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Astronomy&Astrophysicsmanuscriptno.paper (cid:13)c ESO2012 January31,2012 ∼ ∗ Interstellar gas within 10 pc of Sgr A KatiaFerrie`re1 1IRAP,Universite´deToulouse,CNRS,14avenueEdouardBelin,F-31400Toulouse,France Received;accepted ABSTRACT Aims.Weseektoobtainacoherentandrealisticthree-dimensionalpictureoftheinterstellargasouttoabout10pcofthedynamical 2 centerofourGalaxy,whichissupposedtobeatSgrA∗. 1 Methods.WereviewtheexistingobservationalstudiesonthedifferentgaseouscomponentsthathavebeenidentifiednearSgrA∗,and 0 retainalltheinformationrelatingtotheirspatialconfigurationand/orphysicalstate.Basedonthecollectedinformation,wepropose 2 athree-dimensionalrepresentationoftheinterstellargas,whichdescribeseachcomponentintermsofbothitspreciselocationand n morphologyanditsthermodynamicproperties. a Results.TheinterstellargasnearSgrA∗canrepresentedbyfivebasiccomponents,whichare,byorderofincreasingsize:(1)acentral J cavitywithroughlyequalamountsofwarmionizedandatomicgases,(2)aringofmainlymoleculargas,(3)asupernovaremnant 9 filledwithhotionizedgas,(4)aradiohaloofwarmionizedgasandrelativisticparticles,and(5)abeltofmassivemolecularclouds. 2 Whilethehalogasfills≈80%ofthestudiedvolume,themolecularcomponentsenclose≈98%oftheinterstellarmass. Key words.ISM:structure-Galaxy: center -Galaxy: nucleus- ISM:general -ISM:kinematicsanddynamics -ISM:supernova ] A remnants G h. 1. Introduction years have witnessed dramatic progressat both the low-energy p (radioandinfrared)andhigh-energy(X-rayandγ-ray)endsof There is now compelling evidence, largely based on measured - the electromagneticspectrum.Yet, despitethe wealthof obser- o stellarorbits,thatamassiveblackholewithmass≈4×106 M⊙ vationaldatathathavenowbecomeavailable,achievingaclear r resides at the dynamical center of our Galaxy, which is tradi- t andcompletethree-dimensionalviewoftheinnermostGalactic s tionally identified with the compact nonthermal radio source regionremainsachallengingtask,dueinlargeparttothediffi- a Sagittarius A∗ (Sgr A∗) (see Genzeletal. 2010, for a recent, [ comprehensivereview).SgrA∗sitsintheheartofadensecluster cultyinpositioningtheobservedfeaturesalongthelineofsight. 1 of young, massive and luminous stars, which are concentrated Inthiscontext,wewilltrytounravelatbesttheintricatespa- v within the central parsec and are distributed in two relatively tialdistributionoftheinterstellargaswithin∼ 10pcofSgrA∗, 1 thickdisksthatarehighlyinclinedtowardeachotherandrotate and we will describe the emergingpicture by way of a simpli- 3 inoppositedirections(Krabbeetal.1991,1995;Paumardetal. fiedthree-dimensionalgasrepresentation,whichwemeantobe 0 2006). This central star cluster, which has a few bright con- as realistic as possible. Let us specify from the outset that we 6 densations(notably,the IRS 16 complex at the very center), is willtrulyfocusontheinterstellargas.Starswillonlybealluded . 1 the source of intense UV radiation and powerfulstellar winds. tofortheirdirectimpactonthe interstellargas,andinterstellar 0 Also presentaroundSgr A∗, butextendingmuchfartherout,is magnetic fields will be tackled in a separate paper. Let us also 2 a cluster of old and evolved, cool stars, with nearly isotropic emphasize that our purpose is not to provide a comprehensive 1 distribution and slow, solid-body rotation (Blumetal. 2003; overviewoftheinterstellargasintheregionofinterest.Instead, : v Trippeetal. 2008; Scho¨deletal. 2009). The old cluster domi- wewillpresentwhatwefeelarethemostimportantanddirectly i nates the stellar mass by far, with ∼ 106 M⊙ inside the central relevantobservationalstudies. We will discuss theirsometimes X 1 pc (Scho¨deletal. 2009), as opposed to . 1.5 × 104 M⊙ in divergentresultsandextracttheusefulpiecesofinformationthat r the youngcluster (Paumardetal. 2006). Like in the rest of the canserveasbuildingblocksforourgasrepresentation.Wewill a Galaxy,allstarsareimmersedinaninterstellarmedium(ISM), thenassembleallthepiecesofinformationandtrytoreconstruct whichisessentiallymadeofgas(inmolecular,atomicandion- the overall puzzle. The existing observationswill be presented izedforms)anddust. inSection2,whileourgasrepresentationwillbethesubjectof Thefew-parsecregionsurroundingSgrA∗isofindisputable Section3. interest,notonlyinitsownright,becauseitconstitutesaunique, extremelycomplexandhighlyinteractingGalacticenvironment, Owingtotheconsiderableuncertaintiesintheobservational but also from a broader perspective, because it represents (by resultsandintheirinterpretations,whicharereflectedinthedis- far)thenearestand,therefore,mosteasilyaccessibleexampleof parate conclusions reached by different authors, our gas repre- agalacticnucleus,andassuchmaybethekeytounderstanding sentation will necessarily be approximate. However, we hope theenergeticprocessestakingplaceingalacticnucleiingeneral. thatitcanbeusedasanobservationalinputtotheoreticalstudies Thisiswhythecentralfewparsecshavelongbeenthetargetof thatdealwithSgrA∗ anditssurroundings.Onesuchstudythat numerousobservationsoverawiderangeoffrequencies.Recent wehaveundertakeninparalleltothepresentworkconcernsthe propagationand annihilationof positronsfrom Sgr A∗ (Jean et Sendoffprintrequeststo:KatiaFerrie`re al.,inpreparation)–animportantinvestigationindirectneedof 1 KatiaFerrie`re:InterstellargasnearSgrA∗ arealisticandreasonablyaccuratedescriptionoftheinterstellar 2. Observationsoftheinterstellargas gas. The interstellar gas in the immediate vicinity of Sgr A∗ has Unless stated otherwise, the observational maps presented a complicated morphology (see, e.g., Morris&Serabyn 1996; here will be discussed in the equatorial coordinate system de- Mezgeretal.1996,forearlyreviews).Inbrief,SgrA∗isembed- fined by right ascension, α, and declination, δ. In this sys- dedin a ≈ (2−3)pc sized cavity,the ”CentralCavity”,which tem,east/westreferstothedirectionofincreasing/decreasingα was originally identified as a filamentary Hii region named andnorth/southto the directionof increasing/decreasingδ (see SgrAWest.Thiscavityhasmostlikelybeenevacuatedbystel- Figure 1). We will also use the Galactic coordinate system de- lar winds and photo-ionized by UV radiation from the central fined by longitude, l, and latitude, b, where Galactic east/west starcluster.EncirclingtheCentralCavityisanasymmetrictorus refers to the direction of increasing/decreasing l and Galactic of neutral (mainly molecular) gas and dust, usually referred to north/south to the direction of increasing/decreasing b. At the position of Sgr A∗, the trace of the Galactic plane (b = 0◦) in as the Circumnuclear Disk (CND) or, more appropriately, the the plane of the sky is at positionangle 31◦.40 east of north(in CircumnuclearRing(CNR).TheCNRisgenerallyinterpretedas J2000;Reid&Brunthaler 2004), so that there is a 58◦.60 angle beingpartofanaccretiondiskaroundthecentralmassiveblack hole,eventhoughits pronouncedasymmetrysuggeststhatit is betweenthe(α,δ)and(l,b)systems. atransientfeature.BoththeCentralCavityandtheCNRlie,in Our gas representation will be described in terms of the projection(ontotheplaneofthesky),insidea≈10pcscalenon- Galactocentriccartesiancoordinates,(x,y,z),wherexisthehor- thermalradioshellcalledSgrAEastandwidelybelievedtobea izontal(i.e.,paralleltotheGalacticplane)coordinatealongthe supernovaremnant(SNR).SgrAEast,inturn,issurroundedby lineofsighttotheSun(positivetowardtheSun),ythehorizontal a≈20pcdiameterradiohalo,whichisprobablycomposedofa coordinateintheplaneofthesky(positivetowardGalacticeast) mixtureofwarmionizedgas(thermalcomponent)andrelativis- and z the vertical(i.e., perpendicularto the Galactic plane)co- ticparticles(nonthermalcomponent).Finally,abeltofmassive ordinate (positive toward Galactic north). For consistency with molecularcloudsaroundSgrAEaststretchesover≈30pcalong ourpreviouspapersontheGalacticcenter(GC)region,wewill the Galactic plane; most prominent amongst these clouds are adoptr =8.5kpcfortheGalactocentricradiusoftheSun,such ⊙ thewell-knownM−0.02−0.07(or50kms−1)andM−0.13−0.08 thatangularseparationsof1′and1′′translateintolinearsepara- (or20kms−1)giantmolecularclouds(GMCs)locatedeastand tionsnearSgrA∗of2.5pcand0.04pc,respectively. south,respectively,ofSgrA East. We nowproceedto describe The coordinates of Sgr A∗ in the three different eachoftheabovestructuralcomponentsinmoredetail. systems are (α ,δ ) = (17h45m40.s04,−29◦00′28.