Kinematics and properties of the Central Molecular Zone as probed with [C II]

2.1 MB·

by  W. D. Langer

Astronomy&Astrophysicsmanuscriptno.aa-2016-29497 (cid:13)c ESO2017
January16,2017
Kinematics and properties of the Central Molecular Zone as probed
ii
with [C ]
W.D.Langer1,T.Velusamy1,M.R.Morris2 ,P.F.Goldsmith1,andJ.L.Pineda1
1 JetPropulsionLaboratory,CaliforniaInstituteofTechnology,4800OakGroveDrive,Pasadena,CA91109,USA
e-mail:[email protected]
2 DepartmentofPhysicsandAstronomy,UCLA,403PortolaPlaza,LosAngeles,CA90095-1547,USA
7 Received8August2016;Accepted12December2016
1
0 ABSTRACT
2
Context.TheGalacticCentralMolecular Zone(CMZ)isaregioncontaining massiveanddense molecular clouds, withdynamics
n
driven by a variety of energy sources including a massive black hole. It is thus the nearest template for understanding physical
a
J processesinextragalacticnuclei.TheCMZ’sneutralinterstellargashasbeenmappedspectrallyinmanyneutralatomicandmolecular
gastracers,buttheionizedandCO-darkH regionsarelesswelltracedspectroscopically.
2 2
Aims.ToidentifyfeaturesoftheUVirradiatedneutralgas,photondominatedregions(PDRs)andCO-darkH ,andhighlyionized
1 gasintheCMZastracedbythefinestructurelineofC+at158µm,[Cii],andtocharacterizetheirproperties. 2
Methods.Weobservedthe[Cii]158µmfinestructurelinewithhighspectralresolutionusingHerschelHIFIwithtwoperpendicular
]
A On-the-Flystripscans,alongl=–0◦.8to+0◦.8andb=–0◦.8to+0◦.8,bothcenteredon(l,b)=(0◦,0◦).Weanalyzethespatial–velocity
distributionofthe[Cii]data,comparethemtothoseof[Ci]andCO,andtodustcontinuummaps,inordertodeterminetheproperties
G
anddistributionoftheionizedandneutralgasanditsdynamicswithintheCMZ.
. Results.Thelongitude–andlatitude–velocitymapsof[Cii]traceportionsoftheorbitingopengasstreamsofdensemolecularclouds,
h
thecloudG0.253+0.016,alsoknownastheBrick,theArchedFilaments,theionizedgaswithinseveralpcofSgrAandSgrB2,and
p theWarmDustBubble.Weusethe[Cii]datatodeterminethephysicalanddynamicalpropertiesoftheseCMZfeatures.
o- Conclusions.Thebrightfar–IR158µmC+line,[Cii],whenobservedwithhighspatialandspectralresolution,tracesawiderange
r ofemissionfeaturesintheCMZ.The[Cii]emissionarisesprimarilyfromdensePDRsandhighlyionizedgas,andisanimportant
t tracerofthekinematicsandphysicalconditionsofthisgas.
s
a
Keywords.Galaxy:center—ISM:clouds—ISM:ions
[

1
v 1. Introduction mationrate,consideringthemassof densemoleculargasthere
2
(Tayloretal.1993;Longmoreetal.2013).
8
The Galaxy’s Central Molecular Zone (CMZ) is a roughly
4
3 400 pc × 100 pc region stretching from l ∼ –1◦.0 to ∼ +1◦.5
0 that is a significantly different environment than the Galactic The CMZ’s interstellar medium has been fully or partially
. disk. Among many prominent features in the CMZ are a mas- mapped spectrally in over twenty gas tracers, including sev-
1
sive, ∼4×106 M , black hole, the Galactic Center Bubble con- eral isotopologues (Morris 1997; Jonesetal. 2012). The vast
0 ⊙
taining the Arches and Quintuplet clusters (Morris&Serabyn majority trace the neutral gas; these include the rotational
7
1 1996; Molinarietal. 2014), the Radio Arc and Arched transitions of CO isotopologues (e.g. Okaetal. 1998, 2011,
: Filaments (Yusef-Zadehetal. 1984; Serabyn&Guesten 1987; 2012; Enokiyaetal. 2014), CS (Tsuboietal. 1999; Jonesetal.
v Yusef-Zadeh&Morris1987), and gasstreamsof dense molec- 2012), HCN (Jacksonetal. 1996; Jonesetal. 2012), N H+
i 2
X ular clouds orbiting the Galactic Center at a radius of 100 (Jonesetal. 2012), H CO (Ginsburgetal. 2016), CH CCH
2 3
to 120 pc (Tsuboietal. 1999; Molinarietal. 2011; Jonesetal. (Jonesetal. 2012), as well as NH (Mills&Morris 2013) and
r 3
a 2012;Kruijssenetal.2015;Henshawetal.2016a,b).TheCMZ OH(Yusef-Zadehetal.1999),andthelowerfinestructureemis-
molecularcloudscontainabout10%oftheGalaxy’smolecular sion line ofneutralcarbon,[Ci] (Martinetal. 2004). However,
gas, have high average densities (>104 cm−3), reside in a high thesespeciesdonottracemajorcomponentsoftheinnerGalaxy,
thermal pressure environment, are relatively densely packed wherethegasishighlyionizedorwheretheUVirradiatedneu-
(Morris&Serabyn1996;Ferrie`reetal.2007),andhaveregions tral gas is weakly ionized. These regions include the Photon
with an intense flux of stellar Far-UV radiation as well as X- DominatedRegions(PDRs),theionizedboundarylayers(IBL)
rays(see reviewby Pontietal. 2013). Thereis ampleevidence surrounding UV-irradiated molecular clouds, Hii regions, CO-
of large-scale energetic gas motions in the CMZ, as indicated dark H clouds, and the warm and diffuse ionized gas. Less is
2
bythelinewidthsofCOinGiantMolecularClouds(GMCs)of known aboutthese componentsdue to the difficulty of observ-
10to20kms−1,comparedtoafewkms−1 intheGalacticdisk ingkeytracersoftheionizedgas,thefinestructurelinesofC+
(cf.Okaetal.1998;Dameetal.2001),andbytheflowsofma- andN+,fromtheground.Inthispaperwepresentnewresultson
terialwithinandacrossthisregion(c.f.Morris&Serabyn1996; the propertiesof the ionizedand neutralgasin the CMZ based
Morris1997;Enokiyaetal.2014).TheCMZproduces∼10%of onstripscansusingthespectrallyresolvedfinestructurelineof
theGalaxy’sinfraredluminosity,yethasarelativelylowstarfor- ionizedcarbon.
1
W.D.Langeretal.:TheCentralMolecularZoneasprobedwith[Cii]
Prior to the launch of the Herschel Space Observatory1 the[Cii]tracedgascomparedtofeaturesintheCMZ.Section4
(Pilbrattetal. 2010) the CMZ had been mapped in the fine discussesthepropertiesofseveralfeaturestracedby[Cii]inthe
structure line of C+ at 158 µm, [Cii], with COBE FIRAS CMZ,andSection5summarizestheresults.
