Astro2020ScienceWhitePaperTheMagellanicStreamasaProbeofAstrophysicsThematicAreas:☐PlanetarySystems☐StarandPlanetFormation☐FormationandEvolutionofCompactObjects☐CosmologyandFundamentalPhysics☐StarsandStellarEvolution☐ResolvedStellarPopulationsandtheirEnvironments☒GalaxyEvolution☐Multi-MessengerAstronomyandAstrophysicsPrincipalAuthor:AndrewJFox,SpaceTelescopeScienceInstitute,afox@stsci.edu,4103385083Co-authors:KathleenA.Barger,TexasChristianUniversity,k.barger@tcu.eduJossBland-Hawthorn,UniversityofSydney,jbh@phys.usyd.edu.auDanaCasetti-Dinescu,SouthernConnecticutStateUniversity,casettid1@southernct.eduElenad’Onghia,UniversityofWisconsin-Madison,edonghia@astro.wisc.eduFelixJ.Lockman,GreenBankObservatory,jlockman@nrao.eduNaomiMcClure-Griffiths,AustralianNationalUniversity,naomi.mcclure-griffiths@anu.edu.auDavidNidever,UniversityofMontana,david.nidever@montana.eduMaryPutman,ColumbiaUniversity,mputman@astro.columbia.eduPhilippRichter,UniversityofPotsdam,prichter@astro.physik.uni-potsdam.deSnezanaStanimirovic,UniversityofWisconsin-Madison,sstanimi@astro.wisc.eduThorstenTepper-Garcia,UniversityofSydney,tepper@physics.usyd.edu.auAbstract:Extendingforover200degreesacrossthesky,theMagellanicStreamtogetherwithitsLeadingArmisthemostspectacularexampleofagaseousstreaminthelocalUniverse.TheStreamisaninterwoventailoffilamentstrailingtheMagellanicCloudsastheyorbittheMilkyWay.Thoughttobecreatedbytidalforces,rampressure,andhalointeractions,theStreamisabenchmarkfordynamicalmodelsof theMagellanic System, a case study for gas accretionanddwarf-galaxyaccretionontogalaxies,aprobeoftheouterhalo,andthebearerofmoregasmassthanallotherGalactichighvelocitycloudscombined.IfitsurvivestoreachtheGalacticdisk,itmaymaintainorevenelevatetheGalacticstar-formationrate.Inthiswhitepaper,weemphasizetheStream’simportanceformanyareasofGalacticastronomy,summarizekeyunansweredquestions,andidentifyfutureobservationsandsimulationsneededtoresolvethem.Westresstheimportanceof ultraviolet, optical, and radio spectroscopy, and the need for computational models thatcapturefullparticleandradiationtreatmentswithinanMHDenvironment.

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1) IntroductionSinceitsdiscoveryin21cmemissionover50yearsago(Dieter1965,Wannier&Wrixon1972,Mathewson+1974),theMagellanicStreamhasfascinatedmanycommunitiesofastronomers,fromradioandultravioletobservers todynamicistsandsimulators.At thesimplest level, theStreamisanextendedtailofmulti-phasegasstrippedoutoftheMagellanicCloudsandcovering140degreesinlength,or200degreeswhenincludingitsLeadingArm(seeFigure1;Nidever+2008,2010).Yetitissomuchmorethanthat:acasestudyoftheaccretionofgasandsatellitesontoastar-forminggalaxy,akeyconstraintonthedynamicalhistoryoftheMagellanicClouds,atestbedfortheevaporativeencountersbetweencoolgascloudsandthehotGalacticcorona,alaboratoryforunderstandinghowstarformationoccursintidaltails,andascreenagainstwhichionizingradiationfromtheGalacticCentershines(seereviewbyD’Onghia&Fox2016).Forthesediverse reasons, many sub-fields of Galactic astronomy are directly impacted by ourunderstandingoftheStream.ConsiderableprogressontheStreamhasbeenmadeoverthelasttwodecades,particularlyviatheuseofultraviolet(UV)spectrographsonHST,sensitive21cmradiosurveys,andnumericalsimulations(Figure2;D’Onghia&Fox2016).However,manyopenquestionsremain,includingfundamentalpropertieslikeitsdistance,originandfate.Inthiswhitepaper,weidentifyprogressmade in the last fewyears (Section2),andthenoutlineremainingquestions tobeanswered(Section3).We then focuson futureobservational capabilities andnecessary refinements tostate-of-the-artsimulations(Section4),andwefinishwithsomeconcludingremarks(Section5).ThroughoutthewhitepaperweemphasizetheStream’simportanceasaprobeofastrophysicalprocessesandthenecessityofspectroscopicobservationsasdiagnostictools.

