Dust-to-gas ratio, $X_{\rm CO}$ factor and CO-dark gas in the Galactic anticentre: an observational study

0.69 MB·English

by  B. -Q. Chen

Mon.Not.R.Astron.Soc.000,1–??(2014) Printed16January2015 (MNLATEXstylefilev2.2)
Dust-to-gas ratio, X factor and CO-dark gas in the Galactic
CO
anticentre: an observational study
5
1 B.-Q. Chen,1‹: X.-W. Liu,1,2‹ H.-B. Yuan,2: Y. Huang1 and M.-S. Xiang1
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2 1DepartmentofAstronomy,PekingUniversity,Beijing100871,P.R.China
2KavliInstituteforAstronomyandAstrophysics,PekingUniversity,Beijing100871,P.R.China
n
a
J

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Accepted???.Received???;inoriginalform???
1

]
A ABSTRACT
We investigate the correlationbetween extinction and H i and CO emission at intermediate
G
and high Galactic latitudes (|b| ą 10˝) within the footprintof the Xuyi SchmidtTelescope
.
h Photometric Survey of the Galactic anticentre (XSTPS-GAC) on small and large scales. In
p Paper I (Chen et al. 2014), we present a three-dimensional dust extinction map within the
- footprint of XSTPS-GAC, covering a sky area of over 6,000deg2 at a spatial angular res-
o
olution of 6arcmin. In the current work, the map is combined with data from gas tracers,
r
t including H i data from the Galactic Arecibo L-band Feed Array H i survey and CO data
s
a from the Planck mission, to constrain the values of dust-to-gas ratio DGR “ AV{NpHq
[ and CO-to-H2 conversion factor XCO “ NpH2q{WCO for the entire GAC footprint exclud-

ing the Galactic plane, as well as for selected star-forming regions (such as the Orion,
1
Taurus and Perseus clouds) and a region of diffuse gas in the northern Galactic hemi-
v sphere.Forthe whole GACfootprint,we find DGR “ p4.15˘0.01qˆ10´22magcm2 and
6
X “ p1.72˘0.03qˆ1020cm´2pKkms´1q´1.We havealsoinvestigatedthedistribution
0 CO
6 of“CO-dark”gas(DG)withinthe footprintofGACandfounda linearcorrelationbetween
3 the DG column density and the V-band extinction: NpDGq » 2.2ˆ1021pAV ´ AcVqcm´2.
0 ThemassfractionofDGisfoundtobe fDG „ 0.55towardtheGalacticanticentre,whichis
. respectivelyabout23and124percentoftheatomicandCO-tracedmoleculargasinthesame
1
region. This result is consistent with the theoretical work of Papadopoulos et al. but much
0
5 largerthanthatexpectedintheH2cloudmodelsbyWolfireetal.
1
Keywords: ISM:dust,extinction–ISM:molecules–ISM:clouds
:
v
i
X
r
a 1 INTRODUCTION Heteronuclear diatomic molecule CO is the most abundant
molecule in the ISM after H . The 12CO J “ 1 Ñ 0 tran-
Theinterstellarmedium(ISM)makesupbetween10to15percent 2
sition has a ∆E {k about 5.5K and can be easily excited even
ofbaryonicmassoftheMilkyWay.About99percentofthemate- 10
in cold molecular clouds and observed from the ground. As a
rialisingasformandtherestisinsoliddustgrains.Observationof
consequence, CO has become the primary tracer of the inter-
theinterstellardustandgasisaprimarytoolstotracethestructure
stellar molecular gas. The molecular abundance ratio CO{H is
anddistributionofmoleculeclouds,wherethestarformationtakes about 10´4. The CO (J “1Ñ0) survey of Dameetal. (20201)
place.ThedominantmoleculargasintheISMisH ,ahomonuclear
2 is the most complete survey which covers the Milky Way of
diatomicmoleculewithnopermanentdipolemomentandallowed Galactic latitudes |b| ď 30˝. There are a large number of other
dipolerotationaltransitions.ThemosteasilyexcitedH transition
2 CO surveys covering small sky regions around the individual
is the quadrupole Sp0q: J Ñ J “ 2 Ñ 0, which has an exci-
u l molecular clouds, such as the Orion and the Monoceros clouds
tationenergy∆E {k „ 510K,morethananorder of magnitude
20 (Hartmannetal. 1998; Magnanietal. 2000; Wilsonetal. 2005).
higherthanthetypicalkinetictemperatureT „ 10–50Kofthe
kin PlanckCollaborationetal. (2013) have extracted the full-sky CO
ISM.Asaconsequence,H ishardtoexciteandobserve,andone
2 emissioninthreelines,theJ “1Ñ0,J “2Ñ1andJ “3Ñ2lines
hastotracethemoleculargasanddeterminetheH columnden-
2 at 115, 230, and 345GHz from the Planck data using a compo-
sity,NpH q,byotherindirectmethods.Twoofthemostcommonly
2 nent separation method. The 12CO(J “1Ñ0) emission at wave-
usedtracersareCOgasemissionanddustextinction.
length λ “2.6mm is then utilized to determine the total col-
umn density of molecular gas assuming a CO-to-H conversion
2
factor, the so-called X factor, which relates the surface bright-
‹ E-mail:[email protected](BQC);[email protected](XWL). ness of CO emission and the column density of molecular gas,
: LAMOSTFellow.
2 B.Q. Chen et al.
X “ NpH q{W (cf. Bolattoetal. 2013 for a review), where There isanumber of wellknown largestar-formingregions
CO 2 CO
W is the velocity-integrated brightness temperature of 12CO within the footprint of XSTPS-GAC, such as the Perseus, Tau-
CO
J “ 1 Ñ 0. The X factor works best for cloud ensemble rus and Orion clouds. They form parts of the Gould Belt. Those
CO
averages. The minimum averaging scale would be that of a sin- cloudsareinthesouthernGalacticplane,locatedatrelativelysmall
gle Giant Molecular Cloud (GMC), „10–100pc (Dickmanetal. distances (d ă 800pc; Chenetal. 2014). We study the correla-
1986; Young&Scoville 1991; Bryant&Scoville 1996; Regan tionbetweentheinterstellardustandgasfortheseregions.Within
2000;Papadopoulosetal.2002).ThemostquotedvalueofXfac- these regions there exist diffuse, non self-gravitating clouds as
tor for the Milky Way is X „ 2 ˆ 1020cm´2pKkms´1q´1 well as dense, massive, self-gravitating, star-forming clouds like
CO
(Frerkingetal.1982;Dameetal.2001;Lombardietal.2006). the Orion. Several studies on the dust and gas correlation have
The visual interstellar extinction A has been found to have beencarriedoutforthoseindividualclouds.Pinedaetal.(2008a),
V
a linear relationship with the total column density of hydrogen Goldsmithetal. (2008) and Pinedaetal. (2010) study the Tau-
nuclei NpHq.The relationship isacornerstone of the modern as- rusmolecularcloud,whilePinedaetal.(2008b),Leeetal.(2012)
trophysics. The classic value of the constant of proportionality, and Leeetal. (2014) focus on the Perseus cloud and Digeletal.