′′1) A∗ A∗ (in J2000; Reid&Brunthaler 2004), (l ,b ) = A∗ A∗ (−0◦03′20.′′5,−0◦02′46.′′3) (as calculated using the (α,δ) 2.1.TheCentralCavity coordinates of the origin of the (l,b) system given by Reid&Brunthaler 2004) and (x ,y ,z ) = 0 (by con- TheSgrAWestHiiregionappearsinprojectionasathree-arm A∗ A∗ A∗ struction). In the following, angular offsets with respect to spiral,commonlyknownastheMinispiralandcomposedofthe SgrA∗ willbedenotedby(∆α,∆δ)intheequatorialsystemand so-calledNorthernArm,EasternArmandWestern Arcaswell by(∆l,∆b)intheGalacticsystem. as a short east-west Bar that connects the southern end of the Northern Arm and the western end of the Eastern Arm to the WesternArc(seeFigure2).Itisnowgenerallyacceptedthatthe WesternArcisthephoto-ionizedinneredgeofthewesternpart of the CNR (e.g., Lo&Claussen 1983; Serabyn&Lacy 1985; Gu¨stenetal. 1987; Roberts&Goss 1993), while the Northern and Eastern Arms are the photo-ionized surfaces of tidally stretched streamers of material falling in toward the central massiveblackhole(e.g.,Lo&Claussen1983;Serabyn&Lacy 1985; Ekersetal. 1983; Davidsonetal. 1992; Jacksonetal. 1993). For completeness, we should mention that the Eastern Arm has also been suggested, by analogy with the Western Arc, to be the photo-ionized inner edge of the eastern part of theCNR(e.g.,Aitkenetal.1998;Shuklaetal.2004).However, this suggestion appears to be incompatible with the finding that the Eastern Arm is nearly perpendicular to the CNR (Latvakoskietal. 1999) and to the Western Arc (Zhaoetal. 2010). Lo&Claussen (1983) presenteda VLA 6 cm radiocontin- uummapofthecentral3pcoftheGalaxy,whichclearlyshows thespiralstructureofSgrAWest.Theyestimatedthetotalmass of ionized gas in the Minispiral at ≈ 60 M , with ≈ 10 M in ⊙ ⊙ the Northern Arm, ≈ 10 M in the Eastern Arm, ≈ 35 M in ⊙ ⊙ the Western Arc (which they referred to as the south arm) and ≈ 5 M in the Bar (which they referred to as the west arm). ⊙ Fig.1. Different coordinate systems used in this paper, shown They also estimated the electron density at ≈ 5×104 cm−3 in in the plane of the sky: α (right ascension) and δ (declination) the brightest clumps and ≈ 103 cm−3 in the lower-brightness aretheequatorialcoordinates;l(longitude)andb (latitude)are features.Ekersetal.(1983),whomappedtheSgrAWestregion theGalacticcoordinates;andx(towardtheSun),yandzarethe withtheVLAat2cm,6cmand20cm,foundthattheMinispiral Galactocentriccartesiancoordinates.Seetextformoredetails. has projected dimensions ≈ 60′′ ×25′′ (2.5 pc×1.0 pc), with 2 KatiaFerrie`re:InterstellargasnearSgrA∗ remarked that their derived emission measure of the extended component is perfectly consistent with the emission measure ≈ 106 pccm−6 neededtoexplainthelow-frequencyturnoverin the Sgr A∗ radio spectrum by foregroundfree-free absorption, if Sgr A∗ lies in the middle of Sgr A West and in front of the Minispiral. Yusef-Zadehetal.(1989)madehigher-resolutionVLA2cm and 6 cm radio continuum observations of Sgr A West, which brought out a number of fine-scale morphological details. For instance,theydetectedthe stellar windfromthe redsupergiant IRS 7, located ≈ 0.25 pc north of Sgr A∗, and they suggested that this wind is photo-ionized externally by the UV radiation bathing the Central Cavity. They also observed the so-called Minicavity (first described by Morris&Yusef-Zadeh 1987) – a nearly circular, ≈ 0.08 pc diameter hole in the distribution of ionized gas in the east-west Bar, centered ≈ 0.14 pc south- westofSgrA∗.TheynaturallyproposedthattheMinicavitywas swept out by a spherical stellar wind, although the most obvi- ousstellarcandidate,theIRS16complex,liesatthenortheast- ern periphery of the Minicavity, not at its center. Other possible scenarioswere later putforward.Robertsetal. (1996) sug- gestedthattheMinicavityistheionizedcomponentofacompact molecularcloudmovingalonganorbitthatpassesverycloseto Sgr A∗. Along different lines, Lutzetal. (1993) invoked a fast Fig.2. Composite image showing (in green) the 3.6 cm radio wind fromone or morenearbysourceswhich wouldblow into continuumemission from warm ionized gas in the Sgr A West Hiiregion,withthethree-armMinispiralemergingveryclearly, the Bar streamer and produce an expanding gas bubble there. and(inred)the3.4mmHCN J =1→0lineemissionfromthe Meliaetal. (1996) proposeda more elaborate model, in which the centralmassive black hole gravitationallyfocuses the wind surroundingCircumnuclear Ring (CNR). The radio continuum fromIRS16,partiallyaccretesfromit, andexpelsthe restin a dataarefromYusef-Zadehetal.(2008)andtheHCNdatafrom collimated flow whose collision with the Bar streamer creates Wrightetal.(2001).Figurecredit:FarhadYusef-Zadeh. theMinicavity. Sgr A West was also observed in various emission lines, starting with infrared lines such as the 12.8 µm [Neii] the long dimension along the Galactic plane, and for each of fine-structure line (Lacyetal. 1979, 1980; Serabyn&Lacy thethreespiralarms,theyderivedanaverageemissionmeasure ≈4×106pccm−6,anelectrondensity≈4×103cm−3,andatotal 1985; Serabynetal. 1988; Lacyetal. 1991), the 2.06 µm Hei line (Halletal. 1982; Geballeetal. 1991; Paumardetal. massofionizedgas≈15M (assumingafillingfactorofunity). ⊙ 2004), the 2.17 µm Hi Brγ recombination line (Wrightetal. They also detected diffuse emission from Sgr A West, over an area ≈ 75′′ × 40′′ (3.1 pc × 1.6 pc), but they argued that this 1989; Geballeetal. 1991; Herbstetal. 1993; Paumardetal. 2004), etc. The early infrared line emission images (see, e.g., diffuseemissionismainlynonthermalandthatmostofthether- Wrightetal. 1989; Lacyetal. 1991) were found to closely re- mal emission from Sgr A West (at least at 6 cm) really comes semblethe6cmradiocontinuummapofLo&Claussen(1983). from the Minispiral. Note that both Lo&Claussen (1983) and Besides,Lacyetal.(1991)wereabletoreproducethemorphol- Ekersetal.(1983)assumedr = 10kpc,sothattheirestimated ⊙ ogyandkinematicsofmuchof the [Neii]-emittingionizedgas massesanddensitiesneedtoberescaledtoouradoptedvalueof r =8.5kpc.1 with a one-armedlinear spiral containingthe Western Arc and ⊙ the NorthernArm (butexcludingthe Eastern Arm and mostof Similarly,byanalyzingtheVLA2cmradiocontinuummap theBar),alongwhichthegasisinnearlycircularKeplerianro- of Brown&Liszt (1984), Beckertetal. (1996) found that the tationaboutSgr A∗. The[Neii] dataofLacyetal. (1991) were Minispiralis superimposedontoan extendedGaussian compo- nentwith projectedFWHMdimensions≈ 70′′×52′′ (2.9pc× subsequently re-examined by Vollmer&Duschl (2000), who, in addition to circular rotation, allowed for turbulent motions 2.1 pc). However, they attributed the whole extended compo- plus slow radial accretion. They found that the Western Arc, nenttothermal(free-free)emission,which,foranelectrontem- the Northern Arm and part of the Bar are located in a single perature ≈ 6000 K, gave them a central emission measure ≈ 2×106 pccm−6.Assuming,inaddition,aline-of-sightdepth plane(theplaneoftheCNR),whereastheEasternArmisactu- allycomposedoftwodistinctpiecesbelongingtotwodifferent ≈1pcandafillingfactorofunity,theyderivedanelectronden- sity ≈ 1.4× 103 cm−3 and a total H+ mass ≈ 250 M for the planes,oneofwhichalsoenclosestherestoftheBar.Asidefrom ⊙ thedensegasconfinedtotheMinispiral,tenuousgasappearsto extendedcomponent.FortheMinispiral,theyassumedaline-of- sightdepth≈0.1pcandobtainedanelectrondensity≈104cm−3 fill the plane of the Western Arc + NorthernArm and possibly andatotalH+ mass≈ 9 M .Hence,Beckertetal.