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ACCEPTEDBYTheAstrophysicalJournalLetters PreprinttypesetusingLATEXstyleemulateapjv.04/03/99 THEGASEOUSTRAILOFTHESAGITTARIUSDWARFGALAXY M.E.PUTMAN1,2,C.THOM3,B.K.GIBSON3,L.STAVELEY-SMITH4 AcceptedbyTheAstrophysicalJournalLetters ABSTRACT A possible gaseous component to the stream of debris from the Sagittarius dwarf galaxy is presented. We identify 4- 10×106 M⊙ of neutral hydrogen along the orbit of the Sgr dwarf in the direction of the Galactic anticenter (at 36 kpc, the distance to the stellar debrisin this region). This is 1-2%of the estimated total mass 4 oftheSgrdwarf. Boththestellarandgaseouscomponentshavenegativevelocities,butthegaseouscomponent 0 extendsto highernegativevelocities. If associated, thisgaseousstream wasmostlikelystrippedfromthemain 0 bodyofthedwarf0.2-0.3GyragoduringitscurrentorbitafterapassagethroughadiffuseedgeoftheGalactic 2 diskwithadensity>10- 4 cm- 3. Thisgasrepresentsthedwarf’slastsourceofstarformationfuelandexplains howthegalaxywasformingstars0.5-2Gyrago. n a Subjectheadings:galaxies:individual(SagittariusDwarfGalaxy)- galaxies:ISM- Galaxy:formation- J Galaxy:halo- intergalacticmedium- LocalGroup 2 2 1. INTRODUCTION cantamountofstarformationfuel(MHI<1.5×104M⊙ (3σ); Koribalski,Johnston&Otrupcek1994). ThesearchforHIas- 1 EvidencefortheprocesswhichformedourGalaxyisfound sociatedwiththeSgrdwarfwascontinuedbyBurton&Lock- v throughout our Galaxy’s halo as trails of stars and gas (e.g. 7 Yannyetal. 2003; Putmanetal. 2003; hereafterP03). These man(1999);buttheyalsofoundnoassociatedgasover18deg2 7 Galactic building blocksare currentlyaccreting satellites, and betweenb=- 13◦ to -18.5◦ with limits of MHI <7×103 M⊙ 4 (3σ). These results are surprising considering the Sgr dwarf the Sagittarius dwarf galaxy (hereafter Sgr dwarf) is one of 1 was forming stars within the last Gyr. Since the orbit of the the closest examples of this process (Ibata et al. 1994). The 0 Sgrdwarfisapproximately0.7Gyr(Ibata&Lewis1998),one evidence continues to accumulate that contiguous streams of 4 might expect to find this fuel stripped along the dwarfs orbit, 0 leadingandtrailingstellardebrisarebeingpulledfromtheSgr possibly at a similar location to the stellar trail. The trailing / dwarf as it spirals into the Milky Way (e.g. Newberg et al. h stellartidaltailoftheSgrdwarfhasrecentlybeenfoundtoex- 2003; Majewski et al. 2003; hereafter M03). The stars asso- p tendforover150◦acrosstheSouthGalacticHemispherewitha ciated with the Sgr dwarf span a wide rangeof ages, with the - meandistancebetween20to40kpcfromtheSun(M03).Here o youngestpopulationbetween0.5-2Gyrold(Layden&Saraje- wepresentHIdatafromHIPASS(HIParkesAll-SkySurvey5) r dini2000;Dolphin2002;M03). Thisindicatesthatwithinthe t alongtheentireSgrdwarfgalaxyorbittoinvestigatethepossi- s pastGyr,theSgrdwarfwasformingstarsandhadasourceof bilitythatagaseousSgrtrailisalsopresent. Thegasdetected a starformationfuel. : represents a potential method of tracing the history, make-up, v Neutralhydrogenisaprincipalsourceofstarformationfuel and classification of the Sgr dwarf galaxy, as well as the con- i for a galaxy. Galaxies which contain HI are commonly cur- X structionofourGalaxy. rentlyformingstars(e.g.,Leeetal. 