i.e. the dust-to-gas ratio, DGR “ A {NpHq, derived from the (1999), Ackermannetal. (2012) and Rippleetal. (2013) on the
V
measurements of the ultraviolet (UV) absorption of the H i Lyα Orioncloud.
lineandtheH Lymanbandtowardasmallnumberofearly-type One can measure the amount and distribution of the atomic
2
starswiththeCopernicussatellite(Savageetal.1977;Bohlinetal. and molecular gas as well as the dust grains by combining var-
1978), is DGR “ 5.34 ˆ 10´22magcm2. More recently, Liszt ious tracers. In this paper, we combine the dust extinction to-
(2014a) andLiszt(2014b) trace NpHIqand EpB´Vqacrossthe gether withgasdistributiontraced byH idatafromtheGALFA-
skyof Galacticlatitudes|b| =20–60˝.Theyconsider dataonly HI (Peeketal. 2011) and CO data from the Planck mission
of relatively low extinction, 0.015 ă EpB ´ Vq ă 0.075mag, (PlanckCollaborationetal.2013),toconstrainthevaluesofDGR
for which the hydrogen should mostly be in the form of neutral and X at a high angular resolution of 6arcmin. The exis-
CO
atoms and thus the corrections of NpHIq for saturation and H2 tence of molecular gas of low CO abundance has been noted
formation are unlikely important. From the data, they obtain a previously in theoretical models (vanDishoeck&Black 1988;
DGRvalue(3.73ˆ10´22magcm2)thatissmallercomparedtothe Papadopoulosetal.2002),inobservationsofdiffusegasandhigh-
reference value. The column density of atomic hydrogen, NpHIq latitudeclouds(Lada&Blitz1988)andinobservationsofirregular
can bedetermined by measuring theH i 21cmemission. Thus if galaxies(Maddenetal.1997).ThevaluesofDGRandX deter-
CO
one can accurately determine the extinction and value of DGR, mined inthecurrent work allow usto further tracethis so-called
then the H column density can be derived using the relation, “CO-darkgas”(DG;Reachetal.1994;Meyerdierks&Heithausen
2
NpH2q “ AV{DGR´NpHIq. There are several successful large 1996;Grenieretal.2005;Abdoetal.2010)atbothsmallandlarge
H i 21cm emission surveys of the Galaxy, including the Leiden- scales.Wearenotthefirstoneonthistopicofcourse.Forexample,
Argentine-Bonn (LAB) full-skysurvey (Kalberlaetal. 2005),the PlanckCollaborationetal. (2011) has obtained an all-skymap of
internationalGalacticPlaneSurvey(IGPS,Tayloretal.2003),the dust opticaldepth andcompared itwiththeobserved distribution
GalacticAllSkySurvey(GASS,McClure-Griffithsetal.2005)in ofgascolumndensity.Theresultsareusedtoconstrainthecorrela-
the southern hemisphere, and the Galactic Arecibo L-band Feed tionbetweenthedustandgas,pτ {N qref,aswellasthevaluesof
D H
ArrayHisurvey(GALFA-HI,Peeketal.2011)targetingboththe X ,andtoconstructadistributionmapofDGathighandinterme-
CO
Galacticplaneandhighlatituderegions. diateGalacticlatitudes(|b| ą 10˝).Paradisetal.(2012)carryout
Dust grainscan betraced by extinction inthevisibleand in asimilarstudyusingtheextinctiondatadeduced fromthecolour
thenear-infrared(IR).Chenetal.(2014)estimatevaluesofextinc- excess of the 2MASS photometry instead of those derived from
tionanddistancetoover13 millionstarscatalogued bytheXuyi thefar-IRopticaldepth.BoththeworkofParadisetal.(2012)and
1.04/1.20m Schmidt Telescope photometric survey of the Galac- PlanckCollaborationetal.(2011)arelimitedbytheangularreso-
tic anticentre (XSTPS-GAC; Zhangetal. 2013, 2014; Liuetal. lutionoftheLAB21cmHiemissionsurvey(Kalberlaetal.2005),
2014). They derive the extinction by fitting the spectral energy whichis36arcmin.Inthecurrentworkwehaveadopteddatafrom
distribution(SED)forasampleof30millionstarsthathavesup- the GALFA-HI survey. As a consequence, the angular resolution
plementary IR photometry from the Two Micron All Sky Survey hasbeenimprovedsignificantly.
(2MASS; Skrutskieetal. 1997) and the Wide-FieldInfrared Sur- The paper is structured as the following. In Section2 we
veyExplorer(WISE,Wrightetal.2010).Togetherwiththephoto- present the 2D extinction map integrated to a distance of 4kpc.
metricdistancesdeducedfromthedereddenedXSTPS-GACopti- Section3describesthedataofgastracersusedinthecurrentanal-
calphotometry,Chenetal.(2014)constructa3Dr-bandextinction ysis. The correlations between extinction and gas content are in-
map within the GAC footprint at a spatial resolution, depending vestigatedinSection4withthemainresultsdiscussedinSection5.
onthelatitude, that variesbetween 3–9arcmin. The3D mapof WesummarizeinSection6.
Chenetal.(2014)hasalsobeenusedtoconstructa2Dextinction
map,integratedtoadistanceof4kpc,approximatelythemaximum
distance that dwarf starsinthesample can tracewithsufficiently
2 THEEXTINCTIONMAP
highphotometricaccuracy.Thedustextinctionwithinthisdistance
iswellconstrained.Themaphasanangularresolutionof6arcmin The photometric extinction and distance catalogue of Chenetal.
andisalsopubliclyavailable1.Inthecurrentwork,this2Dextinc- (2014)isfirstusedtoconstructa2Dextinctionmapintegratedtoa
tionmapisusedtoinvestigatethedust-to-gasratioandtheXfactor distanceof4kpcforthefootprintofXSTPS-GAC.Thecatalogue
withinthefootprintofXSTPS-GAC. contains more than 13 million stars with estimates of r-band ex-
tinctionandphotometricdistance.Toensuregoodphotometricac-
curacy,onlyobjectsflaggedwithaqualityflag‘A’,i.e.withaverage
1 http://162.105.156.249/site/Photometric-Extinctions-and-Distances. photometricerrorslessthan0.05magforalltheeightbandsused
Dust-to-gasrationof GAC 3
toderivetheextinctionanddistance(g, r,andifromtheXSTPS-
GAC, J, H and K from the 2MASS, and W1 and W2 from the
S
WISE),areincluded(SampleAofChenetal.2014).Possiblecan-
didates of giants, flagged as ‘G’, are excluded because their dis-
tancesmayhavebeengrosslyunderestimated.Thisleavesjustover
7 million stars and they are divided into sub-fields of size 6 ˆ
6arcmin2. For each sub-field, a sliding window of depth 450pc,
steppingbystepsof150pc,isusedtoobtainthemedianvalueof
extinction for each distance bin. The extinction as a function of
distance,Aipdq “ fpdq,whereiistheindexofsub-fields,isthen
r i
obtained after applying aboxcar smoothing of width450pc. The
extinctionversusdistancerelationsofallsub-fields(sightlines)are
usedtoconstructa3Dextinctionmap,whichisthenintegratedout
toadistanceofd=4kpctoyielda2Dextinctionmaptobeusedin
thecurrentanalysis.Mostofthesamplestarsarewithinadistance
of4kpcandtheextinctionouttothisdistanceiswellconstrained.