(1996)came oneoftheplanesoftheEasternArm. ⊙ Inparallelto infraredemission lines, which are plaguedby totheconclusionthat,despiteitsspiralappearance,SgrAWest interstellar dust extinction, radio emission lines have proven hasmostofitsmassspreadoutthroughoutitsvolume.Theyalso particularly valuable since the early work of vanGorkometal. 1 Asageneralrule,distancesscaleasr ,surfacedensitiesasr0,vol- (1985) and Schwarzetal. (1989). Roberts&Goss (1993) car- umedensitiesasr−1andmassesasr2.How⊙ever,whenvolumeden⊙sities riedoutVLAobservationsofthe8.3GHz(3.6cm)H92αrecom- ⊙ ⊙ andmassesareinferredfromemissionmeasures,theyscaleinsteadas bination line to investigate the global kinematics and tempera- r−0.5andr2.5,respectively. turedistributionofSgrAWest.TheyfoundthattheWesternArc, ⊙ ⊙ 3 KatiaFerrie`re:InterstellargasnearSgrA∗ theNorthernArmandtheextendedbar(composedoftheEastern come three small patchy features, named the Western Bridge, Arm,theBaranditslinearextensiontothenorthwest)constitute theNorthernArmChunkandtheBarOverlay.Focusingonthe threedistinctkinematicentities.IncontrasttotheWesternArc, Northern Arm, Paumardetal. (2004) showed that its kinemat- which appearsto be in near circular rotationaboutSgr A∗, the ics could be mostly modeled with a system of Keplerian or- NorthernArmandtheextendedbardonotappeartohavesignif- bits about Sgr A∗; these orbits are all close to the plane of the icant circular motions. Roberts&Goss (1993) also derived the CNR,albeitnotperfectlycoplanar,suchthattheyformathree- electron temperature in Sgr A West, by combining their H92α dimensional, saddle-shaped surface – like the inner surface of line data with a 8.3 GHz continuum map and making the as- a torus. This kind of geometry would naturally come about if sumptions (all found to be satisfied in the case at hand) that an infalling cloud was tidally stretched by the central massive the emitting gas is in local thermodynamicequilibrium (LTE), black hole and had its inward-facingside photo-ionizedby hot thecontinuumemissionisthermalfree-free,thecontinuumand stars from, e.g., the IRS 16 complex. The warping of the flow recombination-line emissions are optically thin, and pressure surfacewouldthengiverisetoorbitcrowdingintheplaneofthe broadening is negligible. Under these conditions, the line-to- sky, precisely along the bright filament seen in intensity maps. continuumratioimpliesanelectrontemperature≈ 7000Kthat Finally,fromthedetectionoftwospotsofextinctionintheflux isapproximatelyuniformthroughoutSgrAWest. maps of the Northern Arm and the Bar, Paumardetal. (2004) Because radio recombination lines at centimeter wave- concluded that, along the line of sight, the Bar lies behind the lengths can be dominated by stimulated emission and affected NorthernArm,whichitselfliesbehindtheEasternBridge. bypressurebroadening,Shuklaetal.(2004)observedtheSgrA Complementingtheradialvelocitiesextractedfrominfrared Westregioninthe92GHz(3.3mm)H41αrecombinationline. and radio spectral lines are the transverse velocities associated Atthishigherfrequency,thecontinuumandrecombination-line withpropermotions.Yusef-Zadehetal.(1998)measuredproper emissionsarebelievedtoarisemostlyindenserionizedgas.The motionsofionizedgasinSgrAWest,basedonVLA2cmradio morphology of the ionized gas in the 92 GHz continuum and continuum observationsmade at three epochs over a nine-year H41αlineimagesisverysimilartothatinthe8.3GHzcontin- period. Their measurements revealed the existence of several uum image of Roberts&Goss (1993). The Northern Arm ap- features(notably,ahead-tailstructuredubbedthe”bullet”)with pearstobe≈0.9pclong,theEasternArm≈0.4pc,theWestern transversevelocitiesgreaterthanthelocalescapevelocities.The Arc≈ 1.5pc,theBar ≈ 1.1pc,andthefourspiralfeaturesare authorscombinedtheir measuredtransversevelocitieswith ex- roughly0.1pcwide.Underthesameassumptionsasthosemade istingradialvelocitiesfromH92αlinedata,wherebytheyfound by Roberts&Goss (1993), Shuklaetal. (2004) also derived a thatmost,andprobablyalmostall,ofthetotalvelocitiesexceed fairly uniformelectron temperature≈ 7000K. At this temper- the local escape velocities. From this, they concludedthat ion- ature, the intensity of the continuum(supposedlythermalfree- ized gas in the Northern Arm is probablyon an unboundorbit free) emission from the arms correspondsto an emission mea- – in agreement with Robertsetal. (1996), who attributed their sure≈2.5×107pccm−6,which,iftheline-of-sightthicknessof measuredH92αradialvelocitiesintheMinicavitytoionizedgas thearmsiscomparabletotheirwidth(≈ 0.1pc)andiftheion- being on a hyperbolic orbit about Sgr A∗. Yusef-Zadehetal.’s izedgasinthearmsissmoothlydistributed(fillingfactor≈ 1), (1998)conclusion, which contradicts the widely accepted view impliesanelectrondensity≈1.6×104cm−3inthearms.There- thattheNorthernArmisatidallystretchedstreamerofinfalling sultingH+ massesare≈2.8M intheNorthernArm,≈1.2M gas,couldbeexplainedifanenergeticphenomenon,afewpar- ⊙ ⊙ intheEasternArm,≈ 4.6 M intheWesternArc,≈ 3.4 M in secs away from Sgr A∗, pushed a gas cloud into the Galactic ⊙ ⊙ theBar,andhence≈12M intheentireMinispiral.2Otherwise, center. ⊙ thekinematicsoftheH41α-emittinggasareessentiallythesame Similarly,Muzˇicétal.(2007)performedthefirstpropermo- asthoseoftheH92α-emittinggas(seeRoberts&Goss1993). tion measurements of infrared dust filaments in the Minispiral A more complex picture of Sgr A West emerged from the and showed that their shapes and velocities are not consistent workofPaumardetal.(2004),whoobservedtheinnerpartsof with pure Keplerian rotation about Sgr A∗. Instead, they could the Minispiral in the 2.17 µm Hi Brγ and 2.06 µm Hei emis- result from the dynamical interaction between a fast GC out- sionlines.Akinematicanalysisoftheirdataledthemtoidentify flowandtheMinispiral,wheretheGCoutflowcouldeitherorig- nine coherentvelocity components, comprising both extended, inate in the central cluster of young mass-losing stars, or em- continuousflowsandsmaller,isolatedpatches.Themostpromi- anatefromtheaccretingblackhole,possiblyintheformofcol- nent component is the Northern Arm, which spreads well be- limated jets (e.g., Yuan 2006), or even arise from a combina- yond the namesake filament seen in intensity maps, forming a tionofboth.ThusMuzˇicétal.’s(2007)filamentsprovideanew wedgeextendingallthewayovertotheEasternArm.Thelatter piece ofcircumstantialevidenceforthe existenceof a GC out- is divided into three parts: two roughly parallel elongated fea- flow,addingtotheMinicavity,whichseemstobeconnectedto tures, namedthe Ribbonandthe Eastern Bridge,anda smaller SgrA∗byachainofplasmablobs(Wardle&Yusef-Zadeh1992; patchy feature at the western end, named the Tip. The Bar is Meliaetal.1996);thebow-shockstructureoftheextendedion- straightandextendsfromtheRibbontotheWesternArc,which ized envelope of IRS 7, with the apex facing toward Sgr A∗ is onlypartlyvisible in the present,limited field ofview. Then or IRS 16 (Yusef-Zadeh&Melia 1992), and the associated cometarytailofionizedgaspointingdirectlyawayfromSgrA∗ 2 Strictlyspeaking,theseestimatessupposethattheMinispirallies (Yusef-Zadeh&Morris1991);thesimilarbow-shock/cometary- intheplane of thesky, which isalmost certainly incorrect. Inreality, tail morphology of IRS 3 (Viehmannetal. 2005); the ob- if a section of an arm makes an angle θ with the plane of the sky, served waviness of the Northern Arm (Yusef-Zadeh&Wardle its estimated electron density and H+ mass (at given emission mea- 1993);thenarrowchanneloflowinterstellarextinctionrunning sure) should be multiplied by (cosθ)1/2 and (cosθ)−1/2, respectively. northeast-southwestthroughSgrA*,withalignedcometaryfea- However,amoreimportantsourceoferroristheuncertaintyinthetrue tures(Scho¨deletal.2007);etc. width/thicknessofthearms,whichcouldpossiblybeasmuchas2times largerthanquotedbyShuklaetal.(2004).Inthiscase,theelectronden- The dynamics of the three ionized streams in Sgr A West sitywouldbelowerbyafactor20.5 ≃1.4andtheH+masseslargerby werefurtherstudiedbyZhaoetal.