Figure1:All-skyH Imapof theMagellanicSystem, fromNidever+ (2010),usingH Idata fromtheGBT,Arecibo,Parkes,Westerbork,andtheLABsurvey.ThisAitoffprojectionisinGalacticcoordinateswithHIemissioncoloredinpink.Werefertotheentirecomplex(LMC,SMC,Stream,Bridge,andLeadingArm)astheMagellanicSystem.

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Figure2: IllustrationofthechemicalabundancesalongtheMagellanicStream,fromD’Onghia&Fox(2016),andusingdatafromFox+(2013)andRichter+(2013).Thetwomainfilamentsofgas (Nidever+2008)havemarkedlydifferentkinematicandchemicalproperties,indicatingtheStream’sdualnature,withLMCandSMCcontributions.

2) RecentProgressontheMagellanicStreamandMagellanicSystem

a. Over ten ultra-faint dwarf (UFD) galaxies have been discovered in the vicinity of theMagellanicClouds(Koposov+2015a,b,2018,Bechtol+2015,Drlica-Wagner+2016),usingdatafromtheDarkEnergySurvey(DES).TheproximityofthesesatellitestotheMagellanicCloudssuggestsaMagellanicgroupofgalaxies (e.g.Kallivayalil+2018,Fritz+2019),whichmayhaveinfluencedtheformationoftheStream(Tepper-Garcia+2019).MoreworkisneededtoidentifywhichofthebrightdwarfspheroidalshaveaMagellanicassociation(Lynden-Bell1976,D’Onghia&Lake2008,Nichols+2011).TheLMChasafairlymassivecompanion,theSMC,whichis1.5magfainter,butitsnextmostluminoussatelliteappearstobeHydrus1(Koposov+2018),nearly13magfainter.This leavesanunexplained>10maggapintheLMCsatellite luminosityfunction.GaiaDR2propermotionssuggestthattwodwarfspheroidalshaveorbitscloselyalignedwiththeMagellanic-Cloudplane(Pardy+2019).TogetherwiththeSMCthisraisestothreethenumberofLMCsatellitesinthemassrangeofdwarfspheroidals,inagreementwithLCDMpredictions.b.ThemetallicityoftheLeadingArmhasbeenconstrainedviaUVspectroscopyfromHST/COS(Fox+2018,Richter+2018a).ThesestudieshavefoundtheLeadingArmhasanSMC-likechemicalabundance composition, but with considerable variation between the different regions. Theoxygenabundancesvaryfrom4%solarto30%solarbetweenregionsLAIIandLAIII(Fox+2018),suggestingmultiplegasremovalepisodes.WhilethereiskinematicandchemicalevidenceforafilamentinthetrailingStreamthatoriginatesintheLMC(Nidever+2008,Richter+2013),asyetnochemicalevidenceforanLMCfilamentintheLeadingArm,thoughHIstudies(Putman+1998)