2002;Meureretal. 2003) r andthosewithoutdetectableHItendtohaveprimarilyanolder a stellar populationand thusappearto have exhaustedtheir star 2. OBSERVATIONS formationfuel(e.g.,Gavazzietal. 2002).Thisisevidentinthe The neutral hydrogen data are from the HI Parkes All–Sky dwarfgalaxiesoftheLocalGroup.ThestarsintheLocalGroup Survey (HIPASS) reduced with the MINMED5 method (P03). dwarfsvaryfrombeingalmostentirelyancient(>10Gyr;e.g., HIPASSisasurveyforHIintheSouthernsky,extendingfrom Ursa Minor)to a numberof systems whichare activelyform- the South celestial pole to Decl. =+25◦, over velocities from ing stars (e.g. WLM, Phoenix, LMC). The HI content of the - 1280to+12700kms- 1 (Barnesetal. 2001). Thesurveyuti- dwarfsis summarizedby Mateo (1998), Grebel, Gallagher, & lized the 64–m Parkes radio telescope, with a focal–plane ar- Harbeck(2003),and Bouchardet al. (2003). The majorityof rayof 13beamsarrangedin a hexagonalgrid, to scan the sky theLocalGroupgalaxieswithHIhaveformedstarswithinthe in 8◦ zones of Decl. with Nyquist sampling. The MINMED5 past2Gyrandthosewithnoevidenceforrecentstarformation reduced HIPASS data has a spatial resolution of 15.5′ and a donotcontaindetectableHI(e.g.,Mateo1998;Dolphin2002). spectral resolution, after Hanning smoothing, of 26.4 km s- 1. PointedHIobservationsofthecentralregionoftheSgrdwarf The survey was completed with a repetitive scanning proce- (α, δ =19h 00m, -30◦ 25′ (J2000);l,b=6◦, -15◦)indicatethat durewhichprovidessourceconfirmation,mitigatesdiurnalin- ourclosestsatellitegalaxydoesnotcurrentlycontainasignifi- fluences,andaidsinterferenceexcision. Forextendedsources, 1CenterforAstrophysicsandSpaceAstronomy,UniversityofColorado,Boulder,CO80309-0389;[email protected] 2HubbleFellow 3Centre for Astrophysics & Supercomputing, Swinburne University, Mail #31, P.O. Box 218, Hawthorn, VIC, Australia 3122; [email protected], [email protected] 4AustraliaTelescopeNationalFacility,CSIRO,P.O.Box76,Epping,NSW1710Australia;[email protected] 5TheParkesTelescopeispartoftheAustraliaTelescopewhichisfundedbytheCommonwealthofAustraliaforoperationasaNationalFacilitymanagedby CSIRO. 1 2 theRMSnoiseis10mJybeam- 1(beamarea243arcmin2),cor- ofgaswouldamounttoMHI=4.3×106M⊙ at36kpc,theap- respondingtoabrightnesstemperaturesensitivityof8mK.The proximatedistanceto the starsin the Sgrstream. Otheremis- northern extension of the survey, from +2◦ to +25◦, was only sionwhichappearsinthechannelmapsincludeadistinctfila- recentlycompletedandispresentedforthefirsttimehere. The mentbeginningatb=- 10◦inthevelocityrangeof-245to-75 noiseinthesecubesisslightlyelevatedcomparedtothesouth- kms- 1thatwouldhaveamassof5.4×106M⊙ at36kpc. The erndata(11mKvs. 8mK).Thismaybeduetoacombination gasinbothcomplexeshaspeakcolumndensitiesontheorder oflowzenithanglesduringtheseobservationsandaninability of1020 cm- 2 atthe15.5′ resolutionofHIPASSandextendsto to avoid solar interference as effectively. Integrated intensity thecolumndensitylimitsofthedata(5σ∼3×1018 cm- 2;∆v mapsof thehighpositiveand negativevelocitygas(generally =25 km s- 1). There is also