Ionized gas at the edge of the Central Molecular Zone

0.76 MB·English

by  W. D. Langer

Astronomy&Astrophysicsmanuscriptno.CMZ-NII-CII-v4 (cid:13)c ESO2015
January28,2015
Ionized gas at the edge of the Central Molecular Zone
W.D.Langer1,P.F.Goldsmith1,J.L.Pineda1,T.Velusamy1,M.A.Requena-Torres2,andH.Wiesemeyer2
1 JetPropulsionLaboratory,CaliforniaInstituteofTechnology,4800OakGroveDrive,Pasadena,CA91109,USA
e-mail:[email protected]
2 Max-Planck-Institutfu¨rRadioastronomie,AufdemHu¨gel69,53121Bonn,Germany
Received18November2014;Accepted15January2015
5 ABSTRACT
1
0 Context.TheedgeoftheCentralMolecularZone(CMZ)isthelocationwheremassivedensemolecularcloudswithlargeinternal
2 velocitydispersionstransitiontothesurroundingmorequiescentandlowerCOemissivityregionoftheGalaxy.Littleisknownabout
theionizedgassurroundingthemolecularcloudsandinthetransitionregion.
n
a Aims.TodeterminethepropertiesoftheionizedgasattheedgeoftheCMZnearSgrEusingobservationsofN+andC+.
J Methods.WeobservedasmallportionoftheedgeoftheCMZnearSgrEwithspectrallyresolved[Cii]158µmand[Nii]205µmfine
structurelinesatsixpositionswiththeGREATinstrumentonSOFIAandin[Cii]usingHerschelHIFIon-the-flystripmaps.Weuse
7
the[Nii]spectraalongwitharadiativetransfermodeltocalculatetheelectrondensityofthegasandthe[Cii]mapstoilluminatethe
2
morphologyoftheionizedgasandmodelthecolumndensityofCO–darkH .
Results.Wedetect two[Cii] and [Nii] velocity components, one along th2elineof sight toaCO molecular cloud at -207 km s−1
A] associatedwithSgrEandtheotherat-174kms−1 outsidetheedgeofanotherCOcloud.Fromthe[Nii]emissionwefindthatthe
averageelectrondensityisintherangeof∼5to21cm−3 forthesefeatures.Thiselectrondensityismuchhigher thanthatofthe
G disk’swarmionizedmedium,butisconsistentwithdensitiesdeterminedforbrightdiffuseHiinebulae.Thecolumndensityofthe
h. CO–darkH2layerinthe-207kms−1cloudis∼1-2×1021 cm−2inagreementwiththeoreticalmodels.TheCMZextendsfurtherout
inGalacticradiusby∼7to14pcinionizedgasthanitdoesinmoleculargastracedbyCO.
p
Conclusions.The edge of the CMZ likely contains dense hot ionized gas surrounding the neutral molecular material. The high
o- fractionalabundance ofN+ andhighelectrondensityrequireanintenseEUVfieldwithaphotonfluxoforder 106 to107 photons
r cm−2s−1,and/orefficientprotonchargeexchangewithnitrogen,attemperaturesoforder104 K,and/oralargefluxofX-rays.SgrE
t isaregionofmassivestarformationasindicatedbythepresenceofnumerouscompactHiiregions.Themassivestarsarepotential
s
a sourcesoftheEUVradiationthationizeandheatthegas.InadditionX-raysourcesandthediffuseX-rayemissionintheCMZare
[ candidatesforionizingnitrogen.

1 Keywords.ISM:clouds—ISM:HIIregions—Galaxy:center
v
2
3 1. Introduction with scale length of 145 pc (see Ferrie`reetal. 2007), which
8 yields a value at the edge of the CMZ < n(e)>∼ 1 - 2 cm−3.
6
The Central Molecular Zone (CMZ), the central∼400 pc by ∼ Lazio&Cordes (1998) proposed that the scattering of radio
0
80pcoftheGalaxy,isaregionfilledwithmassivedenseGiant waves is not due to a uniform electron distribution but occurs
.
1 MolecularClouds(GMC)havinglargeinternalvelocitydisper- in thin ionized layers at the surface of molecular clouds with
0 sion.Characteristiccloudlinewidthsare≥20kms−1,compared n(e)&103 cm−3 and T (e) ∼ 103 K, or at the interface with the
k
5 to a few km s−1 for GMCs in the disk (e.g. Morris&Serabyn Hot IonizedMedium(HIM) with n(e)∼ 5 – 50 cm−3 and T (e)
1 k
1996; Okaetal. 1998; Ferrie`reetal. 2007). In addition,GMCs ∼105 -106 K.Observationsofthespectrallyresolvedemission
:
v in the CMZ have higher kinetic temperatures,T ∼ 30 to fromionsintheCMZcanhelpdistinguishamongthesedifferent
k
i 200 K (Aoetal. 2013; Mills&Morris 2013), and densities, modelsandprovideinformationaboutthedensity,fillingfactor,
X
n(H ) ≥ 104−5 cm−3 (e.g. Jacksonetal. 1996; Okaetal. 1998; andionizationmechanisms.Theobjectiveofthispaperistoex-
2
r Dameetal. 2001; Martinetal. 2004), than GMCs in the disk. plorethenatureoftheionizedgasattheedgeoftheCMZnear
a
ThegasdistributionwithintheinnerGalaxyandacrosstheedge SgrEusingspectrallyresolvedemissionfromthefinestructure
oftheCMZisthoughttobeduetothegravitationalpotentialand lines,[Nii]and[Cii],ofionizednitrogen,N+, andionizedcar-
dynamicsofthegasthere(seeoverviewbyFerrie`reetal.2007). bon,C+,respectively.
While the atomic and molecular hydrogen distributions in the
The Balloon-borne Infrared Carbon Explorer (BICE)
CMZ have been mapped with spectrally resolved Hi and CO, mapped[Cii]intheinnerGalaxywithmodestspatial(15′)and
respectively,lessisknownaboutthedistributionandproperties low spectral (∼175 km s−1) resolution (Nakagawaetal. 1998).
oftheionizedgasinthisregion. BICEdetectedbright[Cii]emissionfromtheCMZfrom358◦.3
Theelectrondistributionoftheinterstellarmediumhasbeen < l < 1◦ but tapering off outside, and confined to |b| .1◦ (see
determined from dispersion measurements of radio emission their Figures 7 and 8). Within the limitations of BICE’s low
frompulsarsandothercompactradiosources(Cordes&Lazio spectralresolution,Nakagawaetal.(1998)foundthat[Cii]has
2002, 2003). Lazio&Cordes (1998) and Roy (2013) find that a large velocity dispersion towards the CMZ, ∆v≥100 km s−1
the averageelectron density in the inner 100 pc is <n(e)>∼10 (seetheirFigure14),similartothe dispersionseenin12CO. In
cm−3. The radial distribution has been modeled as a Gaussian addition, the Infrared Space Observatory (ISO) observed [Cii]
1
W.D.Langeretal.:IonizedgasattheedgeoftheCentralMolecularZone
and [Nii] along a number of lines of sight in the CMZ (see
Rodr´ıguez-Ferna´ndezetal. 2004) with a spectral resolution ∼
30 km s−1. However,much better spectral resolution is needed
toseparateindividualcomponentsanddeterminegascharacter- CMZ edge
istics. ()$*%
To understand better the distribution of the ionized gas at
the edge of the CMZ we observed several lines of sight in
[Nii] and [Cii] in July 2013 using the heterodyne German
REceiver for Astronomy at Terahertz frequencies (GREAT1; !!!!!"""""!!!""!!""!!!!""""
Heymincketal. 2012) onboard the Stratospheric Observatory
for Infrared Astronomy (SOFIA) (Youngetal. 2012). In addi-
tion, prior to the GREAT observations, we made small scale
strip maps in [Cii] to map the overall distribution of ionized %&’$($
gas as part of a Herschel2 (Pilbrattetal. 2010) Open Time
2 Programme, using the high spectral resolution Heterodyne
InstrumentintheFarInfrared(HIFI;deGraauwetal.2010).