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favoranLMCorigin.FurtherUVstudiesfromsensitivefuturespace-basedinstrumentationcouldmapoutthechemicalabundancesintheLeadingArmandsearchforaLMCfilament.c. Some short-length stellar components to the Stream have been found in the DES data(Belokurov&Koposov2016;Navarrete+2019),oneofwhichoverlapswiththegaseousstream,butwearemissingabreakthroughdiscoveryofanextendedstellarstream(aSagittariusdwarf).Theabsenceofastellarstreamhasbeenalong-standingproblem,sinceatidally-createdStreamshould contain both stars and gas. The Stream somehow survives yet does not form stars(Stanimirovic+2010).Other surveysareneeded, suchas theSurveyof theMagellanic StellarHistory(SMASH)usingtheDarkEnergyCamera,whichhasmapped480squaredegreesoftheoutskirtsoftheMagellanicClouds(Nidever+2017).LSSTwillbeinstrumentalinthiseffort.d.Proper-motionandparallaxmeasurementsforstarsintheLMCandSMCandfortheultra-faintdwarfsarenowpossiblewithGaiaDR2.OngoingHSTproper-motionmeasurementswillbetter constrain the LMC’s andSMC’s internal kinematics (e.g.Oey+2018), further constraindynamicalmodelsoftheentireMagellanicSystem,andexploreevidenceforadirectLMC-SMCcollisioninthepast(Besla+2012).FortheUFDs,systemicpropermotionswilldeterminewhethertheyareMagellanicmembers.ThesemeasurementsarestilluncertainforsomesystemsduetothesmallnumberofstarsthathaveGaiaDR2measurements(e.g.Kallivayalil+2018,Fritz+2019).AnaccuratecensusofMagellanicUFDsrequiresfurtherimprovedpropermotions.Specifically,precisepropermotionsoffaintstarsmustbeacquired,andforthispurposeJWSTiskey.e.Stream-analogsinothergalaxieshavebeenidentified.Forexample,theWhaleGalaxy(NGC4631),showsevidenceforamassivetidalgasstreamwithanoxygenabundanceof13%solarandanestimatedtotalgasmassof109solarmasses(Richter+2018b),whichcloselymatchesthemassandmetallicityoftheMagellanicStream(Fox+2014).Richter+(2018b)concludethatthetidalstreamintheWhalegalaxyrepresentsgasstrippedfromsatellitegalaxies.ThenearbyspiralsM31andM33areconnectedbyabridgeofHI(Braun&Thilker2004,Wolfe+2013),althoughitisunclearwhetheritrepresentsacondensingintergalacticfilamentratherthanatidalfeature.Finally,sometidaltailsareseenaroundinteractingdwarfs(Pearson+2016).f.Small-scalestructureintheStreamhasbeenstudiedviadetailedHIstudies(Kalberla&Haud2006,Stanimirovic+2008,Matthews+2009,Nigra+2012,For+2014).Curiously,coldHIcoresareseenyetheating/coolingequilibriumcalculationspredictthatnocoldneutralmediumshouldexistbeyond25kpc.Thisissueneedstoberesolvedwithfuturehigh-resolutionHIobservations.

3) RemainingQuestionsfortheFuture

f.Distance.ThiskeyparameterhasimplicationsfortheStream’stotalmassandfate,andtheorbitalhistoryoftheMagellanicClouds.YettheonlysoliddistanceconstraintontheStreamisthatoneendisanchoredtotheMagellanicClouds,at55kpc.ThedistancetotheLeadingArmisalsopoorlyknown, thoughsomeregionshaveconstraints ranging from~20-30kpc (McClure-Griffiths+ 2008, Price-Whelan+ 2018). In a first-passage scenario, the LeadingArm should beapproximately at the distance of the Clouds since they are currently near perigalacticon, sodistancemeasurementscandirectlytestthisscenario.UpcomingGaiadatareleasescanbeusedtoselectstellartargets(e.g.bluehorizontalbranchstars)foropticalabsorption-linestudiesoftheStreamusingCaIIandNaI,toprovidedirectdistanceconstraintsontheStream.