The4kpcintegratedmapcanbetreatedasalowerlimitofextinc-
tionmapalongthesightlines.
Fig. 1 presents the 4kpc dust extinction map thus derived.
It has a spatial resolution of 6arcmin. The resolution is chosen
to ensure that most of the subfields have a sufficient number of
stars (ą 10) to derive a robust relation of extinction as a func-
tion of distance. For a small fraction of sub-fields (less than 1.5
Figure1.A2Dextinction map,integrated toadistance of4kpcforthe
percent) ofhighGalacticlatitudesorsufferingfromhighextinc-
footprintofXSTPS-GAC.Themaphasanangularresolutionof6arcmin.
tion,therearenotenoughnumbersofstarsofhighphotometryac-
curacies available. For those fields, we have adopted some stars
of lower accuracy from the catalogue (SampleC of Chenetal. thanzero.Theassumptioncouldintroducesomebiases,inpartic-
2014). Those sub-fields are flagged by ‘C’. The V-band extinc- ularforsightlinesofverylow,closetozero,extinction.Inprinci-
tionAV isconvertedfromArusingtheextinctionlawofYuanetal. ple,both thebest SEDfitalgorithmof Chenetal.(2014) aswell
(2013),AV “ 1.172Ar,assumingatotal-to-selectiveextinctionra- astheBayesianapproachofGreenetal.(2014)couldleadtosys-
tio RV “ 3.1. The uncertainties of the map are estimated from tematicallyover-estimatedvaluesof extinctionforregionsoflow
thestarslocated withinthedistancebinof width450pccentered extinction(Berryetal.2012;Chenetal.2014;Schlaflyetal.2014;
at 4kpc and in a width of 450pc. The typical uncertainties are Greenetal.2014).Photometricerrorsobviously contributetothe
0.18mag in AV, or about 0.06mag in EpB´Vq. The uncertain- uncertainties. Other potential sources of error include the colour-
tiesaremainlydeterminedbythestellardensityanddepthofthe reddeningdegeneracy.Thosesystematicsmightbethecauseofthe
sample. Under favorableconditions, theuncertaintiescouldbeas significantnon-Gaussianityseeninthedistributionofdifferencesof
lowas0.04maginAV.Some“stripe-like”artifactscanbeseenon valuesasgivenbyourmapandthoseofSFD(Fig.2).Bycontrast,
themap.Theyarecausedbythenightlyvaryingsurveydepthand thedistributionofdifferencesofvaluesbetweenoursandthoseof
photometricaccuracyoftheXSTPS-GAC.Anotableemptypatch Schlaflyetal.(2014)followscloselyaGaussianone,reflectingthe
aroundl„160˝ andb„20˝ isduetotheabnormallylargephoto- factthatbothuseasimilarmethodtoestimatetheextinction.
metricerrorsofXSTPS-GACinthatarea. Forhighextinctionregions[EpB´Vqą0.8mag],SFDred-
Chen et al. (2014) have compared their SampleA me- dening values are systematically higher than ours while those of
dian extinction map with those of Schlegeletal. (1998) and Schlaflyetal.(2014)maparelower.Mostofthosehighlyreddened
Froebrichetal.(2007),andfoundsomesystematicoffsetsamongst regionsforwhichourestimatesofreddeningaresignificantsmaller
them.Partsofthedifferencesareclearlycausedbythedifference than given by the SFD map are actually from low Galactic lati-
in distance traced by the three maps [2–3kpc for SampleA me- tudes.Infact,ifwerestricttheplotsofFig.2toabsolutelatitudes
dianextinctionmapofChenetal.,essentiallyinfinitefortheSFD higher than 20˝ (|b| ą 20˝), then the systematic differences be-
map and about 1kpc for that of Froebrichetal. (2007), respec- tween ours and the SFD values disappear. Thus it is very likely,
tively]. The 4kpc extinction map presented here should have in- forthosehighlyreddened,lowlatituderegions,theintegrationdis-
cluded almost all the dust extinction alone thesightlines for sub- tanceof4kpcusedtoderiveourcurrent2Dextinctionmapisnot
fieldsofintermediateandhighGalacticlatitudes(|b|ą10˝),thus deep enough. The systematic lower values given by the map of
should be directly comparable with that of SFD. In addition, it Schlaflyetal.(2014)compared tooursforthosehighlyextincted
shouldalsobecomparablewiththe4.5kpcreddeningmapnewly regionsismuchmoredifficulttounderstand,asbothanalysesuse
derivedfromthePanSTARRS(Kaiseretal.2010) photometry by similar methods, although the results are based on different data
Schlaflyetal. (2014). Fig. 2 compares values of extinction from set.TheintegrationoftheSchlaflyetal.mapis500pcfurtherthan
our 4kpc extinction map with those of SFD and Schlaflyetal. ours. But instead of giving higher values, the map of Schlafly et
(2014) for sightlines of Galactic latitude |b| ą 10˝. Values of al. actually yields lower values compared to ours. Schlaflyetal.
A are converted to EpB´Vq assuming R “ 3.1. Except for (2014)noticethatwhentheirmapiscomparedtothatofSFD,their
V V
someoutliers,ourvaluesagreeswellwithboththoseofSFDand map yields systematic lower values even for regions of latitudes
Schlaflyetal.(2014)forEpB´Vqă0.8mag.Inderivingtheex- |b| ą 30˝.ItispossiblethatSchlaflyetal.(2014)mayhavesys-
tinction,bothouranalysesandthoseofSchlaflyetal.(2014)have tematically underestimated the reddening values for some of the
made the assumption that all reddening values should be no less highlyextinctedregions.ForthewholeXSTPS-GACfootprintof
4 B.Q. Chen et al.
highestangularresolutionsandbestsignal-to-noiseratios(SNRs)
areemployedinthiswork.