Here we report on the results of these observations, which
indicatethatthere are two regionsof highlyionized,hightem- !"#$% !!"#$ !"&$% !"’
perature,densegas:onealongthelineofsightthatincludesaCO
molecularcloudassociatedwithSgrEatV ∼-207kms−1and
lsr
theotheratV ∼-174kms−1 outsidetheedgeofaCOmolec- Fig.1. A portion of a map of CO integrated intensity from -
lsr 180 to -90 km s−1 of the edge of the CMZ from (Fukuietal.
ularcloudlocatedatlargerlintheCMZ.Theelectrondensities
derived from the [Nii] emission, n(e) ∼ 5 to 25 cm−3, are two 2006),andreprintedwithpermissionfromAAAS,basedonthe
NANTENCO(1-0)mapwhichhasabeamsize∼3′.Theregion
tothreeordersofmagnitudehigherthanthatinthedisk’swarm
mapped with HIFI is outlined by the yellow box and the posi-
ionized medium (WIM), but consistent with those found from
analysisof[Nii]emissionintheCarinaNebula,aluminousHii tionsofthe[Cii]and[Nii] observationswith GREATareindi-
cated bycrosses. The approximateCO edgeof the CMZ and a
regionwithnumerousOstars(Oberstetal.2011).Thispaperis
portionofLoop1areindicated,asisthelocationofSgrE.
organizedasfollows.InSection2wepresentourobservations,
whileSection3analyzesthedistributionanddensityoftheion-
izedgas.Section4discussesthepossibleionizationsources,and
(Fukuietal. 2006). This loop, however,is at a Galactic radius,
Section5summarizestheresults.
R ∼670pc(Toriietal.2010),welloutsidetheCMZ.
G
2. Observationsof[Cii]and[Nii] 2.1.SOFIA[Cii]and[Nii]data
In Figure 1 we show a portion of the NANTEN 4-m CO map We observed the lower fine structure transition of ionized ni-
from Figure 1a in Fukuietal. (2006), and reprinted with per- trogen, [Nii] 3P1→3P0 at 1461.1319GHz (205.2 µm) and, si-
missionfromAAAS,thatzoomsinontheCOedgeoftheCMZ. multaneously, the fine structure line of ionized carbon, [Cii]
The yellow box in Figure 1 outlines the region mapped with 2P3/2→2P1/2at1900.5369GHz(157.7µm),usingGREAT.The
HIFI,andthepositionsobservedin[Cii]and[Nii]withGREAT angularresolutionis 15′′ at[Cii] and20′′ at[Nii]. To improve
areindicatedbyredcrosses.[Nii]tracesjustthehighlyionized the signal-to-noise ratio (SNR) the [Cii] and [Nii] data were
gas, while [Cii] traces highly ionized gas and the weakly ion- smoothed to a velocity resolution of 1.20 km s−1 and 1.57 km
ized neutral atomic and molecular gas. Located just inside the s−1 perchannel,respectively.Thespectrawerecalibratedusing
edgeoftheCMZisSgrE,aregionwithmanyradiosourceslo- theprocedureoutlinedinGuanetal.(2012).
catedaroundl ∼358◦.7andb ∼0◦ (Gray1994),whichcontains The GREAT [Cii] observationscontained atmosphericfea-
18compact(<2′)Hiiregionsanddiffuseemission(Liszt1992). turesinthespectralregionobservedduetothefrequencysetting
SgrE is a regionofactivestar formationassociatedwith a CO forlargenegativevelocitiesofthesource,andthe[Cii]linewas
molecularcloudwithV ∼-210kms−1 (Okaetal.1998).The inthewingofastrongwaterline.Moreover,thepositionswitch-
lsr
radio recombinationline survey,Hii Region Discovery Survey ingobservingmodemadethepresenceofthisfeatureasignifi-
(HRDS:Andersonetal.2011,2012)detectedeightoftheseHii cantissuethatneededtobeaddressedinthedatareduction.The
sourcestowardsSgrEatV ∼-208kms−1,thesamevelocity standardatmosphericcalibrationwasnotsufficienttoproducea
lsr
astheCOcloud.AlongthelineofsightoutsidetheCMZthere satisfactory fit to the water feature, and we had to use a set of
are Giant Magnetic Loops, large scale molecular gas loop-like non-standardparametersin the atmosphericmodel(Guanetal.
features,withlargelinewidthsandlargescaleflowsdetectedin 2012) to fit the width andthe peakof the water line. Themain
CO (Fukuietal.2006;Fujishitaetal. 2009).TheedgeofLoop changeto the normalprocedurewas to reduce the contribution
1 interceptsthe plane atl ∼358◦.2 to 358◦.4, close to the line of fromtheDryatmosphereinsteps,allowingtheWetpart(water)
sight to the edge of the CMZ, and has a V ∼ -135 km s−1 tofittheatmosphericfeature.Weimplementedasetofsixmod-
lsr
els,andchosethemodelthatbestfitthewaterfeature.Thisfea-
ture was removedand a zero orderspectralbaseline fit applied
1 GREAT is a development by the MPI fu¨r Radioastronomie
tothedelivereddataproducts.Theresultingspectrawere,with
and KOSMA/Universita¨t zu Ko¨ln, in cooperation with the MPI fu¨r
SonnensystemforschungandtheDLRInstitutfu¨rPlanentenforschung. oneexception,fairlyregularwithresidualloworderpolynomial
2 Herschel is an ESA space observatory with science instruments variationsandonlyslightoffsetsfromzero;onespectrumhada
providedbyEuropean-ledPrincipalInvestigatorconsortiaandwithim- longperiod,butwellordered,variationinthebaseline.Inorder
portantparticipationfromNASA. tofitthebaselinesbetterthanzeroorder,thedelivereddatawere
2
W.D.Langeretal.:IonizedgasattheedgeoftheCentralMolecularZone
thenreducedfurtherinCLASS3 andbaselinescorrectedwitha 1.00
polynomialfittingroutine.ComparisonoftheGREATandHIFI
SOFIA OFF1
[Cii] data at one nearly coincident position (see below) shows G358.50+0.25
[CII] Blue
excellent agreement, lending confidence to the removal of the [NII] Red
waterlineandoverallbaselinefittings.Theintensitieswerecor-
rectedforamainbeamefficiencyof0.67.Thermsnoiseforthe
final[Cii]productsrangesfrom0.16to0.26Kper1.20kms−1
channel,whilethatfor[Nii]rangesfrom0.09to0.23Kper1.57
0.50
kms−1channel.
We observed six lines of sight (LOS) along b=0◦ at lon-
gitudes from l=358◦.45 to 358◦.75. The reference off-position,
OFF1,l=358◦.50andb=0◦.25containsweak[Cii]andveryweak T(K)
[Nii] emission, shown in Figure 2, so that all the on-positions
hadtobecorrectedforthisemission.Tocorrecttheobservations
foremission in OFF1 itwas observedseparatelyontwo nights
0.00
asanon-positionusingadifferentoff-position,OFF2,l=358◦.50
and b=0◦.65. The OFF1 lines appear to be singly peaked and a
Gaussianfittothe[Cii]emissioninOFF1peaksatV ∼-195
lsr
km s−1 and has a full width half maximum(FWHM) ∼ 43 km
s−1.Thenarrow[Cii]featureatV ∼-58kms−1 (FWHM∼9
lsr
kms−1)arisesfromthe3kpcarm.AGaussianfittothebroad,
weak[Nii]lineyieldsapeakV ∼-205kms−1 andaFWHM
∼65kms−1,closetothecharactlesrristicsofthe[Cii]lineconsid- -0.50
-350 -300 -250 -200 -150 -100 -50 0
eringthe weaknessofthelineandthe lowsignal-to-noiseratio
of this detection. No other [Nii] is detected in the band above Vlsr(km/s)
the3-σlevel.