(Bennettetal. 1994) and the Balloon-borne Infrared Carbon
Explorer (BICE) (Nakagawaetal. 1998), but with its emission
spectrallyunresolved.However,withtheHeterodyneInstrument 2. [Cii]ObservationsandDataReduction
intheFar-Infrared(HIFI)(deGraauwetal.2010)itbecamepos-
sibletomapspectrallyresolved[Cii]intheCMZ.TheHerschel Weobservedtheionizedcarbon(C+)2P3/2–2P1/2finestructure
line,[Cii],at1900.5369GHz(λ∼157.74µm),usingHIFIinan
HEXGALprogramme(Gu¨sten2007)madeawellsampledHIFI
[Cii] On-the-Fly (OTF) map of a small region around Sgr A OTF mode to produce position–velocity maps of the Galactic
center region. We observed two strip scans through (l,b) =
(Garc´ıa2015;Garc´ıaetal.2016)consistingoftwoslightlyoff-
(0◦,0◦), onealong longitudesfroml=359◦.20 to 0◦.80 atlatitude
set rectangles totaling ∼0.1 square degrees in area. As part of
the Herschel GOT C+ programme (Langeretal. 2010), [Cii] b=0◦ andtheotheralongb=-0◦.80to+0◦.80atlongitudel=0◦;
detailsoftheobservationsanddatareductionaregivenbelow.
wasobservedwithHIFIalongtwospectrallyresolvedstripscans
traversing 1◦.6 in longitudeand latitude through(l,b) = (0◦,0◦).
The purpose of this paper is to gain insight on the nature of 2.1.Observations
theCMZastracedbythesetwo[Cii]stripscans,whichextend
about half the length and the full width of the CMZ. We limit All [Cii] spectral line mapping observations were made with
our discussion to describing the regions and features detected the high spectral resolution HIFI instrument (deGraauwetal.
in [Cii] emission, their kinematics, and estimates of the prop- 2010) onboard Herschel (Pilbrattetal. 2010) in February and
ertiesoftheC+ gas,asdetailedinformationaboutthestructure March2011.These[Cii]spectrallinemapscansusedtheHIFI
ofthesefeaturesisnotpossiblewithoutspectrallyresolved(l–b) band7bandthewidebandspectrometer(WBS).Thelongitude
mapswellsampledinbothlongitudeandlatitude. scan consisted of four OTF scans taken along Galactic longi-
While Herschel HIFI provides the necessary spectrally re- tude centered on (l,b) = (0◦.6,0◦.0), (0◦.2,0◦.0), (359◦.8,0◦.0), and
solvedinstrumenttostudy[Cii],thesmallbeamsizeat1.9THz, (359◦.4,0◦.0). Similarly, the latitude scan consisted of four OTF
12′′, and single pixel made it unsuited to generate large-scale scans taken in Galactic latitude centered on (l,b) = (0◦.0,-0◦.6),
fullysampledmaps,suchasoftheGalacticdisk,CMZ,orexter- (0◦.0,-0◦.2), (0◦.0,+0◦.2), and (0◦.0,+0◦.6). Each OTF scans was 24
nalgalaxies.However,HerschelHIFI wasable to providewell arcmin long, thus the four together cover a range of 1.6◦. The
sampled detailed maps of small regions or a coarse picture of observingdurationofeachOTFscanwas∼2500secandthe24
[Cii] emission across the Galaxy by conducting a large-scale arcmin-long scan data were read out every 40′′ (i.e. averaged
sparsesurveyofspectrallyresolved[Cii]. overa40′′window).
AllHIFIOTFscansweremadeintheLOAD–CHOPmode
The Herschel Open Time Key Programme, Galactic
Observations of Terahertz C+ (GOT C+; Langeretal. 2010; using a reference off-source position about 2◦ away in latitude
(this HIFI observing mode requires the reference position be
Pinedaetal.2013)hadtwomainsubprogrammes:1)aGalactic
disksparsepointedsurveyof[Cii];and,2)aCentralMolecular within2◦ofthetarget).Forallfourl–scansandthetwob–scans
Zone OTF strip scan surveyof [Cii]. In subprogramme(1) the centered at b ≥0◦.0 we used the OFF (l,b) =(0◦.0,1◦.9) which is
GOTC+ surveycharacterizedthe entireGalactic diskby mak- verycleanof[Cii]emissionatthesensitivityofthesurvey.For
ing a sparse sample covering360◦ in the plane. (The GOT C+ theb–scansatb <0◦.0wehadtouseareferencepositioncloser
to the planeat (l,b)= (0◦.0,0◦.5) whichcontains[Cii] emission.
datasetsareavailableasaHerschelUserProvidedDataProduct
under KPOT wlanger 1). The disk survey contains ∼500 lines However,this referencepositionhasbeenwellobservedin our
of sight of spectrally resolved [Cii] emission throughout the HIFI Pointed observation programme using the reference sky
Galactic Disk (l=0◦ to 360◦ and |b|≤1◦). The sampling in positionat(0◦.0,+1◦.9)sothatthe[Cii]emissionspectrumatthis
positioniswellcharacterizedandcanbeusedasaskyreference
Galactic longitude was non-uniformin order to weight as best
to correctfor OFF source emission. Thus the mappingscheme
as possible a uniform angular volume across the disk, with an
emphasis on the important inner longitude range |l|≤90◦. By used for the CMZ provides [Cii] spectral line map data all of
which are effectively referencedback to a commonposition at
analyzing a large sample of spectrally resolved components
(l,b)=(0◦.0,1◦.9).TheGOTC+surveydata,aswellastheOTF
dispersed throughout the Galaxy, rather than large-scale maps
scanreferencespectraextractedinHIPE-13,showlittleemission
of a few clouds, this survey allows a statistical approach to
characterizing the ISM in [Cii] and the results have been re- ata3-σlevelof∼0.1K.
ported in a series of papers Langeretal. (2010); Pinedaetal. Near the Galactic Center we expect [Cii] emission over a
(2010); Velusamyetal. (2010, 2012); Pinedaetal. (2013); wide range of velocities (>200 km s−1). However, the useful
Langeretal. (2014); Pinedaetal. (2014); Velusamy&Langer instantaneous velocity range observable with HIFI band 7b is
(2014); Velusamyetal. (2015). Here we present the results of ∼160 km s−1. To accommodate this velocity range and extract
theGOTC+[Cii]stripscansoftheCMZ,whichpassthrough, the spectral line data, each OTF scan was observed at two lo-
or close to, several features, including Sgr A, G0.253+0.016 caloscillator(LO)frequencieswithfrequencyoffsetof0.9GHz
(theBrick),SgrB2,theGalacticCenterWarmDustBubble,the (correspondingspectra centeredat VLSR =−80kms−1 and +70
Radio Arc, the Arched Filaments, and the open streams of gas kms−1).
orbitingtheGalacticCenter.