3.1 Hidata
WeusetheHidatafromtheGALFA-HIsurveytotracethecontent
ofatomicgas.TheGALFA-HIsurveyusesAreciboL-bandFeed
Array,aseven-beamarrayofreceiversmountedatthefocalplane
of the 305m Arecibo telescope, to map the H i 21cm emission
in the Galaxy (Peeketal. 2011). The survey aims to cover both
the Galactic plane as well as the high Galactic latitudes with an
angularresolutionof3.4arcmin.TheGALFA-HIfirstdatarelease
(DR1)hasbeenpresentedbyPeeketal.(2011).TheDR1datacov-
ers7520deg2 ofthesky,producedfrom3046hrworthofdataob-
tainedfor12individualprojects.Thelocalstandardofrest(LSR)
velocity of the emission ranges from ´700 to `700kms´1. The
rms noise in a 1kms´1 channel ranges from 140mK to 60mK,
withamedianof80mK.TheDR1datacanbedownloaded from
https://purcell.ssl.berkeley.edu/.
TheGALFA-HIDR1datadoesnotcovertheentirefootprint
of XSTPS-GAC,but only the region of 3h ă RA ă 9h and
0 ă Dec ă 39deg. This unfortunately restricts the sky cov-
erage of thecurrent work. Weuse the data of velocity resolution
0.8kms´1. The H i column density NpHIq is derived from the
profileofbrightnesstemperatureafterapplyingasmallcorrection
fortheopticaldeptheffectsassumingaspintemperatureof145K,
asadoptedbyLiszt(2014a,b).Thetemperaturecorrespondstothe
meanratioofHi21cmemissionandabsorption(Lisztetal.2010)
measured by the Millennium H i Emission-Absorption Survey of
Heiles&Troland(2003).ValuesofNpHIqderivedfortheXSTPS-
GACfootprintrangefrom1.5ˆ1020to1.2ˆ1022cm´2,withame-
dianof1.4ˆ1021cm´2.ThedistributionofNpHIqthusdeducedis
shownintheleftpanelofFig.3.
3.2 COdata
Figure2.Comparisonsofextinctionvaluesasgivenbyourmapandthose
givenbytheSFDmap(toptwopanels),andbythemapofSclaflyetal.
We use the CO-integrated intensity image produced by
(2014;lowertwopanels).Theredplusesinthefirstandthirdpanelsrep-
PlanckCollaborationetal. (2013), which is based on the
resentmedianvaluesintheindividualbins.Ablackstraightlinedenoting
Planck first 15.5month survey data (two full-sky scans) and
completeequalityisalsooverplottedtoguidetheeyes.Inthesecondand
uses the full-sky maps of the nine Planck frequency bands,
fourthpanels,theblackcurveisaGaussianfittothedistributionofdiffer-
encesofvalues. and also the 100, 217 and 353GHz full-sky bolometer maps.
PlanckCollaborationetal. (2013) provide all sky maps of the
velocity-integratedemissionoftheCO J “ 1 Ñ 0, J “ 2 Ñ 1
|b| ą 10˝, our map gives on average only marginally larger val- andJ “3Ñ2transitions.Theyproducethreetypesofmaps.We
choose the Type 3 map generated from the multi-line approach.
uescomparedtothoseofSFDandSchlaflyetal.(2014),by0.013
and0.015mag,respectively.Thescattersofdifferencesbetweenthe The approach assumes fixed CO line ratios in order to achieve
the highest possible signal-to-noise ratios, and has an angular
valuesareabout0.06maginEpB´Vq,comparabletothetypical
uncertaintiesestimatedforourmap(0.18maginA ). resolutionof „ 5.5arcmin. Thestandard deviation of theType 3
V
CO(1Ñ0)map, whenusedatanangular resolutionof 15arcmin,
is typically 0.16K, compared to 1.77 and 0.45K for the Type 1
and2maps,respectively.Forcomparison,theCOsurveyofDame
(2011) has a typical uncertainty of 0.6K. We choose the Planck
3 GASTRACERS
Type 3 CO (1Ñ0) map because of its high angular resolution
The interstellar gas can either be atomic, molecular or ionized. andhighSNR.Type3mapisalsomoresuitableforintermediate
Those three phases are commonly traced by the H i 21cm emis- and high-Galactic latitudes (cf. PlanckCollaborationetal. 2013
sion, themolecular CO pure rotational transitions and by the H i formoredetail).Thereissome13COcontaminationinthePlanck
Hα recombination line, respectively. Theionized gas can also be CO(1Ñ0)map.Tocorrectforthis,wehavedividedthedatabya
traced by thefree-freeemission of thermal electrons. Thecontri- factorof1.16(PlanckCollaborationetal.2013).
butionoftheionized gas,whicharemostlyrestrictedtotheindi- Dameetal.(2001)combinedataofCOsurveysoftheGalac-
vidual,isolatedHiiregions,tothetotalgascontentisneglectedin ticplaneandlocalmolecularcloudsandproduceacompositemap
thecurrentstudy.Onlymostrecentlypublisheddatathathavethe ofanangularresolutionof8.4arcmin.Thedatawereobtainedwith
Dust-to-gasrationof GAC 5
Figure3.Contours of NpHIq(leftpanel) andWCO (rightpanel) inthefootprint ofXSTPS-GAC.Thegrey-scale background imageillustrates the4kpc
extinctionmap.Thesquaresdenotedifferentregionsanalyzedinthecurrentwork.Themapshaveanangularresolutionof6arcmin.
the1.2mtelescopeoftheHarvard-SmithsonianCenterforAstro- fittingstrategysimilartothatof Paradisetal.(2012).Thefitting
physics.Thedatapublished byDameetal.(2001)aremainlyfor processcanbedescribedasfollowing.
theGalacticplane.DataofhighGalacticlatitudes(|b| ą 20˝)are
notavailable.Inthispaper,thedataofDameetal.(2001)areonly (i) We first assume that there is no DG in all subfields (pix-
usedasareferencetocheckthedataweactuallyuse. els).WethenselectpixelswheretheCOemissionislow(WCO ă
ThePlanckType3CO-integratedintensityimageispresented 0.2Kkms´1). The column density of molecules of those pixels
intherightpanelofFig.3.Weonlyshowthefootprintwheredata NpH2q must be less than 2.0 ˆ 1019cm´2 if one assumes that
ofAV,N(Hi)andWCO areallavailable.Itshowsexcellentagree- XCO “1ˆ1020cm´2pKkms´1q´1.Giventhattheminimumcol-
mentwiththedustextinctionmap.Inthispaper,wefirstbringall umn density of atomic gas within the XSTPS-GAC footprint is
inputdatasetstoacommonangularresolutiondeterminedbythe 1.5ˆ1020cm´2(seeSection3.1),thegascolumndensityofthose
lowestangularresolutiondataset,whichis6arcminoftheextinc- pixelscanbeconsidered asbeingcontributedbyatomicgasonly
tion map. This is done by performing a Gaussian kernel smooth. [N(H)=N(Hi)].Undersuchassumptions,wethenderivethebest-
Thankstothehighresolution,wecananalysisthecorrelationbe- fitvaluesofDGRandA0 ofthosepixels.