a populationof cloudsappearing |v |>80 kms- 1,aslongasthegaswasclearlyseparatefrom at-125 kms- 1 thatmergeintointermediatevelocityemission lsr Galacticemission)weremadeforthe24deg2 cubeswhichlie andare notincludedin Figure1 forthis reason. The distance alongtheSgrorbit. Thepositivevelocitycubeshadverylittle of36kpcwasadoptedbasedonthecontinuousstellartrailpre- emissioninthem, soweconcentratedonthenegativevelocity sentedin M03atthosedistancesandthematchingdistanceof cubes. The noise at the edges of the negative velocity maps a population of carbon stars (Ibata et al. 2001). The carbon were blankedwithin AIPS and the imageswere then read into starsassociatedwiththeSgrdwarfatthispositionhaveveloc- IDL tocreatethemapoftheentireorbitshowninFigure1. At itiesbetween-140to-160 kms- 1 (LSR;Dinescuetal. 2002; 20-40kpctheMINMED5reducedHIPASSdatahasasensitivity Totten&Irwin1998;Greenetal. 1994). to clouds of gas with MHI >80- 320 M⊙ (∆v = 25 km s- 1; 3σ). 4. DISCUSSION 3. RESULTS ThepresentorbitoftheSgrdwarfisestimatedtobe0.7Gyr The largescale HI map which includesall of the high neg- (Ibata & Lewis 1998). Using this orbit, the core of the Sgr ativevelocitygasalongthe orbitoftheSgr dwarfis shownin dwarfwasatthepositionoftheHIcomplexpresentedhereap- Figure1. Thisplotis inCelestial coordinatesasitdepictsthe proximately0.2-0.3Gyrago. Itwouldmakesenseifthegas main features found along the orbit better than Galactic coor- waspartoftheSgrdwarf0.3-0.7Gyragoconsideringtheage dinates. The Galactic Plane, the orbit of the Sgr dwarf (Ibata ofpartoftheSgrdwarfstellarpopulation(0.5-2Gyr;Layden & Lewis 1998), and the current position of the Sgr dwarf are &Sarajedini2000;Dolphin2002;M03). Notsurprisinglystar shown. Thestream ofM giantspresentedbyM03hasquitea formation surveys find that those galaxies which are actively broadwidth(commonlyseveraldegrees)alongthisorbit. The formingstarsalsocontainsubstantialamountsofstarformation orbit of the Sgr dwarf crosses the Galactic Plane at ℓ∼185◦ fuelin the form of neutralhydrogen(Lee et al. 2002(KISS); andℓ∼0◦whichcorrespondstoapproximately(α,δ)=6h,25◦ Meurer et al. 2003 (SINGG)). This paper addresses the issue and18h, -30◦, respectively. AttheGalactic Centerhighnega- ofwhenandhowthestar formationfuelofthe Sgrdwarfwas tive and positive velocity gas is present, thus we have labeled strippedfromthegalaxy. the negativevelocityHIemissionwithin+/- 5◦ oftheGalac- Thegasbetween-380and-180 kms- 1 atb<- 20◦ inFig- ticplaneatthatlocation. TheSgrorbitcrossestheMagellanic ure2isthegasweproposewasmostlikelyoncepartoftheSgr Stream(labeled)attheSouthGalacticPoleor(α,δ)=0h,-15◦. dwarf,asitfollowstheorbitoftheSgrdwarfandiscompletely ThemajorityoftheSgrdwarforbitdoesnothavebothstarsand isolatedfromlowervelocityemissionthatmayhavearelation HIgasathighnegativevelocities,withtheexceptionoftheHI to disk gas (e.g., Tamanaha 1995). This gas lies at a position gas between α=3- 4.5 h and δ =0- 30◦, or ℓ≈155- 195◦ where the Sgr dwarf would have passed through the extreme and b≈- 5-- 50◦. A close up of the velocity distribution of edge of the Galactic disk, and at this position the Sgr dwarf theseHIcloudsinGalacticcoordinatesisshowninthechannel stellar debris is approximately40 - 50 kpc from the Galactic mapsofFigure2. TheMgiantsoftheSgrstellarstreamfilla Center (M03). We have adopted 45 kpc as the typical dis- largepercentageoftheregionshownFig. 