Very Large Array Observations of Ammonia in Infrared-Dark Clouds II: Internal Kinematics

1 MB·English

by  Sarah E. Ragan

DraftversionJanuary9,2012
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
VERYLARGEARRAYOBSERVATIONSOFAMMONIAININFRARED-DARKCLOUDSII:INTERNALKINEMATICS
SarahE. Ragan1,2,FabianHeitsch3,EdwinA.Bergin2,&DavidWilner4
1MaxPlanckInstituteforAstronomy,Ko¨nigstuhl17,69117Heidelberg,Germany
email:[email protected]
2DepartmentofAstronomy,UniversityofMichigan,830DennisonBuilding,500ChurchStreet,AnnArbor,MI,48109USA
3DepartmentofPhysicsandAstronomy,UniversityofNorthCarolina-ChapelHill,CB3255PhillipsHall,ChapelHill,NC,27599USA
4SmithsonianCenterforAstrophysics,MailStop42,60Gardenstreet,Cambridge,MA,02138USA
DraftversionJanuary9,2012
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1 ABSTRACT
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Infrared-darkclouds(IRDCs)arebelievedtobethebirthplacesofrichclustersandthuscontaintheearliest
2
phasesofhigh-massstar formation. We use the GreenBank Telescope(GBT) andVeryLargeArray(VLA)

n mapsofammonia(NH )insixIRDCstomeasuretheircolumndensityandtemperaturestructure(Paper1),and
3
a here,weinvestigatethekinematicstructureandenergycontent. WefindthatIRDCsoveralldisplayorganized
J velocityfields,withonlylocalizeddisruptionsduetoembeddedstarformation. ThelocaleffectsseeninNH
3
6 emissionarenothighvelocityoutflowsbutrathermoderate(fewkms−1)increasesinthelinewidththatexhibit
maximanearorcoincidentwiththemid-infraredemissiontracingprotostars. Theselinewidthenhancements

] could be the result of infall or (hidden in NH emission) outflow. Not only is the kinetic energy content
A 3
insufficienttosupporttheIRDCsagainstcollapse,butalsothespatialenergydistributionisinconsistentwitha
G scenarioofturbulentcloudsupport. WeconcludethatthevelocitysignaturesoftheIRDCsinoursampleare
duetoactivecollapseandfragmentation,insomecasesaugmentedbylocalfeedbackfromstars.
.
h
Subjectheadings:techniques:interferometric,spectroscopic, stars: formation, ISM: clouds, ISM: kinematics
p
anddynamics,radiolines:ISM,Galaxy:structure
-
o
r
t 1. INTRODUCTION disentangle the problem along the line of sight. Ammonia
s
a Starformationhasbeenthefocusofobservationalandtheo- (NH3) has been a particularly useful probe in molecular
[ clouds (Ho&Townes 1983), as it not only provides kine-
reticalstudiesfordecades,butstilltheconditionsunderwhich