Fig.2.[Cii] and[Nii] emission inthe referenceOFF1 position
As a checkon the qualityof the GREAT data, given thatit
at l=358◦.50 and b=0◦.25. The data have been smoothed to 2.4
hadtobecorrectedfortheatmosphericwaterlineandemission
kms−1 for[Cii]and3.1kms−1 for[Nii]toimprovethesignal-
intheOFF1position,weshowinFigure3aspectrumfromthe
to-noise.The[Cii]emissionpeaksatV ∼-195kms−1 while
HIFIGalacticObservationsofTerahertz(GOTC+)[Cii]survey lsr
thatfrom[Nii]peaks∼-207kms−1,withinthefittingerrorsof
(Langeretal. 2010; Pinedaetal. 2013) at the closest position,
theweaker[Nii]signal.Thenarrow[Cii]emissionatV ∼-58
l=358◦.696andb=0◦.0,totheGREATobservationatl=358◦.700 lsr
kms−1arisesfromthe3kpcarm.[Nii]isnotdetectedfromthis
and b =0◦.0. The offset in longitude of 14′′ correspondsto 1.2
arm.
HIFI and 0.9 GREAT beam widths, and some portions of the
mainbeamsareonlyaboutanarcsecondapart.The[Cii]emis-
sionfromHIFIandGREAThavethesamedistributioninveloc-
ity, both show a strong peak at ∼ -210 km s−1 with intensities componentswerefitwithadoubleGaussianwithonepeakat∼
thatgenerallyagree within the uncertaintiesof the baselinefit- -215kms−1andaFWHM∼35kms−1andtheothercomponent
ting,andbothshowweakemissioncenteredaround∼-175km is at ∼ -170 km s−1 with a FWHM ∼25 km s−1. All the [Cii]
s−1 thatblendsintothestrongerfeature.Theintegratedintensi- featureshavealargeSNRwhilethe[Nii]SNRrangesfrom∼4
ties of these two observationsover the velocity range -150 km to20forthefourLOSinwhichthelinewasdetected.
s−1 to -250 km s−1 differ by only 10%. Furthermore, the GOT
C+ spectrumused a differentoff position than the GREAT ob- 2.2.Herschel[Cii]mapdata
servations,lending furthersupportfor our procedureto correct
the GREAT [Cii] and [Nii] for emission in the GREAT OFF1 The GREAT observations provide [Cii] at six and [Nii] at
position. four longitudinal positions in the plane (b = 0◦). To get
The [Cii] and [Nii] spectra along b=0◦ are shown in insight into the distribution of [Cii] above and below the
Figure 4. [Cii] emission was detected at all six positions and plane we use [Cii] maps made with the high spectral resolu-
there are two adjacent, and at times blended, features arising tionHIFI(deGraauwetal.2010)instrumentonboardHerschel
from the CMZ in the velocity range ∼-150 km s−1 to -250 km (Pilbrattetal. 2010). We used a number of On-The-Fly (OTF)
s−1. At the two positions where these components are clearly scansto mapthe0◦.6 × 0◦.4 regionshowninFigure1. AllHIFI
separated, the peaks occur at V ∼ -174 and -207 km s−1. In OTFscansweremadeintheLOAD-CHOPmodeusingarefer-
lsr
general the -174 km s−1 component is weaker than the -207 enceoff-sourcepositionatl=358◦.80,b=1◦.90,about1◦.7awayin
km s−1 component, and both features are broad with a typical latitudefromthe topof the map.Themapconsists of13 scans
FWHM of ∼ 25 kms−1. No emission is detectedat V ∼ -135 inGalacticlatitude,centeredatb=0◦andsteppinginlongitude
lsr
kms−1,thevelocityofLoop1. at 3′ (0◦.05) intervalsfrom 358◦.20 to 358◦.80. The OTFb-scans
[Nii]isonlydetectedatfourofthesixLOS,andisgenerally are24′long,sampledatevery40′′.At1.9THztheangularres-
much weaker than [Cii] except at l=358◦.45 where the compo- olutionoftheHerscheltelescopeis12′′,correspondingtoaspa-
nentatV ∼-207kms−1 isabouthalfasintenseas[Cii].The tialresolutionof∼0.5pcat8.5kpc,thedistancetotheGalactic
lsr
fourLOSwhere[Nii] is detectedhavean rmsnoise of∼0.1K Center. However the effects of scanning rate and the 40′′ sam-
per 1.57 km s−1 channel whereas the two lines of sight where plingoftheOTFmapsdegradestheeffectiveresolutioninb to
[Nii]isnotdetectedhavealargerrmsnoise,∼0.23K.The[Nii] 80′′ while it is 12′′ in l. Thus spatial structures on scale sizes
smaller than 80′′ in b will be strongly attenuated. Furthermore
3 http://www.iram.fr/IRAMFR/GILDAS the total duration of each OTF scan was typically ∼2000 sec
3
W.D.Langeretal.:IonizedgasattheedgeoftheCentralMolecularZone
3 3 3
G3[C5I8I]. 7B5l+ue0.0 G3[[CN5I8III].] 7 BR0le+ude0.0 G3[C5I8I]. 6B5l+ue0.0
2 2 2
T(K) 1 T(K) 1 T(K) 1
0 0 0
-1 -1 -1
-350 -300 -250 -200 -150 -100 -350 -300 -250 -200 -150 -100-350 -300 -250 -200 -150 -100
Vlsr(km/s) Vlsr(km/s) Vlsr(km/s)
3 3 3
G358.60+0.0 G358.55+0.0 G358.45+0.0
[CII] Blue [CII] Blue [CII] Blue
[NII] Red [NII] Red [NII] Red
2 2 2
T(K) 1 T(K) 1 T(K) 1
0 0 0
-1 -1 -1
-350 -300 -250 -200 -150 -100 -350 -300 -250 -200 -150 -100 -350 -300 -250 -200 -150 -100
V (km/s) V (km/s) V (km/s)
lsr lsr lsr
Fig.4. [Cii] (blue) and [Nii] (red) spectra at the six positions observed with GREAT along b=0◦.0. The Galactic longitude and
latitudeareindicatedbyGXXX.XX+Y.Y.Thetwofeaturescenteredat∼-174kms−1and∼-207kms−1(whichareblendedatsome
positions)arisefromtheedgeoftheCMZ.[Cii]isdetectedatall6positionsintheCMZ,but[Nii]isonlydetectedat4positions.
Thetwonon-detectionsin[Nii]areassociatedwithgreaterrmsnoise.While[Nii]isgenerallymuchweakerthan[Cii],the[Nii]
peaktemperatureatl=358◦.45isabouthalfthatof[Cii].
whichprovidesonlya smallintegrationtime oneachspectrum sitivity of the b–V maps we use a velocity resolution of 3 km
(pixel). Thus the rms in the OTF spectra, 0.22 K in T in an s−1.InalltheOTFb–Vmapsthevelocityfeaturenear-207km
mb
88′′ beamand3kms−1 channel,islargerthanthatinthecorre- s−1 is prominent,but that of the weaker -174km s−1 feature is
spondingGREATspectra. less so, possibly due to the combined effect of low sensitivity
andbeamdilution.However,thebright[Cii]features(>1K)are
The OTF map data presented here were processed follow-
welltracedintheOTFmaps.Tobringoutthespatialstructureof
ing one of the procedures suggested by the Herschel Science
Center science staff, by running the HIPE pipeline without an the -207km s−1 velocityfeaturewe generatea [Cii] integrated
off-source subtraction to produce level 1 data. Then the heb- intensitymapcovering-220kms−1 to-200kms−1.Thisveloc-
ity range facilitates comparison with the Nobayama CO maps
Correctionroutinewasappliedtothelevel1datatoremovethe
ofOkaetal.(1998),whicharepresentedin10kms−1 intervals.
