOD((l,b)=(300,65))asreportedbyJuri´cetal.(2006).Takingintoaccounttheuncer-taintiesintheSgrstreammodeling(seeSec.2)andintheSDSSdataasnotedabove,theagreementinposi-tionisremarkable.Thisresultalsoprovidesanaturalexplanationforthehugeapparentextensionofthistidalremnant,sinceweareobservingthroughastellartidalstreamthatapproachestheSuninanalmostradialdi-rectioninalocalizedareaofthesky.10

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theEarth;AnanalogyseeNakamuracanbefoundetal.2000.

whenameteoroidstreamcrossesTheprolate(qducesaslightlyh=1.4)halomodel(bottompanel)pro-worseestimateofthelocationofVOD

centeranditsgeneralshape.Itdoespredictsasubstan-tialamountoffaint,di usedebrisinthe180<l<240region.ThisistoadegreeobservedinSDSSdata–e.g.thedensityin180<l<210(seeFig.1)isapparentlygreaterthanthatinitssymmetriccounterpartwithre-specttothel=0,l=180direction.

Interestingly,Gondoloetal.(2005)estimatedalo-calstellardensityofSgrleadingtailofbetween270-740stars/kpc33,correspondingtoarangeof2.1to7.4×10 7stars/pcor-15.4<ln(δ)<-14.1.ThisiswithinthemeasuredlimitsofstellardensityintheVOD(seeFig.23inJuri´cetal.2006).

Wenotethattheinterpretationofthefaintdensityexcessismuddiedbytheexistenceofadi erentnarrowstreaminthatregion(Juri´cetal.2006),apparentlyun-relatedtoSgrstream.Analternativescenarioisthatthenarrowstreammightbethetrueoriginoftheden-sityexcessseeninSDSSdata,insteadoftheSgrdebris.However,asitwillbecomeevidentinthenextSection,thethree-dimensionalstructureoftheVODstronglydis-favorsthishypothesis.

3.2.2.Three-dimensionalspatialstructure

Wecomparenowthethree-dimensionalstructureoftheVODtothestructureobtainedfromthetheoreti-calmodelsoftheSgrstreaminthatregionofthesky.Weexaminevariouscross-sections(horizontalandper-pendiculartotheGalacticplane)oftheVODandtheequivalentcross-sectionsoftheSgrstreammodel.

InFig.4weplotthemeasuredstellardensity(left)andthatobtainedfromtheSgrstreammodelsfortwodi er-entvaluesofhalo attening,qThedensityshownonh=0.8(middle)andqthetopleftpanelish=1.4(right).thatshowninFig.8(middle-leftpanel)ofJuri´cetal(2006).Beforediscussingitsimplications,wewillverybrie yde-scribethemethodusedhere(foramoredetaileddiscus-sionwereferthereadertoJuri´cet.al2006):TheSDSShasobserved～4.2millionGstars(0.1<r i<0.15)withinthesurveyarea.ToconverttheSDSSstarcountstospatialnumberdensity,weestimatedthedistancestoindividualstarsusingthephotometricparallaxrelationofJuri´cetal.(Eq.1).Fromtheestimatedabsoluteandmeasuredapparentmagnitudes,andfromastrometry,wecalculatethethree-dimensional(X,Y,Z),positionsofallstarsintheSDSSstellarsample.Wethenbinthemtodx=500pcpixelsinthecoordinatesystemasde nedabovetoobtainathree-dimensionalstar-countmap.Fi-nally,bydividingthemapbythevolumethatwasob-servedwithineachvoxel,weobtainathreedimensionalmapofnumberdensitywithintheMilkyWay.

ThedensityshownintheleftpanelofFig.4isaplanarcross-sectionofthe3DmapalongaplaneparallelwiththeGalacticplane,andatZ=10kpcaboveit.Thecol-orsencodethestellarnumberdensity(starspc 3)onanaturallogscale,rangingfromexp( 15.46)=1.910 7(blue)toexp( 14.03)=8.310 7starspc 3(red).TheblackregionsareplaceswhereSDSShasnotobserved.TheyellowdotmarksthepositionoftheSuninX-Yplane(X=8kpc,Y=0kpc)ThecirclesvisualizethepresumedaxialsymmetryoftheGalaxy.IftheGalacticnumberdensitydistributionwasaxiallysymmetric,thecolorcontoursintheobservedregionswouldtracesuch

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

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circlesaroundtheGalacticcenter,andthedensitydis-tributionwouldrespectthesymmetrywithrespecttore- ectionalongtheXaxis(ρ(X,Y,Z)=ρ( X,Y,Z)).Asseenintheleftpanel,thedensitydistributionisneithercircularnorsymmetric,andexhibitsastrongdensityen-hancementcenteredapproximatelyatX=6,Y=4kpc,theVOD(Juri´cetal.2006).

WetestunderwhichconditionstheVODmightbeplausiblyidenti edwiththeleadingarmoftheSgrtidalstreambyover-plottingtheequivalentX-YslicesatZ=10kpcofsimulatedSgrdebrisoverthecontoursoftheSDSSobservedarea(thegrayareainmiddleandleftpanels).The guresinthemiddleandleftpanelsareconstructedinamannersimilartothemethodusedforSDSSdataoutlinedabove.However,wenotethattheparticlecountshavebeenaveragedinthe9kpc<Z<11kpcslice.Thedi erenceinthickness,comparedtodx=500pcthicknessusedforSDSSdensityestimationistoaccountforthee ectofradialsmearingintroducedbytheintrinsicσrelation.Mrricparallax～0.3magThise ectdispersionincreasesofthethephotomet-e ectiveradialSDSSvo …… 此处隐藏：6253字，全部文档内容请下载后查看。喜欢就下载吧 ……