V
tweendust andgasatintermediateGalacticlatitudesof theGAC (ii) The best-fit values of DGR and A0 obtained from the last
V
fortheindividualmolecularclouds,suchastheOrion,Taurusand step are then applied to all pixels, and this time the value of
Perseus cloud. We divide the clouds into eight different regions. XCO is fitted as a free parameter. We have smoothed the data
Their positions and ranges on the sky are marked by the squares to an angular resolution of 2˝, corresponds to about 140pc at
inFig. 3, together witharegion of diffuse gaswithout any obvi- a distance of 4kpc and 15pc at a distance of 400pc (where
ousmolecularcloudsinthenorthernGalactichemisphere,notedas most of the molecular clouds analyzed in the current work, the
“North”inthecurrentwork. Orion, Taurus and Perseus clouds locate within) in this step
to obtain the statistical cloud-ensemble-average values of X
CO
(Dickmanetal. 1986; Young&Scoville 1991; Bryant&Scoville
4 EXTINCTIONANDGASCORRELATION 1996;Papadopoulosetal.2002).
(iii) TheabovederivedvaluesofDGR,A0 andX areapplied
V CO
The observed visual extinction AV can be modelled by contribu- toEq.(1)tocalculatevaluesofAmodofallpixels.Aχ2valueisthen
V
tionsfromtheatomicgasandtheCO-tracedmoleculargas,as calculatedasχ2 “ř pA ´Amodq,whereiisthepixelindex.
i V,i V,i
AmVod “DGRpNpHIq`2XCOWCOq`A0V, (1) (iv) ThecolumndensityofDGisthencalculatedfromformula,
whereA0 isaconstanttoaccountforpossiblemeasurementoffset NpDGq“pA ´Amodq{DGR. (2)
V V V
of the extinction map and/or possible contribution to the extinc-
tionfromtheionizedgas.Thetotalgascolumndensityisgivenby, (v) Wethengobacktothefirststep,butthistimeusetheDG
NpHq“NpHIq`2NpH2q.Weconstrainthethreefreeparameters, distribution derived from the previous step. We select pixels that
DGR, X andA0,usingthevisualextinction,Hicolumndensity bothhavelowCOemission(W ă0.2Kkms´1)andnoDGcon-
CO V CO
andCOintegratedlineintensitymapsdescribedinthelastSection. tent[NpDGq ď 0cm´2].Thefieldsareusedtoupdatethebest-fit
Considering the high resolution of the maps and thelarge uncer- valuesofDGRandA0 andtheprocessisiterateduntilaminimum
V
taintiesofextinctionforlowextinctionregions,wehaveadopteda χ2valueisreached.
6 B.Q. Chen et al.
presented in Fig. 5. Clearly, different clouds seem to have differ-
ent dust and gas properties. The values of DGR deduced for the
individualregionsofmolecularcloudsrangefrom3.24ˆ10´22to
7.74ˆ10´22magcm2.Forthediffusegasregion“North”,thevalue
is 1.38ˆ10´22magcm2. The values of X for different clouds
CO
range from 0.84 ˆ1020 to 2.39ˆ1020cm´2pKkms´1q´1. The
correlationbetween A andNpHqisquitegoodfortheindividual
V
clouds.Inthe“North”region,thelackofhighCOcontent pixels
makesitverydifficulttofitX .Thedustextinctioninthisregion
CO
is also low (most of the pixels have A ă 1mag), leading to a
V
value of DGR, much smaller than those found for the regions of
molecularclouds.
5 DISCUSSION
5.1 Comparisonwithpreviouswork
Figure4.CorrelationbetweenthevisualextinctionAVandthetotalcolumn The average value of DGR deduced here for the XSTPS-GAC
densityofhydrogen, NpHq “ NpHIq`2NpH2q,forthewholeXSTPS- footprint that excludes the Galactic plane, p4.15 ˘ 0.01q ˆ
GACfootprintof|b|ą10˝thatexcludestheGalacticplane.Thebluecon- 10´22magcm2, is in good agreement with the results of pre-
toursrepresentthedensityofpixelsonalogarithmicscale.Theredpluses vious work. It is slightly smaller than the most cited value,
aremedianvaluesfortheindividualN(H)bins.Theredsolidlineisafitto 5.3ˆ10´22magcm2,derivedbyBohlinetal.(1978)fromtheUV
allthedatapoints.
and optical absorption line measurements toward hot stars. It is
alsoslightlysmaller thanvalues of p6.0´6.5qˆ10´22magcm2
derivedfromthereddening andtheLyαabsorption towardearly-
Inthecurrentwork,wehaveomittedtheGalacticplanewhich type stars (Shull&vanSteenberg 1985; Diplas&Savage 1994).
showsverystrongatomicgasemissionandwherethecontribution On the other hand, Liszt (2014a) finds an even smaller value of
of ionized gas can not be ignored. Thus only data from Galactic 3.7ˆ10´22magcm2fromtheSFDmapcombinedwith21cmHi
latitudes|b| ą 10˝ areusedthroughout thepaper.Sincewehave measurements.ThevaluesofDGRfortheindividualcloudregions
excludedtheGalactic-planeclouds,thoseincludedarerepresenta- varybyafactoroftwo,from3.2to7.7ˆ10´22magcm2.Theyare
tiveofthediffusecloudcomponentoftheGalaxy.Usuallyamini- also consistent with values from the previous work. Paradisetal.
mumχ2 isreachedafterthefirstiterationthatassumesthereisno (2012) obtain an average value of 5.46ˆ10´22magcm2 for the
DGcomponentatallpixels.WelisttheresultsinTable1forboth whole region of the outer Galaxy (|l| ą 70˝) and values of
thewholeXSTPS-GACfootprintof|b|ą10˝andfortheindivid- p3.4´8.7qˆ10´22magcm2forindividualregions,ingoodagree-
ualselectedregions.Errorsoftheresultantparametersarederived mentwithourresults.Avarietyoffactorsmayleadtovariationsof
usingabootstrapmethod.Foreachparameter,weapplythealgo- dust-to-gasratiosamongst differentregions,includingdifferences
rithmto1,000randomlyselectedsamplesandderivetheparame- industcontentandmetalabundance.Thedustextinctionlawofdif-
tersofthosesamples.TheresultantparametersfollowaGaussian ferentregionsmayalsodiffer.Froebrichetal.(2007)discussvari-
distribution.Thevarianceof theGaussiandistributionistakenas ationsoftheextinctionlawamongstindividualcloudsintheGAC
theerrorofthecorrespondingparameter. area. In fact, we notice that the value of DGRdecreases within-
We present in Fig. 4 the correlation between visual extinc- creasingpower-law index β of extinctionlaw inthenear-IR.The
tionAV andtotalcolumndensityofhydrogen, NpHq “ NpHIq` index β is known to correlate with the dust properties, including
2X W ,forallXSTPS-GACpixelsof|b| ą 10˝,excludingthe thegrainsizedistribution(Draine2003).