(2009),whocombinedproper afactor21.5 ≃2.8. motion measurements for 71 compact Hii features with radial 4 KatiaFerrie`re:InterstellargasnearSgrA∗ velocitymeasurementsfromarchivalH92αlinedata.Theywere observed the Northern Streamer in dust far-infrared emission. able to model the three ionized streams with three bundles of Alternatively,onlytheNorthernArmwouldbordertheNorthern Keplerian elliptical orbits about Sgr A∗, all three bundles be- Streamer,whiletheEasternArmwouldbeanionizedrimatthe ing confined within the central 3 pc. They confirmed that the surfaceofanotherinfallingneutralstreamer.3Thesouthern[Oi] Western Arc stream is nearly circular, while the Northern and emission peak, for its part, is close to (but apparently slightly Eastern Arm streams have high eccentricities, and they sug- outside) the most prominentradio continuumemission peak in gested thatthe latter may collidein the Bar region,which they theWesternArc,which,werecall,isgenerallybelievedtobethe located mostly behind Sgr A∗. They also found some support ionizedinneredgeofthewesternportionoftheCNR.Byusing for Liszt’s (2003) suggestionthatthe Eastern Arm and the Bar their 63 µm [Oi] data toward the northern peak together with togetherforma single streamer. For futurereference,the mod- previous146µm[Oi]and158µm[Cii]data(fromGenzeletal. eledorbitalparametersofthethreeionizedstreams(rescaledto 1985;Poglitschetal.1991)asinputtomodelcalculationsofcol- r = 8.5 kpc and adjusted to our line-of-sight vector pointing lisional excitation and radiative transport, Jacksonetal. (1993) ⊙ towardtheSun)areasfollows:theNorthernArmhassemima- estimatedthatneutral(presumablyatomic)gasinsidetheCentral jor axis a = 1.05 pc, eccentricity e = 0.83, inclination (be- Cavity has a temperature ≈ (170 ± 70) K, a true hydrogen tween the angular momentumvector and the line-of-sightvec- density ≈ 3 × 105 cm−3, a space-averaged hydrogen density tor) i = 41◦ and total length(calculatedfrom the quotedrange ≈ 1.6×103 cm−3 (beam-averagedhydrogencolumndensityin oftrueanomaly)l=2.7pc;theEasternArm(orEasternArm+ a 55′′ beam divided by an assumed line-of-sightpathlength of Bar)hasa = 1.5pc,e = 0.82,i = 58◦ andl = 3.5pc;andthe 1 pc; note, however, that the value reported in their Table 2 is WesternArchasa=1.2pc,e=0.20,i=63◦andl=3.5pc. toolowbya factorof2)andatotalhydrogenmass(associated Relying on the Keplerian model of Zhaoetal. (2009), withtheNorthernandEasternArms)≈300M . ⊙ Zhaoetal. (2010) drew a three-dimensional view of the three Latvakoskietal. (1999) found that the three arms of the ionizedstreams,whichclearlyshowsthattheNorthernArmand Minispiralseeninthermalradiocontinuumemissionhavecoun- WesternArcarenearlycoplanar,thattheirmeanorbitalplaneis terpartsin dust far-infraredcontinuumemission, which tend to nearlyperpendiculartotheorbitalplaneoftheEasternArm,and lie ≈ 1′′−3′′ fartherfromSgrA∗. Thisconfiguration,theyex- thattheEasternArmcollideswiththeNorthernArmintheBar plained,isconsistentwiththeradioandfar-infraredfeaturesbe- region,justbehindSgrA∗.Moreimportantly,Zhaoetal.(2010) ing photo-ionized and photo-dissociated, respectively, by UV presentednewobservationsofthe232GHz(1.3mm)H30αre- sources very close to Sgr A∗. Latvakoskietal. (1999) also de- combinationline,whichtheyinterpretedtogetherwithprevious tected a far-infrared ring running along the inner edge of the H92αline and22 GHz continuummeasurementsin the frame- CNR,withradialthickness≈8′′−11′′(0.32pc−0.44pc),and workofanisothermal,homogeneous,non-LTEHiimodel,tode- intersectingthefar-infraredminispiralatitswesternarc.4 They terminethephysicalparametersofthehigh-densityionizedgas naturallyidentifiedthisfar-infraredringasthephoto-dissociated atselectedpositions(towardknowninfraredsources)alongthe inner layer of the CNR. The masses of (presumably photo- NorthernandEasternArms.Theyobtainedelectronkinetictem- dissociated)hydrogentracedinthefar-infraredare≈110M in peraturesintherange≈(5000−11000)Kandelectrondensities ⊙ thenorthernarm,≈ 50 M intheeasternarm+bar,≈ 660 M intherange≈(104−105)cm−3,withvaluesupto≈13000Kand ⊙ ⊙ in the western arc, ≈ 1320 M in the full far-infraredring and ≈ 2×105cm−3 intheBarregion.Thehigherelectrontempera- ⊙ ≈ 16 M in the Central Cavity outside the minispiral. Note ⊙ turesanddensitiesintheBarregioncouldresulteitherfromgas thatthenorthernarmstretches≈ 20′′ (0.8pc)northbeyondthe heating and compression by powerful winds from stellar clus- CNRandthatthe110 M massrefersonlytoitsportioninside ⊙ terssuchasIRS16andIRS13orfromthecollisionbetweenthe theCNR. Themeanhydrogendensitiesnearthesouthwestand NorthernandEasternArms. northeastendsofthefar-infraredring,asobtainedfromthehy- ***** drogencolumndensitiesdividedbyaline-of-sightdepthof1pc, In addition to warm ionized gas, the Central Cavity are≈1.6×104cm−3and≈4.0×104cm−3,respectively.Lastly, also contains substantial amounts of neutral atomic gas, de- thedusttemperatureisfoundtobeintherange≈(60−120)K. tectable through atomic line emission (Genzeletal. 1985; Latvakoskietal. (1999) proposeda simple, self-consistentdust Poglitschetal. 1991; Jacksonetal. 1993) and through dust model,adjustedtoreproducethefar-infrareddataaswellaspos- thermalcontinuumemission (Davidsonetal. 1992; Zylkaetal. sible.Intheirmodel,thefar-infraredring(includingthewestern 1995;Telescoetal.1996;Latvakoskietal. 1999).Onthe other arc)isnearlycircular,withinnerradius1.58pc,axialthickness hand, little molecular gas seems to be present, except possibly 0.4 pc and inclination 65◦ to the plane of the sky. The north- forahotanddensemolecularcomponentdetectedinNH3(6,6) ern arm and the eastern arm + bar are both on parabolic or- emissionverynearSgrA∗(Herrnstein&Ho2002).Thegeneral bits about Sgr A∗, 10◦ and 85◦ out of the plane of the CNR, lack ofmoleculargascanbe understoodif anyinitially molec- respectively.The(poorlyconstrained)meanhydrogendensities ular cloud inside the Central Cavity has been largely photo- are∼ 1.2×104 cm−3 inthefar-infraredring,∼ 4×104cm−3 in dissociated bythe intense ambientUV radiation(Jacksonetal. thecoreofthenorthernarm,∼ 2×103 cm−3 inthecoreofthe 1993;Latvakoskietal.1999;Shuklaetal.2004). easternarm+barand∼ 25cm−3 in theCentralCavity outside The 63 µm [Oi] 3P1 → 3P2 line observations of theminispiral. Jacksonetal. (1993) indicate that neutral atomic gas exhibits twoprominentemissionpeaksonoppositesidesofSgrA∗.The northernpeak is partof a north-south,≈ 1′ (2.5pc) long ridge 3 Clearly, the recent work of Zhaoetal. (2010), which finds the of [Oi] emission, which the authors interpreted as a streamer NorthernandEasternArmstobeonnearlyperpendicularorbits,makes ofneutralatomicgasfallingthroughagapintheCNRintothe thesecondscenariomuchmorelikely. Central Cavity, between the Minispiral’s Northern and Eastern 4 To avoid any possible ambiguity, we reserve upper case Arms.Thelatter,theysuggested,couldsimplybebrightrimsof (Minispiral, Northern and Eastern Arms, Western Arc) for the origi- ionized gas at the surface of this neutral ”Northern Streamer” nalradiofeatures,anduselowercase(minispiral,northernandeastern – a suggestion already made by Davidsonetal. (1992), who arms,westernarc)fortheirfar-infraredcounterparts. 