2. Thishighvelocity tance from the Galactic Center at the position of the bulk of HIgaswaspreviouslyidentifiedaspartoftheAnticentercom- thegas. Thisdistanceissignificantlybeyondthetypicalradius plexes (ACHV and ACVHV; Wakker & van Woerden 1991) quoted for our Galaxy (∼26 kpc), however it is possible that andwasdiscoveredalmost40yearsago(e.g.,Mathewsonetal. anextendedionizeddiskexistsforourGalaxy(e.g.,Savageet 1966),butitsrelationshiptotheSgrdwarfwasnotpreviously al. 2003), as found in other systems (Maloney 1993; Bland- noted.Therearesomesmallnegativevelocitycloudsalongthe Hawthorn, Freeman & Quinn 1997; Steidel et al. 2002). The Sgrdwarforbit,butbesidesthelabeledgasinFig. 1,theonly passagethroughanextendeddiskofourGalaxy,inadditionto othersubstantialcomplexofnegativevelocitygasalongtheor- the tidal forcesalready obviouslyat work as evidentfrom the bit of the Sgr dwarf is in a region which has predominantly stellar tidal stream, mighthave been enoughto disruptthe HI positive velocity stars (Ibata et al. 2001). This is the group in the core of the galaxy and cause the dwarf to lose all re- of high-velocitycloudsknown as Complex L at α≈ 15.25 h, maining star formation fuel. The gas will be stripped from a δ≈-19.5◦. Theamountofhighpositivevelocitygasalongthe galaxy if ρIGMv2 >σ2ρgas/3 (Mori & Burkert 2000; Gunn & orbit of the Sgr dwarf is verylimited, and notcorrelatedwith Gott 1972). We can use this equation to estimate the density the position of the positive velocity stars along the Sgr stellar neededattheedgeofthedisktostripthegasfromthecoreof stream. theSgrdwarf. Weuseatangentialvelocityof280 kms- 1 for Figure2 showsthe gasalongthe orbitofthe Sgr dwarfbe- the Sgrdwarf anda velocitydispersionof11.4 km s- 1 (Ibata tweenα=2- 5.5handδ=0- 30◦inGalacticcoordinatesover etal. 2003;Ibata&Lewis1998). Ifthecolumndensitiesand thevelocityrangeof-380to-85 kms- 1 (LSR).Theemission sizeoftheHIdistributioninthecoreoftheSgrdwarfwereon extendingfrom-380to-180 kms- 1 atb<- 20◦ isorientated theorderof5×1020cm- 2 (averagedoverthecore)and1kpc, alongtheorbitoftheSgrdwarfandiscompletelyisolatedfrom thetypicalρgas is0.16cm- 3. Anextendeddiskdensitygreater gaswhichmergeswithlowervelocityemission. Thiscomplex than3×10- 4cm- 3isthenneededtostripthegasviarampres- 3 sure stripping. Based on previous estimates of the density of - 2 Gyr old, the HI gas presented here most likely represents the Galactic halo, thisdensitywouldbeeasily achievedin the the high column density gas from the core of the Sgr dwarf planeofGalaxy,20kpcfromthecurrentlyobservededgeofthe whichwasfinallystrippedwhenthe dwarfpassed throughthe disk. ExamplesofestimatesforthedensityoftheGalactichalo higher density medium in the extended Galactic disk. This is which are consistent with ∼10- 4 cm- 3 at distances of 50 kpc supported by the relatively high peak column densities of the andmoreinclude: explainingtheexistenceofhead-tailclouds HIgascurrently(∼1020 cm- 2). FortheSgrHI gastosurvive which can be associated with the Magellanic Stream (Quilis for0.2-0.3Gyr(thetimesincethelastpassageofthecoreSgr & Moore 2001); the confinementof the tip of the Magellanic dwarf),nottomentiontheMagellanicStreamwhichisthought Stream (Stanimirovicetal. 