CO CO
Galacticplane.Despitethelargescatters,inparticularforpixelsof The value of CO to H conversion factor, X , deter-
2 CO
lowextinction,thedataexhibitasignificantlinearcorrelationbe- mined hitherto differs from one analysis to another. Our best-fit
tweenthetwoquantities.Thedatayieldabest-fitvalueofDGRof value of 1.72 ˆ 1020cm´2pKkms´1q´1 for the whole XSTPS-
p4.15˘0.01qˆ10´22magcm2.Thebest-fitvalueofX isfound GAC footprint of |b| ą 10˝ is close to the Galactic aver-
CO
tobep1.72˘0.01qˆ1020cm´2pKkms´1q´1. age value (Strong&Mattox 1996). The value is slightly lower
The southern Galactic hemisphere covered by the XSTPS- but nevertheless quite close to the value of p1.8 ˘ 0.03q ˆ
GACsurvey(b ă ´10˝) includeseveral prominent giant molec- 1020cm´2pKkms´1q´1, derived for Galactic regions of |b| ą
ular clouds, including the Orion, Perseus, Taurus clouds and the 5˝ by Dameetal. (2001) based on extinction data from SFD,
Tauruscloudextensionregions.Wehaveearmarkedeightregions H i data from the Dwingeloo-Leiden H i survey and CO data
andexaminethegasanddustcorrelation.Wehavealsoappliedour from Dameetal. (2001). Glover&MacLow (2011) demonstrate
algorithmtoaregionofdiffusegasinthenorthernGalactichemi- by numerical simulations that X falls off with extinction as
CO
sphere,earmarkedas“North”.Theranges,meanextinction,mean X 9 A´3.5 for clouds of mean extinction smaller than 3mag.
CO V
NpHIq and mean WCO of theseregions arelistedinTable2. The However, our current results do not show that kind of relation.
positionsofthoseregionsaremarkedbysquaresinFig.3.TheCO In addition, our derived values of X are significantly smaller
CO
and extinction maps show remarkably similar features, although thanpredictedbythenumericalsimulationsofGlover&MacLow
thosefeaturesarenotsoobviousintheHimap.Forthoseindivid- (2011).Polketal.(1988)considerasimpletwo-componentcloud
ualregions,theresultantparametersarealsolistedinTable1and model which consists of GMC cores and lower column density
therelevantdatashowingthecorrelationbetweenA andNpHqare (“diffuse”) molecular gas. The conversion ratios for the dense
V
Dust-to-gasrationof GAC 7
Table1.DusttogasratioDGRandCO-to-H2conversionfactorXCOderivedforindividualregionstowardtheGalacticanticentre.
Region (10´22DmGaRgcm2) (maAg0V) (1020cm´2XpKCOkms´1q´1) MMHHppDHGIqq MMHHppHDC2GOqq fDG 1021pAcNmVp´´D2GAmqcVaqg´1 (mAacVg)
Allof|b|ą10˝ 4.15˘0.01 0.06˘0.002 1.72˘0.01 0.23 1.24 0.55
Orion1 7.74˘0.21 ´0.75˘0.05 1.54˘0.01 0.09 0.45 0.31 2.15˘0.07 1.10˘0.05
Orion2 5.53˘0.08 ´0.29˘0.02 1.34˘0.03 0.10 1.43 0.59 1.69˘0.07 0.97˘0.06
Taurus 3.65˘0.10 0.34˘0.02 1.38˘0.01 0.22 0.31 0.24 2.75˘0.12 0.91˘0.04
TaurusE1 6.42˘0.10 ´0.15˘0.02 0.84˘0.01 0.11 1.43 0.59 2.28˘0.02 0.76˘0.04
TaurusE2 5.62˘0.08 ´0.14˘0.01 1.69˘0.04 0.11 4.01 0.80 1.89˘0.07 0.64˘0.05
TaurusE3 6.33˘0.15 ´0.13˘0.02 1.41˘0.02 0.13 3.42 0.77 2.28˘0.03 0.57˘0.04
Perseus1 5.01˘0.08 0.10˘0.01 1.04˘0.01 0.18 0.89 0.47 2.10˘0.04 0.59˘0.05
Perseus2 3.24˘0.28 0.22˘0.03 2.39˘0.01 0.22 1.05 0.52 2.36˘0.04 0.55˘0.04
North 1.38˘0.02 0.20˘0.002 0.95 2.40˘0.03 0.22˘0.04
Table2.Individualregionsstudiedinthecurrentwork.
Region longitude latitude AV(mag) NpHIq(1020cm´2) WCO(Kkms´1)
Orion1 211˝ălă201˝ ´17˝ăbă´10˝ 1.44 23.93 1.61
Orion2 201˝ălă188˝ ´18˝ăbă´10˝ 1.04 22.31 0.56
Taurus 178˝ălă164˝ ´19˝ăbă´10˝ 1.38 16.47 4.22
TaurusE1 189˝ălă171˝ ´31˝ăbă´17˝ 1.02 16.78 0.75
TaurusE2 182˝ălă165˝ ´45˝ăbă´32˝ 0.70 14.62 0.12
TaurusE3 197˝ălă183˝ ´41˝ăbă´28˝ 0.66 11.96 0.16
Perseus1 165˝ălă150˝ ´25˝ăbă´10˝ 0.93 13.35 1.30
Perseus2 171˝ălă152˝ ´32˝ăbă´25˝ 0.65 11.49 0.49
North 220˝ălă175˝ 10˝ăbă30˝ 0.31 7.70 0.00
cloud cores and the diffuse clouds are different, estimated at „
4 ˆ 1020 and 0.5 ˆ 1020cm´2pKkms´1q´1, respectively. Thus
themeasuredvaluesof X inthecurrentwork(around„ 1.5ˆ
CO
1020cm´2pKkms´1q´1 )areclosertothoseofthediffuseclouds
of the work by Polketal. (1988) than the dense ones, indicat-
ing that the clouds in our sample are dominated by the diffuse
phases. Pinedaetal. (2008a) find that X varies from 0.9 to
CO
3.0 ˆ 1020cm´2pKkms´1q´1 amongst several regions of the
Perseus cloud. Ackermannetal. (2012) find that for the Orion
cloud, X „ p1.3 „ 2.3q ˆ 1020cm´2pKkms´1q´1. Finally,
CO
Abdoetal. (2010) report a significant variations ranging from
„0.9ˆto„1.9ˆ1020cm´2pKkms´1q´1,fromtheGouldBeltto
thePerseusarm.Allthosefindingsareallingoodagreementwith Figure6.Leftpanel:Comparisonofatomichydrogencolumndensitiesde-
ourresults. rivedassumingaconstantspintemperatureofTs “ 145Kandthosede-
duced assuming optically thin emission. Right panel: Comparison ofthe
correctedPlanckCOdataandthoseofDameetal.(2001).Theredpluses
aremedianvaluesfortheindividualbinsandtheblacksolidlinesrepresent
5.2 Validationofthegastracers thelineofequal.
Both PlanckCollaborationetal. (2011) and Paradisetal. (2012)
assumethatthe21cmlineisopticalthinwhencalculatingthecol-
umndensityofatomicgas.Fukuietal.(2014)infersthattheDGin
theGalaxyisdominatedbyopticallythickandcoolHigas,which tiesderivedassumingopticallythinandthoseassumingaconstant
implies that the average density of H i is two times higher than T “145KispresentedintheleftpanelofFig.6.Thelattervalues
s
thatderivedassumingoptically-thininthelocalinterstellarspace. are1.1–1.3timeshigherthantheiropticallythincounterpart.