5 KatiaFerrie`re:InterstellargasnearSgrA∗ The last gaseous component present inside the Central (1985) used their 0.37mm CO J = 7 → 6 line emission mea- Cavity is a hot plasma, which was discovered with Chandra surements in conjunction with previous measurements of two through its diffuse X-ray thermal emission (Baganoffetal. lower and two higher CO rotational lines to determine the H 2 2003). This hot X-ray emitting plasma extends across the cen- density and gas temperature in CO-emitting regions. They ob- tral ≈ 20′′ (0.8 pc) of the Galaxy. A fit to its X-ray spectrum tainedbest-fitvalues≈3×104cm−3and≈300K,respectively, yields a temperature ≈ 1.3 keV (1.5×107 K) and an electron and they restricted the rangeof acceptabledensity-temperature density ≈ (26 cm−3)φ−1/2, where φ is the hot plasma filling combinations to (5 × 105 cm−3,150 K) − (104 cm−3,450 K). h h factor. If the plasma is fully ionized and has twice the solar They also concluded that the CO-emitting gas is very clumpy, abundances,its totalmassis≈ (0.1 M )φ1/2. Rockefelleretal. with a volume filling factor ∼ 0.1. Serabynetal. (1986), who ⊙ h observed the CNR in 2.6 mm CO J = 1 → 0 and 3.1 mm (2004) showed with three-dimensional numerical simulations CS J = 2 → 1 emission, inferreddensities of a few 105 cm−3 thatthishotplasmacouldbeentirelyexplainedasshockedgas fortheCS-emittinggas,andderivedamassofmoleculargasin producedincollisionsbetweenstellarwinds. the CNR & 1.5×104 M (rescaled to r = 8.5 kpc) from the ⊙ ⊙ measuredCOflux.Suttonetal.(1990)combinedtheir0.87mm 2.2.TheCircumnuclearRing CO J = 3 → 2 observations with existing CO J = 1 → 0 and J = 7 → 6 data to find that the H density and gas tem- 2 TheCNRhasbeenobservedindustthermalcontinuumemission peratureinCO-emittingregionsvaryfrom≈ 2×105 cm−3 and (e.g. Becklinetal. 1982; Mezgeretal. 1989; Davidsonetal. ≈ 200 K in the inner partsof the CNR to ≈ 2×104 cm−3 and 1992; Dentetal. 1993; Telescoetal. 1996; Latvakoskietal. ≈ 100Kintheouterparts.Theyalsoconfirmedtheclumpiness 1999) as well as in a wide variety of atomic and molec- of the CO-emitting gas and estimated its volume filling factor ular tracers, including the 21-cm line of Hi (Lisztetal. at ∼ 0.05. The more recent CO J = 7 → 6 observations of 1983),fine-structurelinesof[Oi]and[Cii](Genzeletal.1985; Bradfordetal.(2005),whichtheauthorsanalyzedtogetherwith Poglitschetal.1991),andvarioustransitionsofH2(Gatleyetal. publisheddataontwolowerandtwohigherCOrotationaltran- 1984, 1986; Depoyetal. 1989; Burton&Allen 1992; sitions,takingradiativetransferintoaccount,yieldedanH den- 2 Yusef-Zadehetal. 2001), CO (Lisztetal. 1985; Genzeletal. sity≈(5−7)×104cm−3andagastemperature≈(200−300)K. 1985;Harrisetal.1985;Serabynetal.1986;Gu¨stenetal.1987; Two other frequently used diagnostic molecules are HCN Suttonetal. 1990; Bradfordetal. 2005), OH (Genzeletal. and HCO+. In the 3.4 mm HCN J = 1 → 0 emission map of 1985), CS (Serabynetal. 1986, 1989; Montero-Castanõetal. Gu¨stenetal. (1987), the CNR emerges as an almost complete 2009),HCN(Gu¨stenetal.1987;Marretal.1993;Jacksonetal. molecular ring centered ≈ 8′′ (0.32 pc) southeast of Sgr A∗, 1993;Marshalletal.1995;Wrightetal.2001;Christopheretal. which has projected major and minor mean diameters ≈ 95′′ 2005; Montero-Castanõetal. 2009), HCO+ (Marretal. 1993; and50′′ (4.0pc and2.1pc),respectively.Themajoraxishasa Wrightetal. 2001;Shuklaetal. 2004;Christopheretal. 2005), position angle ≈ 30◦ east of north, so that it is approximately NH (Coil&Ho 1999; McGaryetal. 2001; Herrnstein&Ho 3 alignedwiththeGalacticplane(seeFigure1).Moreover,ifthe 2005), etc. Collectively, these tracers lead to the picture of an ring is intrinsically circular, the aspect ratio ≈ 2:1 implies an asymmetric, extremely clumpy, torus-like CNR, with a sharp inclination angle ≈ 60◦ out of the plane of the sky. The ring’s inner boundary at radius ≈ (1 − 1.5) pc and a more blurry, inner and outer radii along the major axis are ≈ 30′′ and 65′′ irregularouterboundaryatradius≈(2.5−3)pctothenortheast (1.2 pc and 2.7 pc), respectively,but on the southwestside the and ≈ (4 − 7) pc to the southwest (see Figure 2). Besides, HCN emission extends out to ≈ 100′′ (4.2 pc) – for compari- the CNR appears to be orbiting about Sgr A∗ and to have son,Serabynetal.(1986)derivedanouterradius≈7pcforCO considerableinternalmotions. J = 1 → 0 emission on the southwest side. The axial thick- Shortly after Becklinetal. (1982) discovered the CNR in nessof the wholeHCN structureincreasessteadily withradius dust far-infrared continuum emission, Genzeletal. (1985) ob- from ≈ 0.42 pc at 1.7 pc to ≈ 1.2 pc at 4.2 pc (rescaled to served it in severalfar-infraredemission lines, namely,in fine- r = 8.5kpc).Gu¨stenetal.’s(1987)studyalsoprovidesimpor- ⊙ structure lines of [Oi] and [Cii] and in rotational lines of CO tantkinematicinformation.Themeasuredradialvelocitypeaks and OH. Their observations revealed a disk or torus of neu- at ≃ 100 kms−1, and its variation with position angle on the tral gas around the Central Cavity, with inner radius ≈ 1.4 pc, sky agrees reasonablywell with that expected for rotation of a outer radius & 4.2 pc (both rescaled to r⊙ = 8.5 kpc), incli- warpedringwithrotationvelocity≈110kms−1/135kms−1and nation ≈ 69◦ to the plane of the sky, and tilt ≈ 20◦ to the inclinationangle≈70◦/50◦inthesouthwest/northeastparts.In Galactic plane. Atomic and molecular gases were found to be additiontothisoverallrotation,thegasexhibitsstrongturbulent mixedthroughoutthedisk/torus,withthefractionofatomicgas motionswith a velocity dispersion decreasing from an average decreasing outward – as expected for a photo-dissociation re- of ≈ 55 kms−1 near the inner edge to ≈ 37 kms−1 near the gion illuminated from inside. From the intensities of the [Oi] southwestouteredge. and [Cii] lines, Genzeletal. (1985) estimated the temperature Jacksonetal.(1993),whomappedasomewhatsmallerarea of the atomic gas at ≈ 300 K and its true hydrogen density inthe1.1mmHCN J = 3 → 2emissionline,reachedslightly at ≈ 105 cm−3. Furthermore, from existing dust far-infrared differentconclusions.Theyobtainedavelocity-integratedHCN continuum emission data, they derived a space-averaged hy- mapsimilar to thatof Gu¨stenetal. (1987),buttheyinterpreted drogen density ≈ 7 × 103 cm−3 in the [Oi] emission region the kinematic data in a different manner. Instead of invoking a (assuming a size ≈ (2 − 3) pc) and a total hydrogen mass single rotating ring that is warped, they appealed to four sep- ≈ 1.5×104 M⊙ within a radius of 4.2 pc, while from existing arate kinematic components,the most prominentof which is a CO J =1→0lineemissiondata,theyderivedatotalhydrogen rotating circularring of peak radius≈ (1.5−2) pc, inclination mass≈(1.5−3.7)×104 M⊙ inthepurelymoleculargasbeyond angle ≈ 65◦ − 75◦, position angle of the projected major axis 4.2pc(allmasseswererescaledtor⊙ =8.5kpc). ≈ 25◦ east of north and rotation velocity ≈ 110 kms−1. The Subsequent CO and CS observations offered additional in- other,weakercomponentsaretheso-called50kms−1Streamer, sight into the physical conditions of the CNR. Harrisetal. 70 kms−1 Feature and−20 kms−1 Cloud.For the physicalpa- 6 KatiaFerrie`re:InterstellargasnearSgrA∗ rameters of the molecular gas in the CNR, model calculations J = 1 → 0 lines of CO, HCN, HCO+, N H+, HNC and SiO, 2 ofHCNexcitationandradiativetransportyieldedatemperature the J = 2 → 1 line of SiO and the J = 3 → 2 line of CO. A ≈ (50− 200) K, a true H density ∼ (106 − 108) cm−3 and a one-zonelarge-velocity-gradientradiative-transferanalysisofa 2 space-averagedH density∼(104−105)cm−3(assumingaline- restrictedsetoflines(CO J = 1 → 0,CO J = 3→ 2andHCN 2 of-sightpathlengthof1pcthroughtheCNR). J = 1 → 0),assuming[CO]/[H ]=2.4×10−5 and[HCN]/[H ] 2 2 Wrightetal. (2001) imaged the central 12 pc simultane- = 4.8× 10−8, concludes that the bulk of the CNR is made of ously in the 3.4 mm HCN and HCO+ J = 1 → 0 transitions, moleculargaswith kinetictemperature& 40 K and H density 2 bothofwhicharesupposedtotracemoleculargaswithdensity ≈ (5×103−2×104) cm−3,the best-fitvaluesbeing63K and ∼(105−106)cm−3.Thetwotracerspresentessentiallythesame 1.26 × 104 cm−3, respectively. Comparisons between the var- velocity-integratedemission,andbothindicatethattheCNR is ious line-intensity maps (most importantly, the CO and HCN notadisk,butawell-definedringpeakedatradius≈45′′(1.9pc) J =1→0maps)showthattheinnermostring,atradius≈2pc, thatextendsradiallyfrom≈ 20′′ to60′′ (0.8pcto2.5pc),with isbothwarmeranddenserthanthebulkoftheCNR.ThetotalH 2 a southwestextensionoutto ≈ 120′′ (5pc). Kinematically,the massoftheCNR,estimatedfromthe13CO J =1→0intensity CNRisfoundtoconsistoftwoorthreedistinctstreamersrotat- map,is≈(2.3−5.2)×105 M⊙,whichcorrespondstothetypical ingaroundSgrA∗ onseparateorbits.Thedifferentinclinations massofGMCsintheGCregion.Muchlargeristhevirialmass oftheseorbitswouldbethereasonforthewarped-ringappear- oftheCNR,estimatedat≈ 5.7×106 M⊙.Accordingtotheau- anceoftheCNR. thors, the importantdiscrepancybetween both masses strongly Christopheretal. (2005) performed additional HCN and suggeststhattheCNRisnotboundbyself-gravity,butratherby HCO+ J =1→0observationswithenhancedspatialresolution, the central mass. Finally, the CO J = 3 → 2 data, interpreted which enabled them to study the internalstructure of the CNR withasimplekinematicmodel,pointtoatwo-regimesituation, ingreaterdetail.Theirvelocity-integratedmapsareonthewhole wherethebulkoftheCNR isinfallingataspeed≈ 50kms−1, similartothoseofWrightetal.