Thispaperisorganizedasfollows.InSection2wedescribe
2.2.Datareduction
the data reduction, while Section 3 presents the distribution of
WeprocessedtheOTFstripscandataandproducedthel–andb–
1 Herschel is an ESA space observatory with science instruments scanposition–velocitymaps,(l–V)and(b–V),followingthepro-
providedbyEuropean-ledPrincipalInvestigatorconsortiaandwithim- cedurediscussedinVelusamyetal.(2015).However,theresults
portantparticipationfromNASA. presentedherewereprocessedinHIPE-13,ratherthanHIPE-12
2
W.D.Langeretal.:TheCentralMolecularZoneasprobedwith[Cii]
usedinVelusamyetal.(2015),takingadvantageofthehebCor-
rectiontool,whichremovestheHEBstandingwavespresentin
band7bdata.hebCorrectionisappliedinHIPE-13aspartofthe
pipelinetoextractspectraatthereferenceskyposition.Fromthe
Level2data,the[Cii]stripscansweremadeinto“spectralline
cubes”usingthe standardmappingscriptsin HIPE.Anyresid-
ualHEBandopticalstandingwavesinthereprocessedLevel2
datawereminimizedbyapplyingfitHifiFringetothe“gridded”
spectraldata.We tooktheadditionalprecautioninfitHifiFringe
ofdisablingDoAverageinordernottobiasthespectrallinewin-
dow.
The H– and V–polarizationdata were processed separately
andwerecombinedonlyafterapplyingfitHifiFringetothegrid-
deddata.Thisapproachminimizesthestandingwaveresiduesin
the strip scans by takinginto accountthe standingwave differ-
encesbetweenH–andV–polarization.Inviewofthefactthatthe
spectrumofasingleLO observationmaynotprovideadequate
Fig.1. Exampleof combiningthe two LO spectrain OTFscan
baselinesatbothendsofthespectrallineprofileinHIPE-13,we maps. The spectra at l=0◦.06 in the individual longitude scan
generated two sets of spectral cubes with and withoutbaseline
maps consist of two local oscillator settings, LO 1 (blue) and
subtraction.Wethenusedtheprocessedspectrallinedatacubes
LO 2 (green). The final averaged spectrum is shown in black.
intwopolarizations(HandV)andtwoLOsettingstomake(l–
The double arrowsmark the velocity rangeof a constantbase-
V)and(b–V)maps,bycombiningthemafterfittingbaselineand lineoffsetusedinthecorrespondingmaps.Theredboxdenotes
velocityoffsets.For|l|>0◦.4and|b|>0◦.4combiningthespectral thevelocityrange(±10kms−1)formatchingthetwospectraand
cube data was straightforwardas these spectra had clean base- thebaselineoffsetsherewereestimatedusingtheiraveragedin-
linesatbothendsinoneorbothsetsofthetwoLOsettings.
tensity. The quasi–horizontaldashed lines represent a linear fit
tothebaselinesusedforthespectraforfinalaveraging.Thever-
However,for|l|<0◦.4and|b|<0◦.4thespectracontainedalong ticaldashedlinesmarkvelocitylimitsabove(green)andbelow
andcleanbaselineonlyatoneend,withemissionpresentatthe (blue) where the data are unusable (near the edges of the band
oppositeend.InthiscasethespectraoftheLO-pairswerecom- pass). The spectra for velocities within ±60 km s−1 were aver-
bined, fixing the linear baseline at the cleaner end and match- agedwithbaselinecorrectionsforV<-60andV>+60kms−1
ing the profiles at the overlapping velocities. In Figure 1 we forLO2andLO1,respectively.Notethattheabsorptionfeature
show an exampleof this procedurefor l=0◦.06,where it can be around0kms−1 isduetolocalgas.
seenthatthematchingproducesagoodbaselinecorrection.For
matching we use the region near V ∼0 km s−1 where [Cii]
LSR
isabsorbedbyforegroundemission.Tocheckthisapproach,we 3. Results
comparedthe resultingspectra for |l| >0◦.4, where the emission
isnarrowenoughtofitwithin thebandof asingle LO,andthe Inthissectionwepresentthe[Cii]OTFstripscansandauxiliary
baselines can be set at both ends with no matching of profiles. spectralline mapsof [Ci] and CO, anddescribe their morpho-
The resulting baselines and spectra fit both ways are in good logicalrelationshipstoCMZfeatures.
agreement,thuslendingconfidencethatthematchingapproach
produces good baselines for |l|<0◦.4 and |b| <0◦.4. We find that
3.1.OTFmaps
this matching procedure does introduce a baseline uncertainty
of∼0.5Kandproducesmoreuncertaintyinthefinalspectrafor InFigure2weshowourHIFIGOTC+position–velocity[Cii]
the l scan than for b above and below the plane because these maps along with corresponding Herschel 70 µm dust and Hi
bscansgenerallyhavenarrowerlinesand,thereforelongerand emission maps from Molinarietal. (2011). In Figure 2(a) we
cleanerbaselines. plot the longitude–velocity,l–V, [Cii] emission and in (d) the
latitude–velocity,b–V,emission. Thevelocitydispersionalong
At1.9THztheangularresolutionoftheHerscheltelescope both the longitude and latitude strips is largest near (l,b) =
is 12′′, but the [Cii] OTF observations were averaged over a (0◦,0◦) and the highest detected velocities, as outlined by the
40′′ window along the scan direction (i.e. read out at 40′′ in- white rectangles, reach about ±150 km s−1. The locations in
tervals). Such fast scanning broadens the effective beam size position–velocity space corresponding to a few features in the
along the scan direction (Mangumetal. 2007). Therefore all CMZ, including Sgr B2, the Brick, the Arched Filaments, and
[Cii] maps have been restored with effectivebeam sizes corre- the open orbit streams of clouds are marked on the l–V plot.
spondingto twice the sampleaveragingintervalalongthe scan Thelow-intensityverticalstriationsinFigure2(a)inthel–scan
direction (∼80′′). We used the HIFI Wide Band Spectrometer areartifactsofthebaselineuncertainties.Thesefeaturesareless
(WBS)withaspectralresolutionof1.1MHz(0.17kms−1)for prominent in the b–scan, as seen in Figure 2(b), because the
all the scan maps. The final l–V and b–V maps presented here baselineuncertaintiesaresmaller(seeSection2.2).
weresmoothedtoavelocityresolutionof2kms−1.Thermsin Wehavealsohighlightedtheellipse(redsolidline)inl–V at
themainbeamtemperature,T ,isintherange0.21to0.31K b=0◦correspondingtothekinematicsofatwistedellipticalring
mb
forthel–V mapand0.18to0.22Kfortheb–V map,thesmaller atb=0◦ whichMolinarietal.(2011)usedtoexplainthedistri-
rms for the b–V map is due to the cleaner baseline that results butionofCSvelocitiesinFigure2(b).Thedashed(red)ellipseis
fromhaving(generally)narrowerlineprofilesalongbthanalong ourfittothe[Cii]emissionandwillbediscussedinSection4.1.
l. Panel(b)showsanatomichydrogencolumndensityspatialmap
3
W.D.Langeretal.:TheCentralMolecularZoneasprobedwith[Cii]
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Fig.2. HIFI position–velocity [Cii] maps of the CMZ tracing several notable features, including the Brick, open orbit streams
of molecular gas, Sgr B2, and the Arched Filament. (a) The [Cii] longitude–velocitymap, l–V, covering1◦.6 centeredat (0◦,0◦).