HEBstandingwavesandafterwardswefollowedtheprocedure
Togeneratethe[Cii] mapweintegratetheintensitiesintheb–
described in Velusamy&Langer (2014) to produce the level 2
V mapsat eachlatitude pixeloverthe rangeof velocitiesfrom
data and spectral line maps. The fact that hebCorrection sub-
-220kms−1 to-200kms−1 aroundthe[Cii] spectralpeak.We
tractsthematchingstandingwavepatternsfromalargedatabase
of spectra eliminates the need for off-source subtraction. Thus then use these intensities at all longitudes to generate the 2–D
inouranalysistheprocessedspectraldataarefreefromanyoff mapshowninFigure5.Notethatwhiletheintensitiesaresam-
pled contiguously in latitude, we only have data at 13 discrete
source contamination. We use the processed spectral line scan
longitudeswithapixelwidthof12′′.Tomakethedatamorevis-
map data to make latitude–velocity(b–V)mapsof the velocity
structure of the [Cii] emission as a functionof latitude at each ibleonthemapwebroadenedtheirdisplayinlongitudeto60′′.
Thereislittleorno[Cii]forl<358◦.30.
of the 13longitudesof the OTFpositions.To increase the sen-
4
W.D.Langeretal.:IonizedgasattheedgeoftheCentralMolecularZone
3
!"
G358.70+0.0
HIFI Red
GREAT Blue
#" #" #"
2 #"
#"
#"
#"#" #" #"
#"
#" #"
K) #" #"
(b 1
m
T
0 Fig.5. Comparison of the [Cii] OTF intensity maps integrated
over the velocity range -220 km s−1 to -200 km s−1 shown as
coulored strips with the CO map from Okaetal. (1998) inte-
gratedoverthevelocityrange-220to-210kms−1.TheCOin-
tegrated intensity maps from Okaetal. (1998) are shown only
in 10 km s−1 steps, but to improve the signal to noise in the
-1
[Cii] OTF maps we use a range of 20 km s−1. The CO map
-300 -250 -200 -150 -100
V (km/s) integrated from -210 to -200 km s−1 shows the same extent in
lsr b and the same edge at l ∼ 358◦.5 as the -220 to -210 km s−1
Fig.3. GREAT [Cii] spectrum at the position observed with map,butextendsfurtherto359◦.0,andwouldgivethesamemor-
GREAT, l=358◦.700 and b=0◦.0, that is closest to a [Cii] spec- phological relationship to the [Cii] contours. The color bar at
trum from GOT C+. The GOT C+ position l =358◦.696 and thetopindicatesthe[Cii]integratedintensityinKkms−1.The
b =0◦.0 is offset in longitude by 14′′ from the GREAT point- [Cii]OTFmapssampleinbevery0◦.05inlongitudefrom358◦.2
ing,whichisabouttheGREATbeamwidth.TheHIFIdatahave to358◦.8,so generatinga contourmapisimpracticaldueto the
beensmoothedtotheresolutionoftheGREATdata,1.2kms−1. sparsesamplinginl.Insteadwehaveindicatedbythewhiteline
The [Cii] GREAT spectrumis similar to thatfromHIFI show- theconnectionofthepeakintensityineachstrip.TheCO con-
ingboththe-207kms−1and-174kms−1components.Thetotal tours,indicatedinred,areinstepsof15Kkms−1.The[Cii]is
integratedintensitiesagreewithin±10%. strongestslightlybeyondtheedgeoftheCOemissionabovethe
plane.ThecompactHiisourcestowardsSgrE(Liszt1992)are
indicatedbywhiteandredXsymbols,dependingonbackground
colour.
3. Results
The relationship between the distribution in [Cii] and CO is pact Hii sources detected by Liszt (1992) towards Sgr E (the
broughtoutinFigure5wherewesuperimposeonthe[Cii]strip remaining three are outside the mapped region). In addition to
mapstheCOintegratedintensityfrom-210kms−1 to -220km thesecompactHiiregionsthereappearsto belowleveldiffuse
s−1 (Okaetal. 1998). The only caveat is that the region at the emissionfroml∼358◦.8to359◦.0betweenb=±0◦.1(seeFigure
edgeoftheCMZisirregularlymappedinCOintheNobayama 8inLiszt(1992)).ThecompactHiisourceshaveelectronden-
maps. However, the AT&T Bell Labs CO survey mapped the sitiesoforderseveralhundredcm−3andrequireionizingstarsof
CMZ out to l = ±2◦ (see Figure 1; Morris&Serabyn 1996) typeB0orbrighter(Liszt1992).
and also shows a sharp edge in CO emission at l ∼358◦.5. All six [Cii] and the four [Nii] spectra in Figure 4 show
The -207 km s−1 cloud is clearly seen in the position-velocity emission in the velocity range -140 to -240 km s−1. The de-
maps from the AT&T Bells Labs CO survey (see Figure 4; tection of broademission in [Cii] and [Nii] indicates the pres-
Morris&Serabyn 1996). The CO emission comes from the ence of warm, and perhaps dense, ionized gas at the edge of
cloud associated with Sgr E, an active regionof star formation the CMZ.To gainfurtherinsightinto thenatureofthe gasand
with bright compact Hii regions. Though our [Cii] map is not itsassociationwithdifferentinterstellarmedium(ISM)compo-
contiguous in longitude (scans are spaced by 3′) the brightest nentswe compare[Cii] and[Nii] to theCO(1-0)andHispec-
emission(whiteinthecolorwedge)appearstobeanarcabove tra along b = 0◦ taken from the Three-mm Ultimate Mopra
the planeslightly outside the lowestCO contour.The [Cii] arc Milky Way Survey4 (ThrUMMS) (Barnesetal. 2011) and the
ismostlikelylimbbrightenedemission.Theredoesnotappear AustraliaTelescopeCompactArrayHiGalacticCentersurvey5
tobeasimilarlimbbrighteningforb <0◦.Althoughwedonot (McClure-Griffithsetal.2012),respectively.
have any [Nii] observations along this arc, we conclude from InFigure6weplottheCO(1-0)spectraatb =0◦ alongthe
thepresenceof[Nii] alongthe line ofsightto the CO cloudat strip observedwith GREAT, l = 358◦.40 to 358◦.75 and b = 0◦.
b = 0◦ that the [Nii] and [Cii] emission arise from an ionized The correspondingHi spectra are plotted in Figure 7. The CO
regionsurroundingthecloud,butthereisalsoalikelycontribu- and Hi spectra show that there are two different environments
tionto[Cii]fromaphotondominatedregion(PDR)sandwiched
between the highly ionized hydrogenand dense molecular gas 4 www.astro.ufl.edu/thrumms
tracedbyCO.Alsoindicatedonthemapare15ofthe18com- 5 www.atnf.csiro.au/research/HI/sgps/GalacticCenter
5
W.D.Langeretal.:IonizedgasattheedgeoftheCentralMolecularZone
14 100
358.75
358.75
358.70
12 358.65 358.70
358.65
358.60
80 358.60
358.55
355.55
358.50
10 358.45 358.50
358.45
358.40
358.40
60
8
T(K) 6 T(K) 40
4
20
2
0
0
-2 -20
-260 -250 -240 -230 -220 -210 -200 -190 -180 -170 -160 -150 -270 -260 -250 -240 -230 -220 -210 -200 -190 -180 -170 -160 -150
V (km/s) V (km/s)
lsr lsr
Fig.6. CO(1-0) spectra at b = 0◦ at the longitudesobservedin Fig.7. Hi spectra at b = 0◦ along the latitudes observed in
[Cii] and [Nii]. The data is taken from the ThrUMMS survey [Cii] and [Nii]. The data are taken from the ATCA Hi survey
(uncorrectedformainbeamefficiency)andtheinsetisakeyto (McClure-Griffithsetal.2012)andtheinsetisakeytothelines
the linesof sightobserved.Movingtowardsthe CMZ, largerl, ofsightobserved.Hiisdetectedatallpositionsat-207kms−1
CO is notdetecteduntill ∼ 358◦.55(althoughthereis a hintof and is strongestclose to Sgr E. Hi is also detected at -174 km
CO at358◦.50)at the inneredge of the CMZ, andthat onlythe s−1butisweakerthanat-207kms−1.