(2001),andthey,too,displaya whiletheinnermostringat≈2pcispredominantlyrotating. well-defined ring, although with slightly different dimensions. Numerical simulations have greatly contributed to enhance Here,theazimuthally-averagedHCNemissionisfoundtopeak ourunderstandingoftheCNR. Forinstance,the sticky-particle at radii between ≈ 40′′ and 50′′ (1.7 pc and 2.1 pc), drop off calculationsofSanders(1998)showedthatthemorphologyand steeply(over≈10′′)oneithersideofthepeak,andthendecline kinematicsof the CNR couldbe explainedby the tidal capture muchmoregraduallyoutpast≈150′′(6.2pc).Christopheretal. and disruption of a low angular-momentum cloud by the cen- (2005) were able to resolve 26 dense molecular cores within tralmassiveblackhole.Thecloudwouldfirstbe stretchedinto the CNR, with typical sizes ≈ 7′′ (0.3 pc), and estimated their a long filament, which would wrap aboutthe dynamicalcenter masses in two independent manners: virial masses were de- and collide a few times with itself. Under the effect of viscous rived from the measured sizes and velocity widths, assuming dissipation,the tidal debriswould thensettle into an asymmet- thecorestobegravitationallybound,andoptically-thickmasses ric,precessingdispersionring,whichwouldpersistfor&106yr. were derived from the measured sizes and HCN column den- A similar scenario could apply to the Northern Arm (with its sities, assuming an HCN-to-H ratio of 10−9 (as opposed to westwardextension)insidetheCentralCavity,althoughtheorig- 2 2×10−8 in Jacksonetal. 1993) andmultiplyingby 1.36to ac- inal cloud would have to be smaller and on a lower angular- count for helium. Both masses were found to agree well, with momentum orbit. It is interesting that, in the best-fitting simu- median values ≃ 1.7 × 104 M and ≃ 2.4 × 104 M , respec- lation, the orbital plane of the extended Northern Arm lies at ⊙ ⊙ tively,correspondingtomeanH densitiesinsidethedensecores 10◦ ofthatoftheCNR–whichagreeswellwiththefindingsof 2 ≈ 4×107 cm−3 and ≈ 5×107 cm−3. From their derived core Latvakoskietal. (1999), Paumardetal. (2004) and Zhaoetal. masses,Christopheretal.(2005)estimatedthetotalmassofthe (2010)(seeSection2.1). CNRat≈106 M . ⊙ Remarking that the HCN and HCO+ J = 1 → 0 emis- 2.3.TheSgrAEastSNR sion lines from the GC region are subject to self-absorption duetotheintervening(coolerandmorediffuse)moleculargas, ThenonthermalradiosourceSgrAEastclearlyhasashell-like Montero-Castanõetal. (2009) turned to the higher 0.85 mm structure.TheVLA20cmradiocontinuummapofEkersetal. HCN J = 4 → 3 transition, which they observed toward the (1983) shows that this shell is elongated along the Galactic CNR, alongwith the 0.87mm CS J = 7 → 6 transition.They plane,withprojecteddimensions≈ 3.′6×2.′7(9.0pc×6.7pc), detected very clumpy emission from both tracers, with clumps andthatitisoff-centeredby≈2.1pcslightlynorthofeastfrom arrangedinanecklace-likefashionaroundtheCNR.Thesouth- Sgr A∗. In projection, the western side of the Sgr A East shell ern part of the CNR has stronger emission and is found to be overlapswith the Sgr A West Hii region,and the Western Arc denserandwarmer(fromacomparisonwiththepreviouslymea- appears to smoothly merge into the shell. The shell-like mor- suredHCNJ =1→0line)thanthenorthernpart.Similarly,the phology of Sgr A East, its measured size and its nonthermal inneredgeoftheCNRappearstobewarmerthantheouterparts (supposedly synchrotron) radio emission all converge to indi- –asexpectediftheCNRisheatedbytheintenseUVradiation catethatitisanSNR–asinitiallyproposedbyJones(1974)and fromthecentralstarcluster.Theclumpspresentwiderangesof Ekersetal.(1975). physicalcharacteristics,withsizes≈3′′−13′′(0.12pc−0.5pc), In the VLA 90 cm radio continuum image of Pedlaretal. virial masses ≃ (0.4 − 60) × 104 M⊙ and virial H2 densities (1989),theSgrAEastshellhasprojecteddimensions≈3.′3×2.′1 ≈ (2 − 40) × 107 cm−3. Summing the virial masses of all the (8.2 pc×5.2 pc) and its major axis is at position angle ≈ 40◦ HCN (4 → 3) clumps listed by Montero-Castanõetal. (2009) east of north, i.e., roughly parallel to the Galactic plane (see givesatotalCNRmass≈1.3×106M⊙,comparabletotheCNR Figure1).Onthewesternsideoftheradioshell,thespiralpat- massestimatedbyChristopheretal.(2005). tern ofthe Sgr A West Hii regionclearly standsoutin absorp- The physical parameters of the CNR were very recently tion(free-freeabsorptionbythermalgas)againstthenonthermal re-estimated by Okaetal. (2011), based on several millime- emissionfromSgrAEast.ThisdefinitelyplacesSgrAWestin ter and submillimeter molecular emission lines, including the front,orclosetothenearsurface,ofSgrAEast–asarguedbe- 7 KatiaFerrie`re:InterstellargasnearSgrA∗ fore by Yusef-Zadeh&Morris (1987), based on a comparison metals), they inferred a temperature ≈ 2.1 keV (2.4× 107 K) of 6 cm and 20 cm radio continuum maps. Yet not all of the and an overabundanceof heavy elements by a factor ≈ 4 with 90cmemissionfromSgrAEastisactuallyabsorbedinthisdi- respect to solar levels, with an inward gradient in the abun- rection, which may indicate that Sgr A West lies within Sgr A dance of iron relative to the other metals. Assuming a spher- East and close to its near surface (Yusef-Zadehetal. 2000; see ical volume of radius 1.6 pc, they derived an electron density alsoMaedaetal.2002).Ontheeasternside,theboundaryofthe ≈ (6 cm−3)φ−1/2, a total gas mass ≈ (2 M )φ1/2 and a total h ⊙ h radioshellisstrikinglystraight,whichsuggeststhatSgrAEast thermal energy ≈ (2 × 1049 ergs)φ1/2, where again φ is the iscollidingwiththeneighboringM−0.02−0.07GMC. h h hot plasma filling factor. This estimated gas mass and thermal Additional support for this suggestion comes from the energy,togetherwith the strongenrichmentin heavyelements, 1.3 mm observations of Mezgeretal. (1989), which reveal a lends credence to the long-standing idea that Sgr A East is an dust ring surrounding the Sgr A East radio shell. This dust SNR. Moreover, the combination of shell-like nonthermal ra- ring, with major inner diameter ≈ 10 pc along the Galactic dioemissionandcentrallyconcentratedX-raythermalemission plane,wasalsodetectedinOHandH COabsorption(Sandqvist 2 classifythisSNRasamixed-morphologySNR. 1974; Whiteoaketal. 1974) as well as in CO emission (see Sakanoetal. (2004) obtained a higher-quality X-ray spec- Mezgeretal. 