The color bar indicates the [Cii] main beam temperature,T (K). The red-solid ellipse is the solution at z=0◦.0 for the elliptical
mb
ring model from Molinarietal. (2011) in longitude–velocityspace and the red-dashed ellipse is our fit to the [Cii] observations
assuminganellipse.Theverticalstriationsareartifactsofthebaselineuncertainties(seeSection2).(b)Atomichydrogencolumn
densitymapfromMolinarietal.(2011)highlightingtheirellipticaltwistedringmodel.Theline-of-sightvelocityalongtheringwas
derivedbyMolinarietal.(2011)fromCSobservationsandismarkedalongthering(seetheoriginalfigure).(c)AHerschelPACS
70µmimageoftheCMZfromFigure1ofMolinarietal.(2011).The[Cii]OTFstripscansinlandbareindicatedbywhitedashed
lines.ThelabelsingreenarefromMolinarietal.(2011),exceptfortheBrick(G0.253+0.016).(d)The[Cii]latitude–velocitymap,
b–V,covering1◦.6centeredat(0◦,0◦).ThecolourbarindicatesT ([Cii])indegreesK.Inbothl–V (panela)andb–V (paneld)we
mb
highlighttheposition–velocityextentofthehighvelocitygasnearthecenter(whiteboxes).Thearrowindicatesaverybright[Cii]
peakthatislikelyaforegroundHiisource(seeSection4.8).
of the CMZ with velocities from CS superimposed along por- 0◦.11).ThedusttemperaturemapshowninFigure3coversabout
tionsoftheellipticalringfromMolinarietal.(2011).Panel(c) 0◦.76 in longitude and 0◦.96 in latitude. The white dashed lines
showstheextentofthetwoOTFstripscans(dashedwhitelines) are the [Cii] OTF strip scans. The [Cii] longitudinalstrip scan
superimposed on a Herschel 70 µm map from Molinarietal. goesthroughthewarmdustdoughnut,whilethelatitudinalstrip
(2011).Figure2(d)plotsthe b–V [Cii] emissionatl = 0◦. The scan appears to be at the edge of the warm dust doughnut.
velocitygradientissmallinthismapcomparedtothatofthel–V Also shownisa radiocontinuummapofthe innermostportion
map. oftheCMZ(Yusef-Zadeh&Morris1987;Simpsonetal.2007)
InFigure3weshowthecentralportionofthedusttemper- revealing the Radio Arc, the Arched Filaments, and the clus-
ature map of the CMZ from Figure 3 in Molinarietal. (2011), tersofmassivestarsassociatedwiththeArchesandQuintuplet
whichwasderivedfromtheHerschelHi-GAL70–350µmsur- Clusters. The red dashed lines designate the [Cii] OTF strip
vey, (see also Figure 5 in Molinarietal. (2014)). The Galactic scans throughthis regionand it can be seen that the [Cii] lon-
Center Bubble is surrounded by warm dust with a doughnut- gitudestripscangoesthroughtheArchedFilamentsandpasses
like temperature distribution seen in projection, located to the neartheArchesCluster.
left and below the Galactic Center. The central portion of the The Arched Filaments have mostly negative velocities in
doughnut is an ionized gas bubble centered at about (0◦.11,– the range −70 to +15 km s−1 (Langetal. 2001; Simpsonetal.
4
W.D.Langeretal.:TheCentralMolecularZoneasprobedwith[Cii]
!"
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Fig.3.ThecenterimageisthedusttemperaturedistributionoftheinnerCMZderivedfromHerschelHI-GAL70–350µmimages
(Molinarietal. 2011). It has a doughnutshaped distribution as seen in projection marking a Warm Dust Bubble (Molinarietal.
2011,2014).OntheleftistheradiocontinuummapgeneratedbySimpsonetal.(2007)basedonthedataofYusef-Zadeh&Morris
(1987)ofthecentralportionoftheBubble(theholeinthedoughnut)containingtheRadioArcandArchedFilaments.The[Cii]
integratedintensity,I([Cii])(Kkms−1),stripscansasafunctionoflongitudeandlatitudeareplottedtothebottomandtheright,
respectively,andarealignedwiththetemperaturemapoftheinnerCMZ.Theseplotsshowthevelocity–integrated[Cii]intensity
profilesasfunctionsoflongitudeorlatitude:theblacklinesrepresentthetotalintensityinthevelocityrangeV =−200to+140
LSR
kms−1,theredandbluelinesrepresentI([Cii])integratedemissioninthehigh–velocityredandbluewingsfromV =+80to
LSR
+130kms−1and−80to−130kms−1,respectively.
2007).AscanbeseeninFigure2(a),[Cii]isdetectedalongthe sity,I([Cii]),asafunctionoflongitudeandlatitudefortwodif-
line-of-sightnearthebaseoftheArchedFilaments,l∼0◦.06(see ferentvelocityranges.TheV velocityrangeis80to130km
LSR
Figures1inLangetal.2001,2002)withavelocityrangewithin s−1 (red) away from and −80 to −130 km s−1 (blue) towards
thoseofthefilaments.Itisalsodetectedalongl∼0◦.16,thatmight the sun. The l– and b– scan intensity profiles in Figure 3 indi-
alsobeassociatedwiththeArchedFilaments,butismorelikely cate significantly large [Cii] emission present at the lower ve-
associated with the molecular clouds with V ∼ –20 to –40 locities (|V | < 80 km s−1) in comparisonto that in the high
LSR LSR
kms−1.Thereisalso[Cii]emissionatl ∼0◦.20andwithveloc- velocity wings (|V | > 80 km s−1). The strip scan in latitude
LSR
ities ∼+20 to +30 km s−1 that is probably associated with the shows a shift in b in the peak [Cii] emission between the blue
molecularcloudM0.20-0.033(Serabyn&Morris1994). andredshiftedcomponents.Alikelyexplanationforthelatitude
offsetbetweenthesetwovelocityregimesisthetiltintheCMZ
In Figure 3 we plot the total integrated intensity I([Cii]) = (Morris&Serabyn1996,andreferencestherein).