-207kms−1componentispresent.The-174kms−1COcompo-
nentisdetectedat358◦.75,thepositionfurthestfromtheedgeof Table1.Integratedlineintensities
the CMZ and it is considerablyweaker there than the -207km
s−1component. LOS I([Cii])a,b I([Nii])c I(CO) I(Hi)
V =-207kms−1
lsr
358.45+0.0 63.4 26.4 6.7 1047
358.55+0.0 45.4 15.4 26.6 1904
detectedin[Cii]and[Nii].TheemissionatV ∼-207kms−1
lsr 358.60+0.0 47.6 7.1 60.6 2189
isassociatedwithbothCOandHi,whilethatfor-174kms−1is 358.65+0.0 38.6 - 101.9 2539
associatedonlywithHiuntill∼358◦.75,wherethereisahintof 358.70+0.0 45.4 12.4 98.9 2923
CO emission.The distributionsofthe [Cii] and[Nii] emission 358.75+0.0 57.4 - 110.5 3235
for the -207 km s−1 component are towards the edge of a CO V =-174kms−1
lsr
molecular cloud associated with Sgr E, while the -174 km s−1 358.45+0.0 21.1 8.2 - 730
distributionisalongtheedgeofanotherCOmolecularcloud,as 358.55+0.0 12.2 8.8 - 1212
can be seen in the Nobayama CO maps in this velocity range 358.60+0.0 43.0 3.7 - 1544
358.65+0.0 25.5 - - 1855
(Okaetal.1998).
358.70+0.0 15.3 5.3 - 2177
Toevaluatethepropertiesoftheionizedgaswederivedthe
358.75+0.0 21.8 - 9.8 2522
parametersofthe[Cii],[Nii],andCOfromGaussianfitstoeach
a) Integrated intensities are in units of K km s−1. We only report de-
linecomponent.Incaseswherethelinesareblendedwederive tections with a SNR ≥ 3, see text. b) Typical rms noise in the [Cii]
theirlineparametersusingadoubleGaussianfitovertheveloc- integratedintensityis∼1.4Kkms−1.c)Typicalrmsnoiseinthe[Nii]
ityrange-100kms−1to-300kms−1.Theintensitiesaregivenin ntegratedintensityis∼1.3Kkms−1.
Table 1. We calculate the correspondingHi intensitiesby inte-
gratingoverthevelocityrangesofthetwo[Cii]componentsbe-
causewecannotgetunambiguousGaussianfitsoversuchbroad 3.1.Radiativetransfersolutionsfor[Cii]and[Nii]
and, at low velocities, flat lines. To avoid double counting the
intensitieswechose-190kms−1 asthedividingvelocityofthe The relationship of the column density of C+ and line inten-
two components, and their intensities are given in Table 1. In sity, I([Cii]), has been extensively discussed in the literature
general,thereisagoodcorrelationof[Nii]intensitywith[Cii] (e.g. Goldsmithetal. 2012). The [Cii] peak antenna tempera-
intensity, which suggeststhat a significant fractionof the [Cii] turesreportedhereare<<0.3timesthekinetictemperatureand,
emission arises in the [Nii] region. We will use the [Cii] and as discussed by Goldsmithetal. (2012), are in the “effectively
[Nii] integrated intensities to characterize the propertiesof the optically thin limit” in which the column density is propor-
ionizedgas. tional to the integrated line intensity. Therefore we have from
6
W.D.Langeretal.:IonizedgasattheedgeoftheCentralMolecularZone
Langeretal.(2014),EquationA.2,therelationshipbetweenthe 100
column density, N(C+) in cm−2 and integrated line intensity,
T(K)dv,inKkms−1,
R
n
N(C+)=2.92×1015[1+0.5e∆E/kT(1+ cr)]I([CII]) cm−2 (1)
n 2e17
1e17
5e16
where ∆E/k =91.25K is the energy needed to excite the 10 2e16
2P3/2 level, n is the density of the collision partner of the m/s) 1e16
bulk of the gas, and n is the corresponding critical den-
cr k
sity. The collision rate coefficients for e, H, and H are sum- K
marized in Goldsmithetal. (2012) with an update2for H2 in NII]) (
Wexiceistaetniofenldo&f CG+owldhsemreithwe(2d0e1t4e)c.tE[Nleicit]r.oInnsthweililondiozmedinraetgeiothnes I([10
1
where [Nii] is detected the temperatures are high enough that
wecanneglecttheexponentialtermand
n (e)
N(C+)=2.92×1015[1+0.5(1+ cr )]I([CII]) cm−2. (2)
n(e)
Therelationshipofthecolumndensityofionizednitrogento 0.1
theintegratedlineintensity,inunitsofKkms−1,intheoptically 0.01 0.1 1 10 100
thinlimit,assuminguniformexcitationalongthelineofsight,is n(e) (cm-3)
givenby,
Fig.8.Theintensityof[Nii]205µmlineversusn(e)forarange
hc3 of ionized nitrogen column densities; the insert gives the key
Iul([NII])=Z Tul(K)dv= 8πk(νul)2AulfuN(N+)cm−2, (3) fporroathcehecsotlhuemcnridtiecnaslidtye.nTsihtey.curvesflattenoutasthedensityap-
where A is the Einstein A-coefficient, ν the transition fre-
ul ul
quency, f is the fractionalpopulationof the upperstate which
u oftheemittingregionandthefractionalabundanceofionizedni-
dependsonthedensityofthecollisionpartner,n,I isinunitsof
trogen,x(N+)=n(N+)/n,wheren=n(H)+2n(H )+n(H+)isthe
Kkms−1,andN(N+)isthecolumndensityofionizednitrogen. t t 2
totaldensityofhydrogennucleiincludingions.Toestimaten(e)
Forthe3P →3P transitionat205µmthisequationreducesto
1 0 wereplacethedensityofionizednitrogen,n(N+),inEquation5
by x(N+)n, andbecause the [Nii] emission arises fromhighly,
I([NII])=5.06×10−16f N(N+) (Kkms−1) (4) t
1 or perhaps completely, ionized gas we set n ≃ n(H+) = n(e).
t
For a uniformdensity region we can take N(e) = n(e)L, where
where f isthefractionalpopulationofN+inthe3P state.Ifwe
assume1thatthe[Nii]emissionregionisuniformwe1canreplace L is the thicknessof the emission region,and can now express
Equation5as
N(N+)by,n(N+)L,whereListhepathlengthincm,
I([NII])=5.06×10−16f n(N+)L (Kkms−1) (5) I([NII])= T dv=5.06×10−16x(N+)Ln(e)f (n(e)), (7)
1 10 1
Z
We can solve for f in the optically thin limit from the de-
tailed balance equation1s for the 3P , 3P , and 3P states along whereIisinKkms−1.Rewritinginconvenientunitsweobtain,
2 1 0
withthecondition f + f + f =1.Inionizedregionselectrons
dominatetheexcitat0iono1fN+.2Thecollisionalde-excitationrate I([NII])=0.156x−4(N+)Lpcn(e)f1(n(e)), (8)
coefficientbyelectronsisgivenby
where x is the fractionalabundanceof N+ is in unitsof 10−4
−4
Cul =8.63×10−6Gg(Tu,0.l5), (6) aasndaLfupnccistioinnpocf.tThheuosbwseervceadnisnotlevnesfitoyrothfetheele2c0tr5onµmdenlisniety,,gniv(ee)n,
u the size of the emissionregionandthe fractionalabundanceof
where gu is the degeneracy of the upper level and G(u,l) N+.Atlowdensities,wheren(e)<ncr(e),itcanbeshownthat
is the temperature dependent effective collision strength (see
α
I(NII)
Lennon&Burke1994;Hudson&Bell2004).Wecalculatedthe n(e)∝ cm−3 (9)
electron collisional rate coefficients using the Hudson&Bell "Lpcx−4(N+)#
(2004) values for the effective collision strengths. The critical
density for the 1-0 transition, where collisions out of the 3P1 where0.5<α<0.7,forn(e)<ncr(e).Inthelimitn(e)<<ncr(e),
statebalancetheradiativetransition,A10,isncr(e)∼44cm−3at such as found in the Warm Ionized Medium in the disk, f1 ∼
8000K.InFigure8weplottheintensityofthe205µmlineasa n(e)/ncr(e)andα=0.5.Thus,ingeneral,n(e)isonlymoderately
functionoftheelectrondensityusingEquation4forarangeof sensitivetouncertaintiesinLandx(N+).
ionizednitrogencolumndensities, N(N+)= 1016 to1018 cm−2.