1996). Its eastern part coincides with a ridge of trum of Sgr A East with XMM-Newton. Both their spectral fit- dense molecular gas in M−0.02−0.07 (see Section 2.5) and tingandtheirline-ratioanalysisrequireatleasttwotemperature its southernpartcoincideswith dense moleculargasbelonging components,at≈ 1keVand≈ 4keV,respectively.Thederived to M−0.13−0.08.Mezgeretal. (1989) interpretedthe observed temperaturesaresomewhatlowerinthecoreoftheX-raysource ringasashellofgasanddustsurroundingtheradioshellthatis (≈0.9keVand≈3keV,respectively).TheFeabundancevaries producedbythesamesupernovaexplosion.Thereasonwhythe from ≈ 4 times solar in the core down to ≈ 0.5 solar in the gas-and-dustshellisparticularlydensetowardtheeastandsouth outer region, whereas other metals (S, Ar, Ca) have more uni- would be because in these directions the SNR has expanded formabundances,allinthe range≈ (1−3)solar.Ifthecoreis into the M−0.02−0.07 and M−0.13−0.08 GMCs, respectively. Towardthewest,theshellmayhaveencounteredSgrA∗ andbe approximatedasa28′′(1.1pc)radiussphere,andifthelow-and high-temperaturecomponentswithin it are in thermal pressure captured(atleastpartially)byitsgravitationalpull,soastoform balance and have a combined filling factor φ , their respective thepresent-dayCNR. Theauthorsalsopointedoutthatthe ex- h plosionseemstohavedispersedmostofthegasinfrontofSgrA electron densities are ≈ (20 cm−3)φ−1/2 and ≈ (6 cm−3)φ−1/2. h h West,whichcouldbeexplainediftheexplosionoccurredinside Thecorrespondingtotalmassandthermalenergyofhotplasma aGMC,closetoitsnearsurface.Thetotalhydrogenmassswept- in the core are ≈ (1.4 M⊙)φh1/2 and ≈ (1.3 × 1049 ergs)φ1h/2, up into the gas-and-dustshell was estimated at ≈ 6×104 M , with65%ofthemassand38%ofthethermalenergyinthelow- ⊙ whichimpliesa meanpreshockdensity∼ 104 cm−3 in thepar- temperaturecomponent.ThedistinctoverabundanceofFeinthe entGMC. core(andnotoutside)suggeststhattheaboveestimatesreferto Thedetectionof1720MHzOHmasers,without1665MHz stellar ejecta, which is consistent with a single supernova ex- and 1667 MHz counterparts, along the periphery of Sgr A plosion. The rest of the X-ray emitting plasma is more likely East (Yusef-Zadehetal. 1996, 1999a) revealed the presence of shockedinterstellarmatter. shockedmoleculargas,therebyprovidingindependentevidence Much deeper Chandra observations than those of that the expanding SNR is interacting with nearby molecular Maedaetal. (2002) enabled Parketal. (2005) to perform clouds. At the southeastern boundary of Sgr A East with the a spatially resolved spectral analysis of Sgr A East. They M−0.02−0.07cloud,1720MHzOHmasersweredetectedwith observedenhancedhardX-rayemissionfroma Fe-richplasma radial velocities between 49 kms−1 and 66 kms−1, i.e., close over a ≈ 40′′ (1.7 pc) diameter region near the center of the to the 50 kms−1 systemic velocity of the cloud, which rein- SNR. Theynaturallyidentifiedthisbright,Fe-richplasma with forces the concept that the Sgr A East shock is propagating stellarejecta.LikeSakanoetal.(2004),theyfitteditshardX-ray into M−0.02−0.07. 1720 MHz OH masers were also detected spectrum with two temperatures (estimated here at ≈ 1 keV toward the CNR, with radial velocities of 134 kms−1 (at the and≈ 5keV)andtheyderivedahighFeabundance(≈ 6times intersection between the Northern Streamer and the CNR) and solar) compared to the S, Ar, Ca abundances (≈ (0.7 − 1.8) 43 kms−1 (alongthe outer western edge of the CNR). To con- solar). Parketal. (2005) also observed soft X-ray emission firmthepresenceofshockedmoleculargasneartheOHmasers, outsidethehardX-raycore,inparticular,inaplume-likefeature Yusef-Zadehetal.(2001)lookedfor2.12µmH2v=1−0S(1) extending toward the north of the SNR. They found that the emission,andtheyfoundthatallbutoneoftheOHmasersde- emitting plasma in this feature could be characterized by a tectedintheregionareindeedaccompaniedbyH2 emission.In singletemperature(≈ 1.3keV) andsolar abundances,andthey particular, the 43 kms−1 OH maser lies in projection along an identified it with shocked interstellar matter. Parketal. (2005) H2 filament,whichextendsover≈ 1′ alongthewesternbound- also provided density estimates, both in the central Fe-rich ary of Sgr A East and peaks at velocities ≈ (50−75) kms−1, core andin the northernplume-likefeature.For the latter, they closetothepeakvelocitiesofthewesternedgeoftheCNR.The adopted a half-conical volume with a circular base of radius location,morphologyandkinematicsoftheH2 filamentandits ≈25′′andaheight≈50′′,andtheyobtainedanelectrondensity likelyassociationwitha1720MHzOHmaserstronglysuggest ≈ (7.4cm−3)φ−1/2. Forthecentralcore,theyassumeda≈ 40′′ thatthefilamentwasgeneratedbythepassageoftheSgrAEast h diameter sphere with pure Fe24+ ejecta, and they obtained shock over the CNR (Yusef-Zadehetal. 1999b). This, in turn, electrondensities ≈ (2.3cm−3)φ−1/2 and ≈ (0.5cm−3)φ−1/2 in impliesthatSgrAEastmusthaveengulfedpartoftheCNR. h h the low- and high-temperaturecomponents,respectively,while Insidethecavity oftheSgrA Eastradioshell,Maedaetal. theyderivedatotalFeejectamass≈(0.15M )φ1/2. (2002) observed a hot X-ray emitting plasma with Chandra. ⊙ h They noted that the X-ray emission is concentrated within the Finally, with Suzaku, Koyamaetal. (2007) acquired a de- central ≈ 2 pc of the radio shell. From the measured X-ray tailedX-rayspectrumofSgrAEast,whichdisplaysallthepre- spectrum(continuum+ Kα emission linesfromhighlyionized viously(firmlyortentatively)reportedemissionlines(Kαlines 8 KatiaFerrie`re:InterstellargasnearSgrA∗ fromHe-like S, Ar, Ca, Fe; Kα linesfromH-like S, Ar, Fe) as by Anantharamaiahetal. (1999) revealed an extended area of well as a number of newly discovered emission lines (Kα line H168αemission encompassingthe entire Sgr A East shell and fromHe-likeNi;KβlinesfromHe-likeS,Ar,Fe;Kγlinefrom coveringabroadrangeofradialvelocitiesfrom≈ −200kms−1 He-like Fe). The measured line ratios confirmed the necessary to ≈ +50 kms−1. The fact thatthe ionized gasobservedin the presenceofatleasttwotemperaturecomponents,whilethecom- H168α line is detected neither in the lower-frequency H270α pletespectralfittingrequiredanadditionalhardtail.Altogether, line (sensitive to n . 10 cm−3) nor in the higher-frequency e the best-fit spectrum consists of two thin thermal components, H110αandH92αlines(sensitiveton &1000cm−3)constrains e with ≈ 1.2 keV and ≈ 6 keV, plus a power-law component, theelectrondensityto lie inthe rangen ∼ (10−1000)cm−3. e whichcouldbecausedbyeitheracollectionofpointsourcesor Theelectrondensitycanbeadditionallyconstrainedbyconsid- non-thermalclumpsand filaments. Koyamaetal. (2007) found eringtheH168αdatainconjunctionwiththeradiospectrumof that, on average over the SNR, S, Ar, Ca have roughly solar SgrAEastobtainedbyPedlaretal.(1989)andbyassumingthat abundances,while Fe is overabundantby a factor≈ 2−3,and the H168α emission arises in the same thermal ionized gas as theyestimatedthetotalmassofhotplasmaat≈(27M )φ1/2. thefree-freeabsorptionthatisresponsibleforthelow-frequency ⊙ h turnover of Sgr A East. In this manner, Anantharamaiahetal. (1999) found that a modelwith electron temperature ≈ 104 K, 2.4.Theradiohalo emission measure ≈ 3.3 × 105 pccm−6 and electron density ≈ 100 cm−3 gave a good fit to all the data combined. The H+ TheSgrAEastshellappearstobesurroundedbyanextendedra- masspredictedbythismodelis∼ 8×104 M (thisisthevalue ⊙ diohalo.IntheVLA20cmradiographofYusef-Zadeh&Morris quoted by the authors, but we believe they meant an H+ mass (1987), the radio halo has approximately the same shape ∼8×103 M )overthe∼4′×4′projectedareaofSgrAEast. ⊙ (roughlyelliptical),aspectratio(∼ 1.5),orientation(parallelto Maedaetal. (2002) suggested that the halo of ionized gas the Galactic plane) and center (northeast of Sgr A∗) as Sgr A roughly corresponds to the region of non–solid-body rotation East,butitisabouttwiceaslarge(∼20pcalongitsmajoraxis). around Sgr A∗. This region would have a relatively homoge- ThesepropertiessuggestthatSgrAEastandtheradiohaloare neousdensity,becausedifferentialrotationwouldhavesheared partofthesamephysicalsystem.Onepossibilitywouldbethat and smoothed out the interstellar gas on a short (∼ 105 yr) theradiohaloresultsfromaleakageofcosmic-rayelectronsac- timescale. Regarding the source of ionization, Maedaetal. celeratedintheSgrAEastSNR. (2002) ruled out collisional ionization, which would require Pedlaretal.