2002); theO VI high-velocityab- to be > 0.5 Gyr old, the gas should either be confined by an sorptionline results(Sembachetal. 2003);andthe diffusex- existinghalomediumorassociatedwithsignificantamountsof rayemission(Wang&McCray1993)anddispersionmeasures darkmatter. forLMCpulsars(Taylor,Manchester& Lyne1993)whenthe ionizedmediumisextendedover∼100kpc. 5. OVERVIEW StrippinggasfromaGalacticsatellitebypassingitthrough OurGalaxy’shaloismadeupofnumerousstreamsofsatel- theedgeoftheGalaxy’sdiskwasproposedasapossiblesolu- litedebriswhichtraceitsformation. TheassociationofHIgas tionfortheoriginoftheMagellanicStreambyMoore&Davis with the Sagittarius dwarf galaxy emphasizes the number of (1994; hereafter MD94). They invoked an extended ionized dynamicalprocessesoccuringwithintheGalactichalo. Inthis diskat65kpcfromtheGalacticCenterwithcolumndensities paperwefind4- 10×106M⊙ ofHIgasalongtheorbitofthe <1019cm- 2tostripgasfromthesatellitegalaxy. Thestripped Sgrdwarf. Thisisapproximately1-2%ofthetotalmassofthe gasinitiallyhasitsspeedreduced,butthedeceleratedgasfalls dwarf, 10-20 times lower than the HI mass of the Magellanic toalowerorbitandsubsequentlyattainsahigherangularveloc- Stream, and approximately the same HI mass as the Leading ity. Inordertoreproducethepropertiesanddistributionofthe ArmoftheMagellanicSystem(P03).WearguethisHIwasthe Magellanic Stream, MD94 induce a braking effect on the gas lastgasstrippedfromthecoreoftheSgrdwarf∼0.2-0.3Gyr with a diffuse medium in the halo, preventingit from rapidly agoduetoitspassagethroughtheextendedGalacticdiskwith falling into the Galactic potential. Though the HI trail of the densities >10- 4 cm- 3. This is supportedby the agreementin Sgr dwarf has a very different structure from the Magellanic thespatialdistributionofthegasandstarsalongtheSgrorbit, Stream, parallels can be made between the MD94 model and both componentshavinghigh negativevelocities, the location the stripping of the Sgr HI clouds. Both of the systems have ofthegasrelativetotheplaneofourGalaxy,therelativelyhigh similarimpactvelocitiesandhavegasatsimilardistancesfrom columndensities of the HI gas, and the star formationhistory their cores (roughly50-60 kpc) with similar column densities oftheSgrdwarf. TheassociationofHIgaswiththeSgrstellar and velocities. The gas along the Sgr dwarf orbit has slightly streamsuggeststhatthedwarfspheroidalclassificationforthe higher negative velocities than the carbon stars with velocity Sagittariusgalaxymayneedtobereconsidered.Italsosuggests determinations in this region and this may be due to the gas thatslightlyoffsetHIandstellarstreamsmaybeacommonfea- fallingintoaslightlylowerorbitthanthestars. Theexactpre- tureofdisruptedsatellitesintheGalactichalo. Withtheaccre- dictedpositionandvelocityofthegasrelativetothestarswill tionofthisSgrHIstream,theMagellanicStream(P03),Com- beaddressedinafuturesimulationpaper. plex