Theintensity scaleslinearlywith n(e)upto ∼10- 15cm−3 be- 3.2.The-207kms−1component
yondwhichitlevelsoffasn(e)approachesthecriticaldensity.
The [Nii] emission comes from ionized gas and so can be As seen in Figures 4 and 5 there is [Cii] emission along the
used to derivethe electrondensitydirectlyif we knowthe size line of sight to the CO molecular cloud with V ∼ -207 km
lsr
7
W.D.Langeretal.:IonizedgasattheedgeoftheCentralMolecularZone
s−1. Thepeakin[Cii] intensityforthiscomponentisatb > 0◦ = I([Cii]) - I ([Cii]) arises primarily from the CO–dark H
H+ 2
at the edge of the lowest contour of CO and is thus consistent gas (also called the CO–dark gas) and is given in Table 3.
withlimbbrightened[Cii]emission.ThereforetheSOFIA[Cii] Negative or very small values of I ([Cii]) indicate that al-
and [Nii] spectra along b=0◦ likely arise from a layer of gas most all the [Cii] emission arises froHm2 the dense ionized gas.
whose size is related to the thicknessof the [Cii] emission be- Unfortunatelywe do notknow the kinetic temperatureand hy-
yondthelowestCOcontour.Thethicknessofthe[Cii]emission drogendensityoftheCO–darkN(H )tocalculateaccuratelyits
2
is about 6′ which correspondsto ∼15 pc at the distance to the measured C+ column density. For illustrative purposes we cal-
CMZ. To solve for the electron abundancein the [Nii] regions culate the column density of C+ in the H layer, N (C+), and
we need, in addition to L and I([Nii]), x(N+) appropriate to the corresponding column density of CO2–dark H H,2N (H )=
pc 2 C+ 2
the CMZ. To estimate the fraction of N+ in the CMZ we use N (C+)/x(C+) (see Langeretal. 2014), assuming 100K and
the nitrogen abundance Galactic gradient, 0.07 dex kpc−1, for 30H02 cm−3 in Equation (1), along with the appropriate n (H )
cr 2
R > 3 kpc, derived from Rollestonetal. (2000) which leads fromWiesenfeld&Goldsmith(2014).Theseresultsarelistedin
G
to x(N+) ∝ exp(−R /6.2)(see Pinedaetal. 2013; Langeretal. Table3foralltheLOSwithpositiveI ([Cii]).Higherdensities
G H2
2014).Howeverbecausewedonothaveinformationabouthow willresultinlowercolumndensitiesandviceversa.
it scales to smaller R we extrapolate this relationship to the
G
halfwaypoint,1.5kpc.Usingthelocalnitrogenabundancefrac-
Table3.C+ columndensitiesandintensities
tionx(N+)=5.51×10−5fromJensenetal.(2007)yieldsx(N+)=
1n.(6e×)a1r0e−n4oattvReGry=s1e.5nskitpivce. Atosmdoisdceusstsuedncaebrtoavien,titehseinsoxl(uNti+o)n.sTfhoer LOSa N(C+)b IH+([Cii])c IH2([Cii]) NH2(C+) NC+(H2)f
-207d
solutionsforn(e)andN(N+)aregiveninTable2.
358.45 4.9e17 64.1 -0.7 - -
The electron density for the -207km s−1 componentvaries 358.55 3.5e17 38.5 7.0 4.2e17 8.2e20
from∼9to21cm−3 forthefourLOSwith[Nii]detectionsand 358.60 2.2e17 18.3 29.3 1.8e18 3.5e21
has an average value ∼14 cm−3. Thus the gas traced by [Nii] 358.65 - - - - -
is warm dense ionized gas, significantly denser than that de- 358.70 3.0e17 31.3 14.1 8.6e17 1.7e21
rived for the typical WIM in the disk, which has n(e) of order 358.75 - - - - -
afew×10−2cm−3(Haffneretal.2009;Velusamyetal.2012).In Average 3.4e17 38.1 16.8e 1.0e18 2.0e21
the next section we will discuss what might generate and sus- -174d
tainsuchahighdensityionizedgastowardsthecloudassociated 358.45 2.4e17 21.1 0.1 3.3e15 6.4e18
withSgrE. 358.55 2.5e17 22.6 -10.4 - -
358.60 1.5e17 9.9 33.1 2.0e18 3.9e21
358.65 - - - - -
358.70 1.8e17 13.4 1.3 8.2e16 1.6e20
Table2.Electrondensityandnitrogencolumndensity
358.75 - - - - -
Average 2.0e17 16.9 11.5e 7.0e17 1.4e21
V a= -207 -207 -207 -174 -174 -174
lsr
LOS n(e)b N(N+)c N(H+) n(e)b N(N+) N(H+) a) All LOS are at b =0◦. b) Column density in cm−2 and rounded to
358.45d 20.7 1.5e17 9.3e20 9.9 7.5e16 4.6e20 onedecimalplace.c)Intensityof[Cii]inthe[Nii]emissionregionin
Kkms−1.d)V inkms−1.e)Negativeintensitiesarenotincluded.f)
358.55 14.6 1.1e17 6.8e20 10.4 7.9e16 4.8e20 lsr
AssumesT =100Kandn(H )=300cm−3.
358.60 9.1 6.9e16 4.2e20 6.3 4.8e16 2.9e20 k 2
358.65 - - - - - -
358.70 12.8 9.7e16 5.9e20 7.7 5.9e16 3.6e20
358.75 - - - - - -
Average 14.3 1.1e17 6.6e20 8.6 6.5e16 4.0e20 3.3.The-174kms−1component
a) In km s−1. b) Densities in cm−3. c) Column densities in cm−2 and
The -174 km s−1 [Nii] and [Cii] emissions are weaker, and in
roundedtoonedecimalplace.d)AllLOSarealongb=0◦.
some positions much weaker, than those of the -207 km s−1
component, and are associated with CO only at l =358◦.75 in
Assuming that the nitrogen is nearly fully ionized, the Figure6.TheCOmapsinOkaetal.(1998)showaCOmolec-
column density of ionized carbon in this region is given by ularcloudwithanedgeatthislongitudeandextendinginwards
N(C+)=x(C+)/x(N+)N(N+).Forx(C+)/x(N+)weadoptthevalue to larger l. The -174 km s−1 emission is also associated with
of 3.2 (Jensenetal. 2007; Sofiaetal. 2004) and the values for Hi emission(Figure7) thatisweaker thanthatof the -210km
N(C+) are givenin Table 3 along with N(H+). The C+ column s−1 cloud.We donotdetectthis weaker [Cii] componentatall
densitiesareoforderafew×1017cm−2,withanaverage∼4×1017 the positions in the HIFI OTF maps (see discussion in Section
cm−2. 2.2) and so have no estimate of the size of the [Cii] and [Nii]
The [Cii] intensity associated with the [Nii] region, emission region. For comparison we have calculated densities
I ([Cii]),iscalculatedusingtheinverseofEquation(2),from andthecolumndensitiesofthiscomponentassumingthesame
H+
thecolumndensityN(C+),n(e)derivedfrom[Nii],andassum- size,15pc,forthe[Nii]emissionregionasthatforthe-207km
ing T = 8000 K. The exact kinetic temperature is not critical s−1 cloud.TheseresultsaregiveninTables2 and 3. Thesolu-
k
asthecolumndensityisonlyweaklydependentontemperature tionsforthe-174kms−1componentyieldslightlyloweraverage
throughthecollisionalde-excitationratecoefficient,resultingin electrondensitiesandcolumndensitiesthanforthe-207kms−1
n (e) ∼ 50T 0.37 cm−3, where T is the kinetic temperature in component.
cr 4 4
unitsof104K.TheresultsaregiveninTable3.