(1989)obtainedmoreinformationonthenature too high a temperature.Instead, they argued in favor of photo- andphysicalcharacteristicsoftheradiohalobycombiningVLA ionizationbyXrays,andtheyproposedthattheionizingXrays 90 cm, 20 cm and 6 cm continuum observations of the Sgr A wereemittedbySgrA∗ ∼ (102−103)yrago,duringanepisod complex. The radio halo is clearly visible in the 90 cm image, ofintensenuclearactivity.Thisepisodcouldhavebeentriggered whereithasaroughlytriangularshape,withatotalextent≈ 7′ bythepassageoverSgrA∗ofthegas-and-dustshellcompressed (17.5 pc). The entire Sgr A East shell shows a low-frequency by the Sgr A East forward shock. If this scenario is correct, turnoverinitsnonthermalemission,whichcanbeexplainedby SgrA∗shouldpresentlyresideinsidetheSgrAEastcavity,con- free-free absorption by thermal ionized gas with an emission sistent with Sgr A West itself residing inside Sgr A East (see measure ≈ (1 − 2) × 105 pccm−6 (assuming an electron tem- Yusef-Zadehetal.2000). perature≈ 5000K). Pedlaretal. (1989) suggestedthatthe ab- Although the existence of a radio halo around Sgr A East sorbingthermalgasbelongstothe7′ radiohalo.Theradiohalo leaves virtually no doubt, the presence of warm ionized gas itselfhasmainlynonthermalemission(attheconsideredwave- withinit isnotuniversallyaccepted.Forinstance,the ideawas lengths), and it, too, shows a low-frequency turnover explain- called into question by Roy&Rao (2009), who measured the ablebyfree-freeabsorption.However,here,insteadofresiding totalfluxdensitiesofSgrAEastandtheradiohaloatfivediffer- in a separate foreground screen, the absorbing thermal gas is ent frequencies ranging from 154 MHz (195 cm) to 1.4 GHz more probably mixed with the emitting nonthermalgas within (21 cm). They observed similar low-frequency turnovers (at thehalo.Inotherwords,the7′radiohaloislikelytocomprisea ∼ 400 MHz) in the radio spectra of both sources, which they mixtureofthermalandnonthermalgases. argued could be entirely attributed to free-free absorption in a Pedlaretal. (1989) were able to reproducethe spectrum of common foreground screen, without requiring the presence of theradiohalobyadoptingforthe thermalgasan electrontem- warm ionized gas inside the radio halo. From this, they con- perature≃ 5000K,anemissionmeasure≃ 2.7×105 pccm−6, cludedthattheradiohaloisinfactapurelynonthermalsource. andFWHMdimensions≃ 4′×4′ (10pc×10pc),which,fora sphericaldistributionandafillingfactorofunity,implyanelec- trondensity≃ 165cm−3 andanH+ mass≃ 2100 M (rescaled 2.5.Thebeltofmolecularclouds ⊙ to r⊙ = 8.5 kpc). Furthermore, since the thermal-gas free-free The geometry, kinematics and physical state of molec- opticaldepthsrequiredtoexplainthelow-frequencyturnoversof ular clouds around the Sgr A complex have been in- theSgrAEastshellandoftheradiohaloaresimilar,Pedlaretal. vestigated mainly through radio spectral lines of differ- (1989) suggested that the radio halo is mostly situated in front ent molecules, including CO (Solomonetal. 1972), NH 3 of Sgr A East. It should be noted,however,that within the un- (Gu¨stenetal. 1981; Okumuraetal. 1989, 1991; Coil&Ho certainties, the derived free-freeoptical depths are also consis- 1999,2000;McGaryetal.2001;Herrnstein&Ho2002,2005), tent with only the near half of the radio halo lying in front of CS (Serabynetal. 1992; Tsuboietal. 1999, 2006, 2009, see SgrAEast,sothatSgrAEastcouldactuallybedeeplyembed- Figure3), H (Leeetal. 2003, 2008), CH OH (Stankovicetal. 2 3 dedwithintheradiohalo,andevenconcentricwithit. 2007),HC N(Sandqvistetal.2008),SiO(Amo-Baladrońetal. 3 The presence of warm ionizedgasin the radio halo is con- 2009, 2011), etc., and also through dust submm continuum firmed by observations of radio recombination lines, which, emission(e.g.,Mezgeretal.1989;Zylkaetal.1990;Dentetal. in addition, provide useful kinematic information.VLA obser- 1993; Lis&Carlstrom 1994). The early CO emission map of vations of the 1375 MHz (22 cm) H168α recombination line Solomonetal. (1972) already revealed two massive molecu- 9 KatiaFerrie`re:InterstellargasnearSgrA∗ lar clouds peaking ≈ 3′ east and ≈ 2.′5 south of Sgr A∗ and having radial velocities in the range ≈ (45 − 65) kms−1 and ≈ (15 − 35) kms−1, respectively. Solomonetal. (1972) estimated their diametersat ∼ 6′ −20′ and their hydrogenmasses at& 105 M .Later,Gu¨stenetal.(1981)carriedoutNH obser- ⊙ 3 vationsoftheregionandderiveda fairlyuniformgastempera- ture≈(50−120)Kthroughouttheclouds.Theyalsolabeledthe cloudsM−0.02−0.07andM−0.13−0.08,respectively,according totheGalacticcoordinatesoftheirNH emissionpeaks.Today, 3 thesecloudsareoftenreferredtoasthe50kms−1and20kms−1 clouds, respectively, although both denominationsare not necessarily strictly equivalent. For instance, some authors include into the 50 kms−1 cloud not only M−0.02−0.07,but also sev- eralmolecularknotsonitspositive-longitudeside. Zylkaetal. (1990) made a first importantattempt to obtain acoherentthree-dimensionalpictureofmolecularcloudsinthe central∼ 50pc.Tolocategasalongthelineofsight,theycom- paredtheir1.3mmdustemissionmaptoa2.2µmintensitymap showing dust absorption against the emission from the central star cluster, and to determine the gas kinematics, they resorted toisotopicCOandCSspectroscopy.Inthismanner,theyfound thatM−0.02−0.07isactuallycomposedoftwoseparateclouds, which they designated the Sgr A East Core and the Curved Streamer.Thesetwo clouds,togetherwith M−0.13−0.08,were foundtohavethefollowingproperties: •M−0.13−0.08,situatedsouthofSgrAEast,hasprojecteddi- mensions≈ 15pc×7.5pc(withmajoraxisroughlyparallelto theGalacticplane),hydrogenmass≈3×105 M andradialve- ⊙ locities increasing with Galactic longitude from ≈ 5 kms−1 to ≈ 25 kms−1. Along the line of sight, the cloud lies in frontof SgrA∗(asalreadypointedoutbyGu¨sten&Downes1980,based onH COabsorption),atapossibledistance∼50pc. 2 • The Sgr A East Core, which surroundsthe Sgr A East radio shell, is & 15 pc in size and contains & 2 × 105 M of hy- ⊙ drogen.While it exhibitsvery high (positiveand negative)turbulent velocities, its bulk radial velocity typically ranges from ≈ 40kms−1 to≈ 70kms−1.Alongthelineofsight,itliesim- mediatelybehindSgrA∗.Clearly,theSgrAEastCorecontains the gas-and-dustshell observedby Mezgeretal. (1989), and it canbeidentifiedwiththeGMCinsidewhichthesupernovaex- plosionthatcreatedSgrAEasttookplace.5 • The Curved Streamer, which stretches east of Sgr A East from the northern end of M−0.13−0.08 up to the eastern part of the Sgr A East Core, is ≈ 7.5 pc wide in b, contains ≈ (1 − 1.5) × 105 M of hydrogen and has radial velocities in- ⊙ creasing steeply with Galactic longitude from ≈ 25 kms−1 to ≈65kms−1.ItliesinfrontofSgrA∗,withitssouthernendcon- nectingwithM−0.13−0.08anditsnorthernendpointingdeeper inward, though probably not deep enough to connect with the Fig.3. Contourlinesofthe6.1mmCSJ =1→0lineemission SgrAEastCore. inthevelocityranges(10−30)kms−1(top)and(40−50)kms−1 The above GMCs are embedded in a clumpy and highly tur- (bottom), superimposed on a grayscale equivalent-width im- bulent molecular intercloud medium, with estimated hydrogen age of the 6.4 keV low-ionization Fe Kα line emission, in a mass ∼ 106 M (in the inner ∼ 50 pc), average density ∼ 17′ × 17′ field of view centered on Sgr A∗. The CS data are ⊙ 102cm−3andradialvelocitiesintherange≈−40to+90kms−1. fromTsuboietal. (1999) and the 6.4 keV data fromParketal. Serabynetal.(1992),whomappedM−0.02−0.07intheCS (2004).Figurecredit:SangwookPark. J = 5 → 4 and J = 7 → 6 transitions (at 245 GHz and 343 GHz, respectively), also came to the conclusion that this ingat(∆α,∆δ)≈(3.′0,1.′5)withrespecttoSgrA∗ andamolec- cloudconsistsoftwocomponents:adensemolecularcorepeak- ular ridge curving all the way around the eastern edge of the Sgr A East radio shell. As noted earlier by other authors, this 5 The designation Sgr A East Core was taken up by Mezgeretal. spatialconfigurationstronglysuggeststhatSgrAEastiscollid- (1996).However,insteadofenvisioninganextendedM−0.02−0.07that would contain the entire Sgr A East Core (in addition to the Curved ingwith,andcompressing,M−0.02−0.07.Serabynetal.(1992) Streamer),theyregardedM−0.02−0.07asbeingonly”thecompressed foundthatradialvelocitiesinM−0.02−0.07peakat≈45kms−1 easternpartoftheSgrAEastCore”. and span the range ≈ (25−65) kms−1, with a steady increase 10