In this work we have applied a first-order opacity correction, as- Intherightpanel ofFig.6,wecomparethePlanckCOdata
sumingaconstantT fortheemission.Theneedtocorrectforthe withthoseofDameetal.(2001).ThePlanckType3COdataare
s
opacityeffectsinthe21cmlineprofilesultimatelylimitstheaccu- ingoodagreementwiththoseofDameetal.(2001),aftercorrect-
racyof NpHqdeterminations.Wefindthatformostdirectionsto- ingforaconstantslopeof1.16.Wehaveinvestigatedtheparame-
wardtheGACbutoutoftheGalacticplane,thecolumndensitiesof tersfortheoverlappingsubfieldsusingtheCOdataofDameetal.
neutralhydrogen derivedaresystematicallyhigher thanthosede- (2001)insteadofthosePlanck.Ityieldsconsistentresultsverysim-
ducedassumingopticallythin.AcomparisonofHicolumndensi- ilartothosepresentedhere.
8 B.Q. Chen et al.
Figure5.SameasFig.4butfortheindividualregions.Thenameoftheregionismarkedineachpanel.
5.3 TheCOdarkgas per cent of the total gas mass, very close to the value of 22 per
centof PlanckCollaborationetal.(2011) andthevalueof 16per
There is a clear excess of A as given by the extinction map
V centofParadisetal.(2012).Thesmalldifferencesamongst those
compared to the simulated Amod for regions of NpHq ą 1.2 ˆ
V estimatesarehoweverprobablyinsignificantowingtothelargeun-
1021cm´2 (A ą 0.6mag,seeFig.4).Theexcesscorrespondsto
V certaintiesinvolvedintheanalyses.Ontheotherhand,the“close”
the presence of CO dark gas, i.e.DG (PlanckCollaborationetal.
agreement betweentheresultsfromdifferentanalyses, whichuti-
2011; Paradisetal. 2012). The DG could be interstellar gas that
lizedifferentdatasetofdifferingspatialresolutionsandskycover-
exists in the form of H along with C i and C ii, but con-
2 ageprobablyindicatea“trueuniversal”fractionofDGmassrela-
tains little or no CO. The column density of DG, NpDGq,
tivetothatofallgas,„20percentonlargeGalacticscale.
can be determined using Eq.(2). The total mass of hydrogen
of the DG component, MpDGq, the mass of atomic hydrogen, ThespatialdistributionofDGderivedfromthecurrentwork
MpHIq, and the mass of the CO-traced molecular gas, MpHC2Oq, isshown intheupper panel of Fig.7for theentireXSTPS-GAC
canbeestimatedfollowingPlanckCollaborationetal.(2011)and footprintwheretheGALFA-HIdataareavailable.Regionswhere
Paradisetal. (2012): MpDG{HI{HC2Oq9řNpDG{HI{HC2Oq. We WCO ą 1Kkms´1 have been excluded. The distribution agrees
haveassumedthatallgascomponenthavethesamespatialextent. well with that of PlanckCollaborationetal. (2011) and clearly
Wehave MpDGq{MpHIq “ 0.23 and MpDGq{MpHC2Oq “ 1.24, showsthattheDGisdistributedmainlyaroundmolecularclouds,
respectively. The values of those quantities thus derived are pre- suchastheTaurus,PerseusandOrioncloudsandexhibitsfeatures
sentedinTable1.OurestimatesofDGmassarelowerthanthose similartothedustdistribution.SignificantamountofDGisfound
foundbyPlanckCollaborationetal.(2011)butareconsistentwith south of the Galactic plane within the footprint of XSTPS-GAC
thoseofParadisetal.(2012).TheestimatedDGmassisabout19 butlittleinthenorth.IfwecomparethemassofDGtothoseofHi
Dust-to-gasrationof GAC 9
andCOamongstdifferentclouds,weseesignificantvariations.For
example,theratioof MpDGq{MpHIqvariesfromavalueof0.09
forOrion1to0.22fortheTauruscloudandto0.95fortheNorth
region. Similarly, the ratio of MpDGq{MpHCOq increases from a
2
value of 0.31 for the Taurus cloud to 1.43 for Taurus E1/Orion
2 and to 3.42 and 4.01 respectively for Taurus E3 and E2. The
valueof MpDGq{MpHCOqfortheNorthregionisnotlistedinTa-
2
ble 1 given the extremely large uncertainty as a consequence of
the lack of detection of CO emission in this region. All the ra-
tiosquoted above arehowever highly dependent on thevaluesof
DGR and X derived. The mass fraction of DG, f , isdefined
CO DG
as f “ MpDGq{rMpDGq`MpHCOqs.Itsvaluesfortheindivid-
DG 2
ualregionsarealsogiveninTable1.FortheentireXSTPS-GAC
regionof|b| ą 10˝,wefind f =0.55,ingoodagreement with
DG
the value of 0.54 found by PlanckCollaborationetal. (2011) for
thesolarneighborhood,andcomparabletothevalueof0.43found
byParadisetal.(2012)forouterGalaxyof|b| ą10˝.Ourresults
arealsoinqualitativeagreementwiththeanalysisbyGrenieretal.
(2005),Abdoetal.(2010)andAckermannetal.(2011),whofind
that the DG component amounts to 40´400 per cent of the CO-
tracedmassinlocalsmallmolecularclouds,suchastheCepheus,
Polaris,Chamaleoncloudsetc.Maddenetal.(1997)findadditional
molecularmassin[Cii]-emittingregionsequivalenttoatleast100
times the mass of CO traced molecular gas in irregular galaxies.