The [Cii] emission from the neutralPDR, I ([Cii]), is the
difference between the observed total [Cii] inteHn2sity, I([Cii]), 4. Discussion
giveninTable1,andtheintensitycalculatedfromthe[Nii]re- The analysis of [Nii] emission at the edge of the CMZ reveals
gion, I ([Cii]) (Langeretal. 2014). This quantity, I ([Cii]) a hot dense plasma with an electron abundance ranging from
H+ H2
8
W.D.Langeretal.:IonizedgasattheedgeoftheCentralMolecularZone
∼ 6 to 21 cm−3. The densities we derive are much larger than rayionizationrateofnitrogen(includingthesecondaryphotons
those characteristic of the disk’s WIM, but are consistent with generated by cosmic rays interacting with hydrogen) is about
thosederivedforverybrightnebulawithnumerousluminousO- a factor of two larger than that of hydrogen (McElroyetal.
type stars. For example,Oberstetal. (2011) studied the Carina 2013), ∼few×10−15 s−1, but is much lower than the values
Nebula,whichhasaverylargeUVflux,usingemissionfromthe needed to offset the recombination by electrons. Although re-
[Nii]205µmand122µmlinesalongwithanexcitationmodel6, gionsofintensecosmicrayionizationmayexistwithionization
andfindvaluesofn(e)rangingfromafewtoover100cm−3.In rates∼10−14s−1 (Bayetetal.2011;Meijerinketal.2011),rates
the discussion below we will use the results from our analysis ≥10−12 s−1 would be required for cosmic rays to explain high
of [Nii] and [Cii] emission to characterize some of the possi- fractionalnitrogenionizationatdensitiesn(e)≥1cm−3.Indeed
ble energy sources that can maintain such a hot dense ionized Meijerinketal. (2011) consider rates almost that large in their
plasma. modelsof photondominatedregions(PDRs) but onlyconsider
high atomic hydrogendensities, n(H) ≥103 cm−3 such that the
fractionalionizationissmall.Theirmodelscannotbeappliedto
4.1.IonizationofNitrogen theionizedgasprobedbyour[Nii]observations7.
Nitrogen has an ionization potential of 14.534 eV, higher than
thatofhydrogenat13.598eV,andintheISMcanbeionizedby
4.1.2. X-rayionization
cosmic rays, X-rays, Extreme Ultraviolet (EUV) photons with
wavelengths shortward of 853.06 Å, electron collisional ion- The CMZ has several different sources of X-rays, includ-
ization, or via charge exchange with protons (H+) at energies ing an accreting black hole at the center, a bright X-ray
≥0.936 eV, the energy difference between the ionization of H source 1E1740.7-2942 located l ∼ 359◦.1 and b ∼ -0◦.1 (e.g.
andN.InanearlyfullyionizedregionN+ isdestroyedprimar- Heindletal. 1993; Wangetal. 2002), extended diffuse X-ray
ilyviaelectronradiativeanddielectronicrecombination,andin emission(Koyamaetal.2007),aswellasover9000X-raypoint
partially ionized regions also via exothermic charge exchange sources(Munoetal.2009)someofwhichlieattheedgeofthe
with H atoms. We can estimate the required ionization rate to CMZbetweenl=358◦.9and359◦.0aroundb∼0◦andcouldpro-
sustain a highly ionized N+ gas by calculating the electron re- vide the >10−15 s−1 ionizationrate of hydrogenrequiredto ex-
combinationrateforthesolutionsforn(e)giveninTable2.The plaintheH3+observationsofGotoetal.(2014).(Unfortunately,
recombinationratecoefficientsare takenfromthe UMISTdata these Chandra observations of the CMZ survey the Galactic
base (McElroyetal. 2013) andBadnell(2006) forradiativere- Center do not extend into the Sgr E region.) However, unlike
combinationandBadnelletal.(2003)fordielectronicrecombi- ionization by cosmic rays, the cross section for ionization of
nation. (In molecular hydrogen clouds N+ reacts rapidly with heavyatomsby X-rayscanbe muchlargerthanhydrogen,pri-
H andinitiatesachainofreactionsproducingnitrogenbearing marilyduetoKshellionizationatX-rayenergies>0.4keV,and
2
molecules,but thischannelis notrelevantin the WIM, WNM, thus X-rays could play an important role in ionizing nitrogen.
orHiiregions.)ChargeexchangewithHhasareactionrateco- Forexample,at1keVtheionizationcrosssectionofnitrogenis
efficientat8000K∼5.5×10−14cm3s−1(seeTable2ofLinetal. ∼7×10−20cm−2 (Verneretal.1993)overthreeordersofmagni-
(2005))andwhencomparedwithrecombinationwith electrons tudelargerthanthatforhydrogen(Gotoetal.2014;Wilmsetal.
∼8×10−13 cm3 s−1, we see that electron recombination domi- 2000).Therefore,iftheX-rayionizationrateneededtoexplain
nates for n(e)/n(H) > 0.2. To get an estimate of the degree of the productionof H3+ is ∼few×10−15 s−1, thenthe correspond-
ionizationneededtosustainahighlyionizedgasweneglectthe ingionizationratefornitrogenwillbeoforderafew×10−12s−1,
charge exchange of N+ with H in the regime where the gas is sufficient to explain the observed large fractional ionization of
highly ionized and consider only recombination by electrons. nitrogenatn(e)∼5to25cm−3.Thepenetrationdepthof1keV
For large electron densities in the range 5 to 25 cm−3 such as X-rays is a column density N(H+2H2) ∼ 1022 cm−2, which is
derived here, the recombination rate ranges from 4×10−12 to sufficient to penetrate the diffuse gas and envelopes of clouds
2×10−11 s−1. Sustaining a high ionization fraction (≥0.5) re- intheCMZ(Gotoetal.2014).WhetherX-raysaretheprimary
quiresanionizationratecomparabletoorgreaterthantheseval- ionization source or not depends on the details of the distribu-
ues . For typical WIM conditionsin the disk, n(e)∼10−2 cm−3, tion and luminosity of the X-ray sources, and we do not have
the ionization rate required to keep nitrogen highly ionized is sufficientinformationtomakethisdetermination.
twotothreeordersofmagnitudelower.Inthefollowingwedis-
cussthelikelysourcesofionizationattheedgeoftheCMZ.We
4.1.3. Photoionization
excludeelectroncollisionalionizationbecause,due to the high
ionizationpotentialofN,itrequiresaveryhotelectrongaswith Thephotoionizationcrosssectionσforionizingnitrogenranges
kinetictemperaturesoforder105K,aconditionmetonlyinthe fromabout10−17to1.5×10−17cm2fromthethresholdat∼853Å
HIMportionoftheCMZ. to500Å(Samson&Angel1990).Thetotalphotoionizationrate
isgivenby σ(λ)Φ(λ)dλ,whereΦ(=4πJ)istheincident
λ<853Å
4.1.1. Cosmicrayionization flux integraRted in units of photons cm−2 s−1. It has been esti-
mated that to maintain the ionization of hydrogenin the WIM
Gotoetal. (2014) have detected H3+ in the CMZ and use its one needs an ionizing flux of Lyman continuum photons>105
abundance to derive a lower limit on the hydrogen ionization photons cm−2 s−1 (Haffneretal. 2009). Reynoldsetal. (1995)
rate, ζH > 1×10−15 s−1, which they suggest could come from estimate an incident Lyman continuum flux 4πJ ∼ 2×106 to 9
cosmic rays or X-rays. If the ionization is due to cosmic rays,
it is about ten to thirty times larger than the cosmic ray ion- 7 NotethatMeijerinketal.(2011)calculatetheN+ fractionalabun-
ization rate in the disk (Indriolo&McCall 2012). The cosmic dance, x(N+)=n(N+)/n(H+2H ), as ≤10−7, which is a low ionization
2
scenario. They also calculate the line intensities of the [Nii] 205 µm
6 With two transitions the electron density can be derived without and122µmlines,buttheirpredictionsfallfarbelowthemeasuredin-
havingtoassumeacharacteristicsizefortheemissionregion. tensitiesgiveninTable1.