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g.Massinflowrate.WhatisthemassinflowrateintheStreamandhowdoesitcomparetotheGalacticstarformationrate?CurrentestimatesoftheStream’sinflowrateareintherange3-7solarmassesperyear (Fox+2014,Richter+2017), thoughtheyscalewith thedistanceto theStream,somayneedtoberevisedupward.ThiswillallowustoassesswhethertheStreamwilltriggerafuturestarburstintheMilkyWay,dependingonitsinteractionwiththehalo.h.Fate.ThefateoftheStreamisstillunknown.Whetherthecoolgaswilleventuallysettleontothe Galactic disk depends on the nature of the interaction with the hot Galactic corona.Simulationshaverevealedthatthisinteractioncanbebothevaporative(e.g.Hensler&Vieser2002,Heitsch&Putman2009)orcondensative(Fraternali+2015,Armillotta+2017),sothatcoolcloudscaneithershrinkorgrowwithtime,dependingonthemetallicityanddensitycontrastwiththesurroundinghotmedium.BetterconstraintsonthehotGalactichalowithfutureX-rayfacilities will improve our understanding of the externalmedium. Furthermore,maps of themagneticfieldacrosstheStreamareneeded,becausemagneticfieldsmaystabilizethecloudagainstram-pressurestrippingandconductiveevaporation,andthusimpacttheStream’sfate.McClure-Griffiths+(2010)reporteda6µGcoherentfieldinaLeadingArmcloudbasedonFaradayrotationmeasures,andKaczmarek+(2017)finda0.3µGcoherentfieldintheMagellanicBridge.i.IonizationbytheGalacticCenter.Bland-Hawthorn+(2013,2019)havereportedanimprintofaSeyfertflareattheGalacticCenterintheMagellanicStream.Inthisscenario,aflareseveralMyragoreleasedaburstofionizingradiationthationizedtheStream,whichisnowrecombiningandglowing inHaemission.TheStreammay thusprobe recentnuclearactivity fromSgrA*.FurtherUVabsorption-lineratios(e.g.CIV/CII,SiIV/SiII;Fox+2014)andopticalHaemission-lineobservations(Putman+2003,Barger+2017)cantestthisscenario,andexplorewhetheritssignaturecanbedistinguishedfromotherionizationprocessesinthehalo,suchasshocks(Bland-Hawthorn+2007,Tepper-Garcia+2015).Optical spectroscopic facilities (e.g. theWisconsinH-AlphaMapper)thatcanmapemissionfromHaandothernebularlines([SII],[NII],[OII])acrosstheentireStreamareneededtocompleteourmulti-phaseviewoftheStream.j.Totalspatialextent.FurtherUVandradiosurveysareneededtoaddresstheStream’stotalfootprintonthesky,bothinneutralandionizedgas.DoesitstipcrosstheGalacticplane?DotheStreamand LeadingArm together forma great circle?Current all-sky surveys (e.g. theHI4PIsurvey,HI4PIcollaboration,2018)reachcolumndensitiesofafewx1018cm-2;futurefacilitiesandsurveys (GASKAP, FAST) reaching a few x1017cm-2would reveal considerablymore structure,sincetheHIcolumndensitydistributionfunctionrisestolowN(HI).Indeed,GBTobservationsofsmallregionsoftheStreamalreadyrevealstructureatafewx1017cm-2(Howk+2017).ChartingthefullsizeoftheStreamwillfurtherconstrainthedynamicsoftheentireMagellanicSystem,informingwhethertheCloudsareattheirfirstpassagearoundtheMW(Besla+2007,2010).