R Tmbdv (K km s−1) over the velocity range −200 to +140 km In Figure 4 we compare the gas in the CMZ traced by
s−1 (blacklines)asafunctionoflongitudeandlatitude,bottom C+ with their neutral cloud components, C and CO, using the
and right, respectively. We also plot the integrated [Cii] inten- AST/RO maps of [Ci] 2P →2P and CO(4–3) (Martinetal.
1 0
5
W.D.Langeretal.:TheCentralMolecularZoneasprobedwith[Cii]
!!""
!!"
Fig.4. Comparison of the [Cii] position–velocity strip scans with the AST/RO observations of [Ci] and CO(4–3) (Martinetal.
2004). (a) The [Cii] longitude–velocity strip scan and (b) the latitude–velocity strip scan. The [Ci] and CO maps are shown in
grayscale,wherethegreyscalebarsontherighthandsideindicatethemainbeamtemperatures,T (K);thecorresponding[Cii]
mb
emissionmapsareoverlaidascontours.The[Cii]contourlevels(red)areT (K)=1,2,4,6,8,and10.Inthel–V mapsthedashed
mb
lineisroughlyoutlinesthe[Cii]emissionatb=0◦.0fortheellipticalringmodelofMolinarietal.(2011)(seealsoFigure2).This
ellipseisusedonlytodelineatethel–V spacewheretheopenorbitgasstreamscanbefound,andisnotintendedasamodelofthe
gasdynamics.Thearrowsinpanel(b)indicatealocalHiisource(seeSection4.8)
2004).TheAST/ROspatialmapscoveralongituderange−1◦.3 strong, and vice-versa. We also note a strong [Cii] feature at
to+2◦.0,largerthantheGOTC+OTFstripscan,butwerelimited b ∼0◦.15 which is a local Hii region (see Section 4.8). Finally,
inlatitudeto−0◦.3to+0◦.2,smallerthantheGOTC+stripscan. the two local peaks in [Cii] centered at l∼0◦.06 and 0◦.18 with
TheAST/ROmapshaveanangularresolution∼120′′,whichis V ∼ –20 to –40 km s−1 appear associated with the Arched
LSR
about50%largeralongthescandirectionthantheeffectiveres- FilamentsandRadio Arc filaments, respectively,buthavelittle
olution in the GOT C+ strip scans. The AST/RO spectra were [Ci]orCO(4–3)associatedwiththem.
resampled to match the velocity resolution of 2 km s−1 in the
GOT C+ [Cii] OTF scans. CO(4–3) and [Ci] in the AST/RO Theabsenceofa strongcorrelationbetween[Cii] and [Ci]
mapsarestrongatb<0◦(seeFigure3inMartinetal.2004)and and between [Cii] and CO(4–3) seen in many positions in
relativelyweakatb>0◦.Theleftpanelsshowthel–V mapsand Figure 4 indicates that a significant fraction of the [Cii] emis-
therightpanelstheb–V maps.Thefeaturesinthesestripscans sion in the CMZ is tracing a highly ionized gas component.
willbediscussedinmoredetailinSection4. Furthermore, there is a trend for the highly ionized gas to ap-
Thel–V maps(leftpanels)ofallthreetracersshowthesame pear closer to the Galactic Center than further out from it. To
generaltrendinvelocitywithlongitude,butthevelocitydisper- explorethis trendfurtherandto lookforany differencesin the
sion near l = 0◦ is larger in [Cii] than in [Ci] or CO(4–3). A sourceof[Cii]emissionalongtheCMZ,weevaluatedtheglobal
strong peak in the [Cii] longitude strip scan occurs at l∼0◦.25, relative intensity distribution along the Galactic longitude strip
correspondingtothelocationoftheBrick.Inadditionthereare scan of [Cii] with respect to CO(4–3) and [Ci] by computing
some notable anti-correlationswithin the l–V maps, some fea- theiraverageintegratedintensities,I=<(R Tdv)>,inbinsof0◦.5
tureshave strong[Cii] butweak or no [Ci] and/orCO, for ex- width.Theaverageintensityineachbinwascalculatedusinga
ample at l∼0◦.25, and vice-versa. In the latitude–velocity maps velocity range of ±130 km s−1 but excludingthe range −10 to
(right panels) [Ci] and CO(4–3) are generally correlated and, +10 km s−1 which is contaminated with emission and absorp-
again, the [Cii] emission is weak where [Ci] and CO(4–3) are tionfromlocalgas.TheresultsaresummarizedinTable1.Note
6
W.D.Langeretal.:TheCentralMolecularZoneasprobedwith[Cii]
that the [Cii]/CO(4–3) ratio at the center bin, within ±0◦.25 of 8
(0◦.0,0◦.0), is about 2.3 times that on either side. Furthermore, !"#$%"&’(" (0.0o,0.0o)
the [Cii]/[Ci] ratio at the center bin is also about 3 times that 7
on either side. Thus the results in Table 1 clearly show that
thereisadistinctexcessin[Cii]overCO(4–3)and[Ci]within 6
±0◦.25oftheGalacticCenter.UnlikeCOor[Ci],the[Cii]emis-
sioncanariseinboththeneutralmedium,excitedbycollisions 5 )*+"#$%"&’(" !"#$$%#
withH molecules,andintheionizedgas,excitedbycollisions
2 !&#$$%#
with electrons. Therefore,by comparisonwith the regionsout-
side the central bins, we can conclude that the [Cii] emission K) 4 ,-%&,"&’("
within ±0◦.25 of the Galactic Center has a significant contribu- T(
3
tionduetoelectronexcitationofhighlyionizedgas.Thisresult
isfurthercorroboratedbythe detectionof[Nii] in theGalactic
2
Center (Goldsmithetal. 2015; Garc´ıaetal. 2016; Langeretal.
2016).
1
3.2.[Cii]spectra 0
In Figure 5 we show six [Cii] spectra extracted from the OTF
observations.Thespectraat(0◦.51,0◦.0)and(−0◦.58,0◦.0)areatthe -1
-200 -150 -100 -50 0 50 100 150 200
hypotheticaledgesoftheopenorbitgasstreams,(−0◦.085,0◦.0)is V (km s-1)
∼8pcoutsideSgrA,and(0◦.0,−0◦.20)and(0◦.0,−0◦.49)showthat LSR
[Cii]emissionissignificantlynarrowerbelowtheplane.Within Fig.6.Mainbeamtemperatureversusvelocityfor[Cii](black)
eachspectrumthereare narrowfeatureswith linewidthsonthe and[Nii](red)spectratowards(0◦,0◦)takenwithHIFI(Langer
orderof10to15kms−1,alongwithbroadbutweaker[Cii].In
etal.2016).Thevelocitiesoftheforegroundspiralarmsarela-
addition,thereisevidenceofabsorptionbylocalgasnear0km
beled.
s−1intheinnermostspectrumat(−0◦.085,0◦.0).