9
W.D.Langeretal.:IonizedgasattheedgeoftheCentralMolecularZone
×106photonscm−2s−1,sufficienttoexplaintheionizationinthe approximation,n(e)≃n(H+)andthefractionalabundanceofni-
WIM.HowevernitrogenisionizedbyEUVphotonsbeyondthe trogenis
Lymanlimit,forwhichthefluxmustbedeterminedfrommodels
incorporatingadistributionofOandBstarsandattenuationby f(N+)= kx(T) , (13)
thesurroundingISM,includingconsiderationsoftheclumpiness k (T)+k (T)
x r
of the ISM (cf. Haffneretal. 2009). In general the flux drops
sharplybeyondtheLymanlimit,ascanbeseeninthemodelofa which is independentof n(e) and n(H+). However,the fraction
spectrumofastarclusterinFigure17ofKaufmanetal.(2006), f(N+)isverysensitivetothetemperatureoftheplasmabecause
wherethespectralflux(inergs/s/Hz/sr/cm2)dropsbyafactorof oftheendothermicnatureofthereaction.Thechargeexchange
10attheLymanedgeandbyanotherfactorof10by∼400Å.We process for N++ H → N + H+ has been measured (Stebbings
canestimatethefluxneededtosustainahighnitrogenionization et al. 1960), but only above ∼30 eV, and the low energy cross
fractionbybalancingphotoionizationwithelectronrecombina- sectionsmustbe calculatedtheoretically.Linetal. (2005) have
tion, made the most recent calculations of the charge transfer reac-
tion with ground state nitrogen, H+ + N ↔ H + N+, and use
φ(λ)σ(λ)dλ thesetocalculatetheratecoefficients,k forforwardandreverse
n(N+)/n(N0)= Rλ<853kÅ(e)n(e) (10) reactions.Theresults forthe chargeexxchangereactionrate co-
r efficientsaregivenintheirTable2andcanbeusedtocalculate
wherek (e)istheelectronrecombinationratecoefficient,n(N0) thefractionalnitrogenionizationasafunctionofkinetictemper-
r
ature.
is the neutral nitrogen number density, and φ the photon flux
However, one must be cautious here, because the theo-
asafunctionofwavelength.Thisequationcanbesimplifiedby
retical rate coefficients derived by Linetal. (2005) for H+
assuming that the ionization cross section is roughly constant
over the energy range of interest (see above), so n(N+)/n(N0) + N → N+ + H are lower than those of Steigmanetal.
∼ (Φσ)/(k n(e)), where Φ is the total photon flux in cm2 s−1. (1971); Butler&Dalgarno (1979); Kingdon&Ferland (1996)
r (see Figure 7 in Linetal. (2005)) below 2×104 K, which is
Thereforethefractionofionizednitrogenis
the relevant temperature regime for the WIM and Hii regions.
Φσ WesolvedEquation13fortheratesderivedbyLinetal.(2005)
f(N+)= (11)
andKingdon&Ferland(1996)asafunctionoftemperatureand
k n(e)+Φσ
r
these are plotted in Figure 9. Using the reaction rates from
For example, to sustain f(N+) = 0.5 in dense ionized gas with Kingdon&Ferland(1996)leadstoahighfractionalabundance
T=8000 K at n(e) = 1, 10, and 20 cm−3 requires an EUV flux attemperaturesconsistentwiththosemeasuredintheWIMand
∼6.5×104,6.5×105,and1.3×106photonscm−2s−1,respectively. Hiiregions,fortemperatures4000to10000K.Howevertherate
Foranearlyfullyionizednitrogenregimetherequiredintensity coefficientsfromLinetal. (2005)leadto a muchlowerioniza-
of the EUV field increases sharply. For example, to maintain tionfractionofnitrogenatthesetemperatures,andrequirestem-
f(N+) = 0.9 requires an order of magnitude greater flux than peratures&15,000KtoionizenitrogenefficientlyviaH+charge
for f(N+) = 0.5, which can be found near youngmassive stel- exchange. Thus, depending on which calculation of kx is cor-
lar clusters (Kaufmanetal. 2006). Rodr´ıguez-Ferna´ndezetal. rect determines whether or not charge exchange with protons
(2004) estimate that the PDRs in the CMZ are illuminated by cansustainahighionizationfractionofN+ inthe[Nii]region.
afar-UVradiationfieldaboutafactorof103 largerthanthelo-
cal ISM, and if this increase extends to the EUV it would be
4.2.Conclusions
sufficient to explain the ionization of nitrogen. One constraint
on the flux distribution near the -207 km s−1 Sgr E feature Fromouranalysisof[Nii]emissionattheedgeoftheCMZnear
comes from observations of carbon and helium recombination Sgr E we find evidencefor dense, hot, highlyionized gas. The
lines.Wengeretal.(2013)detectedcarbonrecombinationlines association with [Cii] andCO indicatesthatthis dense ionized
in two of the eight Sgr E Hii sources detected in hydrogenre- gassurroundsthe PDRs associated with two molecularclouds,
combinationlines,butdidnotdetectanyheliumrecombination onewith V ∼ -207kms−1 and the other∼ -174km s−1. The
lsr
lines. This resultindicatesthatthe flux atwavelengthsshortof electrondensities,n(e)∼5to21cm−3,areconsiderablyhigher
504.26Å(energies≥24.59eV)issmall. thanthoseintheWIMofthedisk,butconsistentwiththoseex-
pected for bright diffuse Hii regions, such as the Carina neb-
ula(Oberstetal.2011).Theelectrondensitiesareconsiderably
4.1.4. Protonchargeexchangeionization lowerthanthosesuggestedbyLazio&Cordes(1998),n(e)&103
The last important ionization mechanism is charge exchange cm−3,ifthephotoionizedlayersofcloudsareresponsibleforthe
withenergeticprotons.Inahighlyionizedhydrogengas,n(H+) scatteringofradiowavesintheCMZ.However,theyareconsis-
>50n(H),wecanneglecttheneutralizationofN+bychargeex- tentwiththeirothersuggestionthatthescatteringcouldbedueto
changewithHatoms,whichyieldstheratioofionizedtoneutral theinterfacewithahotionizedmediumwithn(e)∼5-50cm−3
nitrogen, and Tk(e)∼ 105 – 106 K. We have noinformationaboutthe ki-
netictemperaturefromourobservations,andhaveassumedtypi-
k (T)n(H+) calWIMkinetictemperatures∼8000K.Furthermore,ourresults
f(N+)=n(N+)/n(N0)= x , (12)
k n(e) apply to the ionized regionsaroundthe molecular cloud and if
r
theseareindeedthesourceofthescatteringofradiowaves,then
where the charge exchange reaction rate coefficient, k (T) is a they occupy a small filling factor in the CMZ and may not be
x
Maxwellianaverageofthecrosssectionovertheelectronveloc- representativeoftheelectrondensityinthebulkofthevolume.
itydistribution(Linetal.2005).Thevalueofk (T)isanexpo- Therearethreewaystosustainsuchadensehighlyionized
x
nential function of temperature because only protons with en- hydrogengas with large nitrogen ionization fraction.One is to
ergies&0.94eV can undergochargeexchange.To a verygood haveastrongEUVradiationfieldwithafluxoforder106to107
10