C (Wakker et al. 1999), and possibly other HVCs, there At 36 kpc from the sun the HI gas between -380 and -180 is ample fuel for our Galaxy’s continuing star formation and km s- 1 at b<- 20◦ has a mass of 4.3×106 M⊙. If the fila- understandingthe distribution of stellar metallicities (e.g., the mentextendingfromb=- 10to- 25◦ andvLSR=- 245to- 85 G-dwarfproblemisnotaproblem). Determiningthemetallic- kms- 1 isincludedasoncebeingpartoftheSgrdwarf,theto- ity,distance, andionizationpropertiesoftheHIgaspresented talHImassgoesupto9.7×106 M⊙. AtypicaltotalHImass here will aid in confirming if this HI gas was indeed stripped for a dwarf galaxy is on the order of 107- 8 M⊙ (Grebel et al. fromthe Sgr dwarfduringits currentorbit. Since we propose 2003),so106- 7M⊙ ofgasoriginallybeingassociatedwiththe thisisthelastgasfromthecoreoftheSgrdwarf,wewillalso dwarfis certainlyplausibleandmayrepresentanywherefrom lookformoleculargasanddustassociatedwiththeseclouds.A themajorityto10%oftheSgrdwarf’soriginalHImass. Using futuresimulationpaperwilladdresstheexactspatialandveloc- a total Sgr dwarf mass of 5×108 M⊙ (M03), this gas repre- itydistributionexpectedforgasstrippedfromtheSgrdwarf. sents 1-2%ofits totalmass. The Sgrdwarfmostlikelyorigi- nally hadmore than106- 7 M of neutralhydrogenassociated ⊙ We would like to thank Phil Maloney, Steve Majewski and with it, as the outer gaseous componentwould have been the Geraint Lewis for useful discussions. We also thank the ref- first thing stripped (e.g., Mihos 2001; Yoshizawa & Noguchi eree for many useful comments. M.E.P. acknowledges sup- 2003). This outer gas would have had column densities be- port by NASA through Hubble Fellowship grant HST-HF- tween 1018- 1019 cm- 2 andhasmostlikelyeitheralreadydis- 01132.01 awarded by the Space Telescope Science Institute, persed or been ionized, althoughremnantsof this gas may be which is operated by AURA Inc. under NASA contract NAS presentas smallHVCs alongthe Sgr dwarf orbit. Since there 5-26555. B.K.G. acknowledges the support of the Australian is currently no HI associated with the core of the Sgr dwarf Research Council, through its Large Research Grant and Dis- (Koribalskietal. 1994),andthedwarfhasstarswhichare0.5 coveryProjectschemes. 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ThecurrentpositionoftheSgrdwarfisshownbythesolidpoint,andtheorbitoftheSgrdwarf(Ibata&Lewis1998)isplottedas thesolidlinethroughthispoint.ThenegativevelocitygasattributedtotheMagellanicStreamandGalacticPlanegasattheGalactic Centerislabeled. b=0◦ isindicatedbythesolidlinethroughthelabeledGalacticPlanegas,withthetwodottedlinesoneachside representingb=+5◦and- 5◦. Thenegativevelocitycarbonstarsextendfromapproximatelyα,δ=0h,- 20◦to12h,20◦alongtheSgr orbit(Ibataetal. 2001).Contoursrepresentcolumndensitylevelsof0.5,1.0,5.0,and10.0×1019cm- 2. 6 Fig.2. Channel maps of the HI clouds found along the trailing stellar stream of the Sgr dwarf in Galactic coordinateswith v LSR labeled in the upper left corner. The M giants (M03) extend across this region in a ∼15◦ band along the orbit of the Sgr dwarf (dashedline;Ibata&Lewis1998).Contoursare0.055,0.11,0.22,0.44,0.88,and1.8K.