Thevaluesof f inthecurrentworkareconsistentwiththethe-
DG
oreticalworkofPapadopoulosetal.(2002),butmuchlargerthan
the value modelled by Wolfireetal. (2010), f “ 0.3, i.e. the
DG
DG mass is only about 43 percent of the CO traced molecular
mass. The reason could be that the clouds we studied are all off
the Galactic plane while Wolfireetal. (2010) consider the self-
gravitatinghigh-A structures(A ě 2mag)thatobeytheLarson
V V
relations.Bycontrast,Papadopoulosetal.(2002)andotherstudies
areconcentrateonthediffuse-typelow-A H cloudsthataremore
V 2
likelytobeCO-dark,i.e.thetypeofmolecularISMthatisbeing
studied in this work. PlanckCollaborationetal. (2011) have also
pointed out that Wolfireetal. (2010) adopted the Solomonetal.
(1987)valueforthemeancolumndensityofGMCs,whichis2-5
timeslargerthantherecentresultofHeyeretal.(2009).Amongst Figure 7. Upper panel: Distribution of excess column densities. White
the different clouds, f decreases from „ 0.7 in the Taurus ex- areasareregionswherenosuitableextinctionorHidataareavailable,and
DG
tensionsE1–E3(witharelativelylowaverageextinction)to„0.2 regionswheretheCOemissionissointensesuchthatWCOą1Kkms´1.
in the Taurus and Orion clouds (with a relatively high extinction Thesquares delineate thedifferent regions analyzed inthe current work.
Themaphasanangularresolutionof6arcmin.Lowerpanel:Correlation
onaverage).AlinearcorrelationbetweentheDGcolumndensity
NpDGqand thevisual extinction A isfound for thedifferent re- between the visual extinction AV and the DG column density. Pluses of
V differentcoloursarebinnedmedianvaluesforthedifferentregions,whereas
gionsanalyzedhere(lowerpanelofFig.7).Theratiosofthosetwo
linesofdifferentcoloursarelinearfitstothebinnedmedianvalues.
quantitiesshowlittlevariationsamongst thedifferentregions.We
find that NpDGq “ p1.8 „ 2.4qˆ1021pA ´Acqcm´2, where
V V
Ac is a constant, representing the minimum extinction required
V EpB´Vq].AcomparisonwiththewidelyusedSFDdustmapand
for the presence of DG. The constant Ac thus represents the ex-
V thenewlyderivedSchlaflyetal.(2014)reddeningmapshowsthat
tinctionoftheHi-to-H transitionregion.Theoretically,thisvalue
2 ourmaphasahighfidelity.
changeswiththetotalHnucleidensityn,metallicityz,FUVfield
The integrated extinction map is combined with high-
G andH formationrateR oftheclouds(Elmegreen1989,1993;
0 2 f resolution HI and CO measurements to study the correlation be-
Papadopoulosetal.2002).Inthecurrentworkwefindthatitsvalue
tween the interstellar gas content and dust extinction within the
increaseswiththeaverageextinctionoftheregion,fromavalueof
XSTPS-GACfootprint.Themainresultsare:
0.2magfortheNorthregionto1.1magforOrion1.
(i) Anaveragevalueofdust-to-gasratioDGR“p4.15˘0.01qˆ
10´22magcm2andanaverageCO-to-H conversionfactorX “
2 CO
p1.72˘0.01qˆ1020cm´2pKkms´1q´1arefoundfortheXSTPS-
6 SUMMARY
GAC footprint of |b| ą 10˝. The results are consistent with the
Inthispaper,wehaveconstructeda2Dextinctionmapintegrated findingsofpreviouswork.
outtoadistanceof4kpcforthefootprintofXSTPS-GACusingthe (ii) We find a factor of „ 2 cloud-to-cloud variations in the
datapresentedinPaperI.Themaphasahighangularresolutionof values of DGR and X amongst the molecular clouds south of
CO
6arcmin and a low noise level of 0.18mag in A [„60mmag in the Galactic plane within the XSTPS-GAC footprint. Variations
V
10 B.Q. Chen et al.
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ACKNOWLEDGEMENTS
Photo-OpticalInstrumentationEngineers(SPIE)ConferenceSe-
We thank the anonymous referee for instructive comments that ries
improved the current work significantly. This work is par- Kalberla, P. M. W.,Burton, W. B.,Hartmann, D., Arnal, E.M.,
tially supported by National Key Basic Research Program of Bajaja,E.,Morras,R.,&Po¨ppel,W.G.L.2005,A&A,440,775
China2014CB845700andChinaPostdoctoralScienceFoundation Lada,E.A.&Blitz,L.1988,ApJ,326,L69
2014M560843. Lee,M.-Y.,etal.2012,ApJ,748,75
ThisworkhasmadeuseofdataproductsfromtheGuoshou- Lee,M.-Y.,Stanimirovic´,S.,Wolfire,M.G.,Shetty,R.,Glover,
jing Telescope (the Large Sky Area Multi-Object Fibre Spectro- S.C.O.,Molina,F.Z.,&Klessen,R.S.2014,ApJ,784,80
scopicTelescope,LAMOST).LAMOSTisaNationalMajorSci- Liszt,H.2014a,ApJ,783,17
entificProjectbuiltbytheChineseAcademyofSciences.Funding Liszt,H.2014b,ApJ,780,10
fortheprojecthasbeenprovidedbytheNationalDevelopmentand Liszt,H.S.,Pety,J.,&Lucas,R.2010,A&A,518,A45
Reform Commission. LAMOST is operated and managed by the Liu,X.-W.,etal.2014,inFeltzingS.,ZhaoG.,WaltonN.,White-
National Astronomical Observatories, Chinese Academy of Sci- lockP.,eds,Proc.IAUSymp.298,SettingthesceneforGaiaand
ences. LAMOST, Cambridge University Press, pp. 310-321, preprint
This publication utilizes data from Galactic ALFA H i (arXiv:1306.5376)
(GALFA-HI) survey data set obtained with the Arecibo L-band Lombardi,M.,Alves,J.,&Lada,C.J.2006,A&A,454,781
FeedArray(ALFA)ontheArecibo305mtelescope.AreciboOb- Madden,S.C.,Poglitsch,A.,Geis,N.,Stacey,G.J.,&Townes,
servatoryispartoftheNationalAstronomyandIonosphereCenter, C.H.1997,ApJ,483,200
whichisoperatedbyCornellUniversityunderCooperativeAgree- Magnani,L.,Hartmann,D.,Holcomb,S.L.,Smith,L.E.,&Thad-
mentwiththeU.S.NationalScienceFoundation.TheGALFA-HI deus,P.2000,ApJ,535,167
surveysarefundedbytheNSFthroughgrantstoColumbiaUniver- McClure-Griffiths,N.M.,Dickey,J.M.,Gaensler,B.M.,Green,
sity,theUniversityofWisconsin,andtheUniversityofCalifornia. A.J.,Haverkorn,M.,&Strasser,S.2005,ApJS,158,178
Meyerdierks,H.&Heithausen,A.1996,A&A,313,929
Papadopoulos,P.P.,Thi,W.-F.,&Viti,S.2002,ApJ,579,270
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