At(l,b)=(0◦,0◦),inadditiontoapointedobservationof[Cii]
fromGOTC+,wealsohaveaspectrumofthe[Nii]205µmfine I ([Cii])∼0.9(C+/N+)I([Nii]), where (C+/N+) is the carbonto
ion
structurelinetakenfromaHerschelHIFIsurveyoftenGOTC+ nitrogen elemental abundance ratio (Langeretal. 2016). The
lines of sight (Goldsmithetal. 2015, Langer et al. 2016). The [Cii]intensityfromtheneutralgas,I ([Cii]),isjustthedif-
neutral
angularresolutionforthe[Nii]spectrais16′′ comparedto12′′ ferenceoftheobservedtotal[Cii]intensity,I ([Cii]),minusthe
tot
for[Cii].InFigure6the[Nii]spectrumisoverlaidonthe[Cii] predictedI ([Cii])derivedfrom[Nii].Wecalculatedthefrac-
ion
spectrum2.Itcanbeseenthatthereisemissionacrossmostofthe tionofemissionof[Cii]fromtheionizedgasalong(0◦,0◦)using
velocityrangeofthe[Cii],butonlyonestrongcomponent.The thespectrashowninFigure6.However,toderivejustthecontri-
lineofsighttotheGalacticCenterinterceptsanumberofspiral butionfromtheCMZwehavetoexcludetheemissionfromthe
arms.Wehaveindicatedthreefeatureslikelyassociatedwiththe 3-kpcarmwhichdominatesthetotal[Nii]intensity.Wefindthat
3-kpcand4.5-kpcarms,andlocalgas.Thestrongpeakin[Nii] thepredictedI ([Cii])derivedfrom[Nii] isabout30%ofthe
ion
alignswiththe[Cii]peaktowardsthe3-kpcarmandlikelymost total [Cii] emission, I ([Cii]), as derivedfrom the [Cii] spec-
tot
oftheemissionarisesthere.AGaussianfittothis[Nii]spectral trum, and the remaining 70% comes from PDRs and CO-dark
featureyieldsVLSR∼ −60kms−1 anda FWHM linewidth∼17 H2 regions.Thisresultholdsonlyforthisonepaththroughthe
kms−1.Thestrongest[Cii]featurehasverysimilarlineparam- CMZbecausewedonothave[Nii]spectraalongtheremaining
eters,VLSR∼–64kms−1andFWHM∼19kms−1.Thecoupleof positionsinthetwostripscans.
kms−1 shiftinV between[Cii]and[Nii]ischaracteristicof
LSR
theemissionarisingfromdifferentregionsacrossthespiralarm
profile(Velusamyetal.2015).Theremaining[Nii]isbroadlow 4. Discussion
levelemissionarisingfromtheCMZanditisdifficulttoextract
In this section we discuss the propertiesof the followingCMZ
anydistinctspectrallineswithGaussian-likeshape.The4.5kpc
armalignswithapeakin[Cii].Theabsorptionbythelocalarm featurestracedbyorneartothe[Cii]OTFstripscans:theopen
andlocalforegroundgasatV ∼−10to+10kms−1,whichis orbit gas streams, the Brick, the Radio Arc, the base of the
LSR Arched Filament, the Warm Dust Bubble, Sgr B2, and an Hii
possibly in the Riegel-Crutcher cold cloud (Riegel&Crutcher
1972),andwhichisveryevidentin[Cii],isnotapparentinthe regionlikelyassociatedwiththelocalsourceS17.
[Nii]spectrum.TheN+ abundanceisafactorofafewlessthan
C+ andprobablyhastoo low anopacityto absorbsignificantly 4.1.Openorbitgasstreams
the[Nii]emissionfromtheCMZ.
The [Nii] line can be used to separate the contributions Molinarietal.(2011)foundthatHerschelfar-infraredimagesof
of [Cii] from highly ionized gas and neutral gas (PDRs and the CMZ revealedcold dense gastracing outa roughly200 pc
CO-dark H ). The [Cii] intensity from the fully ionized gas, size∞–shapedimage,asseeninprojection,centeredslightlybe-
2
I ([Cii]), is approximately proportional to the [Nii] inten- low(l,b)=(0◦,0◦).Thisfeaturecorrespondswellwiththebow-
ion
sity over a wide range of temperature and electron density, like streams seen in CS by Tsuboietal. (1999). Molinarietal.
(2011) used the CS(1–0)spectralline mapsand, assumingthat
2 Wereproducethe[Cii]and[Nii]spectrafromLangeretal.(2016) this line traces dense cores in the clouds forming this feature,
here because the OTF strip scans show the spatial extent of the [Cii] theyassignedaveragevelocitiestothepointsalongthe∞-shape.
componentassociatedwith[Nii]inthe3-kpcarm. They proposed that the shape and velocity structure could be
7
W.D.Langeretal.:TheCentralMolecularZoneasprobedwith[Cii]
Table1.Longitudinal[Cii]versusCO(4–3)and[Ci]intheCMZ
Longitude <I([Cii])>a <I(CO(4–3)b))>a <I([Ci])b>a <I([Cii])>/<I(CO(4–3))> <I([Cii])>/<I([Ci])>
Range (Kkms−1) (Kkms−1) (Kkms−1)
0◦.75to0◦.25 123 1012 345 0.12 0.36
0◦.25to-0◦.25 313 1013 269 0.31 1.16
-0◦.25to-0◦.75 67 570 172 0.12 0.38
a)Averageintensitycalculatedoverthevelocityrange±130kms−1butexcludingtheemissionintherange±10kms−1whereitismostlikelyto
becontaminatedwithemissionfromlocalgas.b)AST/ROobservationsfromMartinetal.(2004).
Fig.5.TheGOTC+[Cii]OTFspectraatsixpositionswhere(0◦.51,0◦.0)and(−0◦.58,0◦.0)areattheedgesoftheopenorbitinggas
stream,(−0◦.085,0◦.0)isnearSgrA,and(0◦.0,−0◦.20)and(0◦.0,−0◦.49)showthenarrowingofthe[Cii]emissionbelowtheplane.
modeled roughly as an orbiting elliptical ring with an oscillat- Furthermore,theirorbitalvelocitiesvaryalongtheorbitandare
ingwarpinthez–direction.TheymadeabestfittotheCSdata larger(upto100–200kms−1),thanthoseintheellipticalring
andfoundanellipsewithmajorandminoraxesof∼100pcand model.Theirbestfityieldsmajorandmajorandminoraxes121
∼60 pc,respectively,projectedon the plane of the Galaxy (see and59pc,respectively,andanorbitalvelocityatapocenterand
their Figure 5), an orbitalvelocityof 80 km s−1 (forsimplicity pericenterof101and207kms−1,respectively.