4) FutureProgressTheUV,optical,andradioarethekeyregimesfordiagnosingtheStream’sphysicalandchemicalconditions. In terms of spectral lines per Angstrom, the UV is the richest portion of theelectromagnetic spectrum with a large number of ionization states available. The Stream’scompositionandphysicalpropertiescanbedeterminedfromfullanalysesoftheseUVabsorptionlinesinthespectraofbackgroundsources.High-resolutionUVspectrographswithmultiplexingcapabilities on large-aperture space telescopes (such as LUVOIR or HabEx) would provide

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significantlyhighersensitivitythanthecurrentUVspectrographsonHubble,yieldinganorderofmagnitudemorebackgroundtargets,allowingtheStream’schemicalabundancepatternstobemappedoutonfinerspatialscales.WealsoneedopticalspectroscopyofanystarsdiscoveredtowardtheStreamandLeadingArm(Casetti-Dinescu+2014,Price-Whelan+2018,Zhang+2017,2019) toverify theirmembershipviakinematicandabundanceanalyses.Spectroscopyof themainsequenceatthedistanceoftheStreamdemandslargetelescopeswithmultiplexingability.Ontheradiofront,continued21cmstudiesareneededtorevealthefullextentandkinematicstructureoftheStream(McClure-Griffiths+2018).TheGASKAPHIsurvey(Dickey+2013)usingtheAustralianSquareKilometerArrayPathfinder(ASKAP)telescopewillmaptheStreamat30”resolution,comparedto16.2’withtheHI4PIsurvey,thecurrentstate-of-the-art.ObservationsfromtheNortharealsoneeded.Progresswillbemadebythenewgenerationoffocal-planearrayreceiversontheGBTandArecibo,whichofferanorder-of-magnitudeincreaseinmappingspeedsandsensitivitiestoN(HI)ofafewx1017.The500mFASTtelescope(China)willcontributeinthenextdecadeat3.5’ resolution.Polarizationmeasurements (e.g. fromthePOSSUMprojectonASKAP)willmakeprogressonthemagneticpropertiesoftheStreamandLeadingArm.OnlybycombiningtheUVandradiodatawithhigh-resolutionhydrodynamicalsimulationsonscales of individual clouds canwe fully understand the fate of the Stream. Full particle andradiation treatments within anMHD environment are needed. Magnetic effects (Gronnøw+2017,2018)andheatingeffects (Hensler&Vieser2002)areboth important inHVCs.RecenthydrodynamicalmodelshaveimprovedourunderstandingoftheLMC-SMCsystem(Pardy+2018)and included the effect of ram-pressure stripping (Bustard+ 2018, Tepper-Garcia+ 2019).Nonetheless, furtherrefinementsareneeded, includingsub-gridphysicssuchasmetal-mixingandnon-equilibriumcooling.Betteralgorithms,codes,andsoftware/hardwarearchitecturesareneededtomakeprogress.TheStreamwillalwaysbethetestcaseforthesemodels.

5) ConcludingRemarks

UnderstandingtheGalactichaloisnolesscomplexthanstudyingtheEarth’satmosphere.Theprocessesofaccretion,outflows,andgasrecyclingcirculatematerialbetweenthediskandthehalo,justasgascirculationisakeyprocessintheterrestrialatmosphere.TheMagellanicStreamaffordsanearbybenchmarkforthestudyoftheseprocesses,allowingustostudygasphysicsandmetalmixinginclosedetail.TheexchangeofgasoccurringwhensatellitesliketheMagellanicClouds approach centrals like the Milky Way (a.k.a. intergalactic metal transfer) may be asignificant andpotentially dominantmodeof gas transfer between galaxies, as suggestedbymodernhydrodynamicsimulations(Angles-Alcazar+2017,Hafen+2018).BycharacterizingtheStream’sproperties,wedirectlyconstrainintergalacticmetaltransferandtherebyaddressthefundamentalquestionofhowgalaxiesgettheirgas.ExtragalacticstudiesoftheCGMandthebaryon cycle have the advantage of large statistical samples, but do not have the spatialresolutionormulti-wavelengthdatasetsavailableintheMilkyWayandLocalGroup.Forthesereasons,weendorsecontinuedmulti-prongedstudiesoftheMagellanicStreamandthegaseoushalooftheMilkyWayusingUV,radio,andopticalspectroscopy.

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