assumedconstant),andthattheminoraxisisoriented40◦ with
Toseewhetherthe[Cii]l–V dataencompassestheopenor-
respect to the line of sight to the Sun. The ellipse is assumed
bit gas streams we derived the [Cii] velocitiesas a functionof
to lie 15 pc below the plane and to have a vertical frequency
longitude for b=0◦ using the elliptical model of Molinarietal.
twicetheorbitalfrequency.Themodelparameters,asnotedby
(2011) to find the envelope of these features in [Cii]. This el-
Molinarietal. (2011), need to be used with caution given the
liptical model also allows us to constrain the limits on orbital
oversimplification of the model and that the assigned CS(1–0)
velocity.(Note thatour use of the Molinarietal. (2011) model
velocitiesarederivedbyeye.Nonetheless,theobservedveloci-
todefinetheapproximate(l,b,V)extentofdensemoleculargas
tiesarewithin±25kms−1 ofthemodelvelocities.
does not imply that the gas is on closed orbits.) The solid red
However,aspointedoutbyKruijssenetal.(2015)itisphys- lineellipseinFigure2(a)denotestheellipticalringinlongitude–
ically impossible to havea closed orbitin an extendedgravita- velocityfromMolinarietal.(2011)calculatedforb=0◦.Ourfit
tionalpotential.Insteadtheyhavereplacedthesingle–orbitellip- tothe[Cii]dataisshownasared-dashedellipseand,inorderto
ticalmodelofMolinarietal.(2011)byconsideringgasstreams accommodatetheemissionatl=±0◦.6,requiresanincreaseinor-
moving in an open orbit in the gravitational potential of the bitalvelocityto∼100kms−1 andorbitsizeofthemajor-axisto
CMZ. Henshawetal. (2016a,b) have expanded on this model. about120pc,aswellasaslightchangeintheminoraxisorien-
They find a best fit to the observed velocity field if they as- tationtothelineofsighttothesun(seeFigure5Molinarietal.
sume four streams (see Figure 2 in Henshawetal. (2016b)). 2011)to∼50◦.Ingeneralthe[Cii]emissionfollowsthetrendin
8
W.D.Langeretal.:TheCentralMolecularZoneasprobedwith[Cii]
theellipticalmodel,buttheoverallextentinlongitudein[Cii]is
largerthanthatderivedbyMolinarietal.(2011)fromCS(1–0),
asisevidentatthelimitsoftheellipseatl ≃±0◦.6.Insummary,
theorbitalvelocityderivedbyMolinarietal.(2011)istoosmall
toaccommodateallthe[Cii]emission.
Ourellipticalfittothe[Cii]emissionisinbetteragreement
withtheKruijssenetal.(2015)andHenshawetal.(2016b)open
orbitgasstreammodels.Togetaclearerpictureoftherelation-
shipof[Cii]emissiontotheopenorbitgasstreams,wecompare
theGOTC+[Cii](l–V)mapwiththewiththefouropenstream
orbital model of Kruijssenetal. (2015) and the Molinarietal.
(2011)ellipticalmodel.Figure7showsthe(l–b)and(l–V)maps
in Kruijssenetal. (2015) reproduced from their Figure 2 (left
panel in Figure 7) and Figure 4 (right panel in Figure 7). The
dotted line in the left panel indicates the Molinarietal. (2011)
elliptical orbital model. The solid open arcs in the right panel
show the four open gas streams and the legend labels them by
Fig.8. Expandedviewofthel–V mapsinFigure4consistingof
color. The grey–scale in the top panels is the integrated inten-
[Cii]contours(red)andtheCO(4–3)grey-scaleimagecovering
sityofNH (1,1)emissionthattracesthedensegas.Thesymbols
3 l = −0◦.7 to 0◦.0 and V = −130 km s−1 to −40 km s−1. The
witherrorbarsrepresentthephase–spaceinformationextracted LSR
[Cii]coutoursareT =1,2,3,4,5,and6K,andtheCO(4–3)
by Kruijssenetal. (2015) from NH (1,1) – see their paper for mb
details.TheGOTC+OTFscanatb3=0◦passesthroughstreams Tmbvaluesareshowninthecolourbarontheright.Wehavesu-
2and4,andpassesclosesttostream3nearl∼0◦.2andtostream perimposedthecentroidvelocitiesofthedensecorespresented
4nearl∼0◦.05. inFigure2bfromthepaperbyHenshawetal.(2016a).(Thever-
InthebottompaneltheGOTC+[Cii]longitudinal–velocity ticaldashedlinesare intheoriginalfigurefromHenshawetal.
(2016a) and identify the correspondence between the gas con-
contourplots are overlayedonthe l–V plots of the fourstream
open orbit model from Kruijssenetal. (2015). The [Cii] (l–V) densationsandthe extremesin velocityfield amplitude.)Itcan
beseenthatthel–V trendsof[Cii]andCO(4–3)correlatewell
maps overlap both the elliptical and open stream models, but,
as discussed above,the outermost(l–b)[Cii] emission fits bet- withthoseofthedensecorestracedbyN2H+.
ter with the open stream model. The [Cii] l–V emission corre-
lateswellwithstreams1and2,andaportionofstream3,while
the correlationwith stream4 is uncleardueto the weaknessof the region studied in Henshawetal. (2016a) in N2H+. The re-
the [Cii] and overlap with stream 2. However, the association gion extendsover l = −0◦.7 to 0◦.0 and VLSR = −130 km s−1 to
of [Cii] emission with the dense gasin streams2 and 3 is per- −40kms−1.Forcomparisontothedensegastracersweoverlay
hapsnotsurprisingasbothofthempassthroughthe[Cii] scan our [Cii] l–V mapson Figure 2b from Henshawetal. (2016a),
atb=0◦,howevertheassociationwiththedensegasinstream whichisalongitude–velocitymapsofthecentroidvelocitiesof
1 (andperhapsstream 4), which pass a few pc below the [Cii] theN2H+ spectralcomponents.TheN2H+ l–V plotliescloseto
stripscan,suggeststhatthereislowerdensitygasinaPDR,CO- several[Cii]localpeaksthatappeartohaveacorrugatedveloc-
darkH ,orionizedregionassociatedwiththedensegasstreams. ity structure in l–V space. In addition, the CO(4–3) color im-
Longm2oreetal.(2013)andHenshawetal.(2016b)proposethat age and[Ci] colorimage(notshown)appearto followthe gas
thegasalongastreamrepresentsatimesequenceintheforma- streams. Thusthe [Cii], [Ci], and CO(4–3)emissions are trac-
tionandevolutionofstar–formingdenseclouds.Ifthereisatime ingthegascloudsintheopenorbitingstreaminthisregion.This
sequencethenperhapssomeofthe [Cii] in inthe (l–V)mapis relationshipistobeexpected,astheneutralCandCOlayerslie
tracingtheCO-darkH phaseinearlycloudformationoralate closeto,andinthecaseofCO,overlapthedensecoresincloud
stagewhereitisbeings2trippedaway.Ifsothenmapsofthe[Cii] chemicalmodels,and[Cii]liesintheassociatedPDRs,CO-dark
emissionmaybeabletotellussomethingaboutthedynamicsof H2gasclouds,andionizedboundarylayers.
theformationofthecloudsinthegasstreams. Thewidespreaddistributionof[Cii]asrevealedbythelarge
One of the more interesting aspects of the orbiting gas distribution in velocity and the blending of [Cii] components
streams is the presenceof a corrugatedvelocity field with sev- makesitdifficulttotellwhetherthereisasystematicshiftinve-
eral regularly spaced features detected over l ≈ −0◦.1 to −0◦.7 locity amongthe diffuse anddense gastracers. Only spectrally
(Henshawetal. 2016a), whose velocity maxima correlate with resolvedl–bmapsof[Cii]oftheorbitinggasstreamcanreveal
massive and compact clouds. Henshawetal. (2016a) suggest whether there is evidence for the dynamical flows responsible
that these featuresillustrate the mechanism of cloud formation for cloud formation in this region. To derive the absolute re-
intheinnerCMZand,inparticular,highlighttheregionswhere lationship between the [Cii] velocity field and that of N2H+ is
gas flow ’traffic jams’ seed cloud formation. The evidence for beyondthe scope of this paper,given the limits inherentin the
the ripples and condensationsis derived from high density gas longitudinalstripscanandtheblendingofmanyfeaturesineach
tracers such as N H+, but direct evidence for the flows could [Cii]spectrum.
2
come from spectral probes that trace the lower density outly-
inggassurroundingthecores.The[Cii] longitudinalstripscan
4.2.G0.253+0.016(theBrick)
also cuts across or passes close to most of the region in which
Henshawetal.(2016a)detecttheripplesinvelocity. G0.253+0.016, also known as the Brick, is an infrared-dark
To examine the relationship of [Cii] emission to the corru- cloud at l ∼0◦.25 centered slightly above b =0◦. There is lit-
gatedvelocityfieldweshowinFigure8ablow-upofthe[Cii] tle evidence of massive star formation in the Brick despite
andCO(4–3)l–V mapsinFigure4(bottom-leftpanel)covering its high mass, ∼(1 – 2)×105 M , and high local and average
⊙
9
W.D.Langeretal.:TheCentralMolecularZoneasprobedwith[Cii]
Fig.7.TheGOTC+[Cii](l–V)mapcomparedwiththeMolinarietal.(2011)orbitalmodelandwiththefouropenstreamorbital
model of Kruijssenetal. (2015). (left panels): (l–b) and (l–V) maps in which the dotted line indicates the Molinarietal. (2011)
orbital model (reproduced from Figure 2 in Kruijssenetal. (2015)). The grey–scale in the top panel is the integrated intensity
of NH (1,1)emission that traces the dense gas. The symbols with error bars representthe phase–spaceinformationextractedby
3
Kruijssenetal. (2015) fromNH (1,1).The smallopenblack circle denotesSgr A∗. (Thecircle in the left panelis in the original
3
figureandindicatesa featureinposition–velocityspacediscussedbyKruijssenetal. (2015)).(rightpanels)(l–b)and(l–V)maps
inwhichthesolidopenarcsindicatethefouropengasstreamsandthelegendlabelsthembycolor(reproducedfromFigure4in
Kruijssenetal.(2015)).TheGOTC+OTFscanatb=0◦passesthroughstreams2and4,andpassesclosesttostream3nearl∼0◦.2
andtostream4nearl ∼0◦.05.InthebottompaneltheGOTC+[Cii]longitudinal–velocitycontourplotsareoverlayedonthel–V
plotsofthefourstreamopenorbitmodelfromKruijssenetal.(2015).
densities, >105 and >104 cm−3, respectively (Lisetal. 1994; spectral line data presented here pass through an edge of the
Longmoreetal.2012;Rathborneetal.2014;Millsetal.2015). Brick where V ∼ 30 to 40 km s−1. In contrast to the high
LSR
The detection of one H O maser and several compact radio dipolemolecules,CO,[Ci],and[Cii]tracethelessdenseouter
2
sources is consistent with the formation of perhaps at most envelopes of G0.253+0.016, and probes an important spatial
a few early B-stars (Longmoreetal. 2014; Millsetal. 2015). regime where dynamical effects (such as gas compression and
Pillaietal.(2015)derivetheorientationandstrengthofthemag- stripping) due the orbital streaming motion of the clouds with
netic fieldin G0.253+0.016fromdustpolarizationmaps.They respecttoGalacticcenterarelikelytobepronounced.Herewe
findthatthemagneticfieldishighlyorderedandliesinanarch present the results of the [Cii] emission and velocity structure
along the length of G0.253+0.016 and is sufficiently strong to andananalysisoftheH moleculargasseenby[Cii],[Ci],and
2
supportthecloudagainstcollapse.Longmoreetal.(2013)sug- COemissions.
gestthatthiscloudwas tidally compressedperpendicularto its
orbitandstretchedalongitsorbitafterpassingthroughpericen- In Figure 9 we compare the [Cii] l–V scans from l = 0◦.1
tre at its minimum in the Galactic gravitational potential. The to 0◦.4 at b =0◦.0, to threeother gastracers:CO(4–3),[Ci], and
[Cii] emission towards the Brick is strong; in fact it has some HN13C. Figure 9(a) shows the [Cii] contourson top of a color
ofthebrightestpeaksintheCMZstripscans,ascanbeseenin image of the AST/RO CO(4–3) emission (Martinetal. 2004),
Figures2(a)and4. where the CO(4–3) emission associated with G0.253+0.016is
strongest at l ∼0◦.25 over a velocity range V ∼20 to 40 km
LSR
G0.253+0.016 has been well studied by Rathborneetal. s−1. The CO(4–3) emission over l ∼0◦.1 to 0◦.3 at velocities 50
(2014) using a suite of optically thin and thick spectral line to80kms−1isnotassociatedwithG0.253+0.016andlikelylies
mapdataofhighdensitygastracers,anddustcontinuumemis- farfromit(see footnote15Rathborneetal. 2014).Inaddition,
sion in the mid-infrared and 450 µm. In Figure 9(d) we show therearetwoverybright[Cii]featuresthatpeakatl∼0◦.21.
the integrated HNCO intensity distribution from Figure 6 in
Rathborneetal. (2014) that traces out the dense core and the Figure 9(c) shows the [Cii] l–V emission contours plotted
velocityfield alongthe Brick.Our [Cii] longitudinalstrip scan over a color image of the [Ci] emission. [Ci] is much weaker
is offset from the main core of the Brick as indicated by the thanCO(4–3)acrosstheBrickandisstrongestatl ∼0◦.32,near
horizontal red line at b =0◦. It can be seen that the [Cii] theedgeofthe[Cii],andweakat∼0◦.21,where[Cii]isstronger.
10