1 this process commences are quite uncertain. The identifica- matic information but also serves as a cloud thermometer
(Walmsley&Ungerechts 1983; Maretetal. 2009). Am-
v tion of objects in different evolutionary stages, such that a
monia has been used widely to study local clouds (e.g.
2 sequence can be constructed, is the essential observational
Myers&Benson 1983; Laddetal. 1994; Wiseman&Ho
0 ingredient needed to test theoretical scenarios. In the so-
1998;Jijinaetal.1999;Rosolowskyetal.2008;Friesenetal.
4 lar neighborhood, it is possible to resolve the precursors to
2009)andIRDCs(e.g.Pillaietal.2006;Devineetal.2011).
1 stars(ormultiplesystems),knownaspre-stellarcores,butthe
These studies focus on the lower metastable states, (J,K) =
. counterpart in massive regions has to date been difficult to
1 (1,1) and (2,2), sensitive to the coldest (<20 K) gas without
isolate. WiththerecentsurveysbySpitzerinthemid-infrared
0 anyevidenceofdepletion.
and advancementof millimeter and radio interferometricar-
2
InRaganetal.(2011,hereafterPaper1),wedetailedVery
1 rays,progressinidentifyingobjectsinvariousearlyphasesof
Large Array (VLA) observationsmapping six IRDCs in the
: massivestarformationhasbeenrapid.
v Infrared-darkclouds(IRDCs), the densest parts of molec- NH3 (J,K) = (1,1) and (2,2). We used the maps to produce
i column density and gas temperature profiles. With between
X ular cloud complexesembedded within Galactic spiral arms 4 and 8′′ angular resolution, we find that ammonia traces
(Jacksonetal. 2008), are believed to host these earliest
r the absorbing structure seen at 8 and 24 µm with Spitzer
a stages of clustered star formation. Studies in the infrared
(Raganetal. 2009), and there is no evidenceof depletionof
(e.g. Peraultetal. 1996; Eganetal. 1998; Raganetal. 2009;
ammoniainIRDCs.Weestimatedatotalammoniaabundance
Butler&Tan 2009; Peretto&Fuller 2009), millimeter con-
of 8.1× 10−7 and found that the gas temperature is roughly
tinuum (e.g. Rathborneetal. 2006; Vasyuninaetal. 2009),
constant, between 8 and 13 K, across the clouds. Here, we
andmolecularlines(e.g.Careyetal.1998,2000;Raganetal.
further our analysis of these ammonia data, focusing on the
2006; Pillaietal. 2006; Sakaietal. 2008; Du&Yang 2008)
velocitystructureof theclouds. Thehighangularresolution
haveshownthatIRDCscontainfromtenstothousandsofso-
larmassesofdense(N(H )∼1022−23cm−2)materialandhave allowsustoprofilethekinematicsandexaminetheirdynam-
therightphysicalconditio2ns(T<15K,n>105cm−3)togive icalstateandstability.
rise to rich star clusters, i.e. clusters which can potentially
hostmassive(M>10M )stars. 2. DATA&METHODS
⊙
Star formation is dynamical by nature (see WeobtainedobservationsoftheNH (1,1)and(2,2)inver-
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McKee&Ostriker 2007, for a review of the important sion transitions with the Green Bank Telescope (GBT) and
processes), but observational tests of dynamics are compli- Very Large Array (VLA). The observationsare described in
cated by the projection of this three-dimensional process detail in Paper 1. The single-dish and interferometer data
onto the two-dimensional plane of the sky. Molecular line werecombinedinMIRIAD(afulldescriptionofthemethod
emission – from a number of molecules excited in the cold isfoundinPaper1). Asummaryofthetargetpropertiessen-
environments of molecular clouds – is the key tool to help sitivity and resolution of the combined data set is given in
2 Raganetal.
TABLE1
Targetsummary
IRDC RA DEC Distancea vlsr rmsb beamsize MIRDCc Area Bcrd
(J2000) (J2000) (kpc) (km s−1) (mJy) (′′×′′) (103 M⊙) (pc2) (mG)
G005.85−0.23 17:59:51.4 -24:01:10 3.14 17.2 2.8 7.7×6.8 5.5 0.55 1.63
G009.28−0.15 18:06:50.8 -21:00:25 4.48 41.4 4.8 8.3×6.4 1.8 1.8 1.43
G009.86−0.04 18:07:35.1 -20:26:09 2.36 18.1 4.3 8.1×6.3 2.6 1.3 0.87
G023.37−0.29 18:34:54.1 -08:38:21 4.70 78.5 2.5 5.7×3.7 10.9 4.4 1.05
G024.05−0.22 18:35:54.4 -07:59:51 4.82 81.4 4.3 8.2×7.0 4.0 1.8 0.99
G034.74−0.12 18:55:09.5 +01:33:14 4.86 79.1 6.8 8.1×7.0 5.5 1.6 1.43
aAssumethekinematicneardistancesfromRaganetal.(2006).
bofthecombineddataset.
cfromabsorptionat8µm(Raganetal.2009).
dCriticalfieldstrength,see§4.3andeq.(8).
Table 1. The combined data set has a velocity resolution of summarizes the kinematic properties of the emission peaks.
0.6 km s−1. In Table 1, we also list the estimated mass and Although we list only the velocity properties of the central
cloud area based on 8 µm extinction, which was computed componentoftheNH (1,1)line,thevelocitystructuretraced
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withSpitzerdatainRaganetal.(2009)andthecriticalmag- by the satellite lines closely follows the trends seen in the
netic field strength required for support, which will be dis- central line. We also list the main line optical depth of
cussedin§4.3. the NH (1,1) transition, τ (1,1), the ratio of thermal to
3 m
Ateachposition,theammoniaspectrawerefitwithacus- non-thermal contributions to the pressure (R ), which will
p
tom gaussian fitting algorithm utilizing the IDL procedure bediscussedinSection4, andnotesaboutthevelocitytrend
gaussfit. TheconfigurationweusedfortheVLAbackend in the cloud or particular characteristics of the integrated
didnotfittheentireNH (1,1)hyperfinesignature(spanning intensity peak. In this section, we discuss the centroid
3
∼3.6 MHz) in the bandpass (3.125 MHz). Our line-fitting velocityandlinewidthtrendsin eachIRDC individuallyand
routine takes a “first guess” line center velocity of the cen- connect the detected 24 µm point sources to the kinematic
tralline(fromRaganetal.2006,seeTable1)whichisoffset signatures.
byapproximately7.7kms−1 fromtheneighboringhyperfine
componentstoeitherside. Wefiteachofthecomponentsin- For the sake of our modeling, we categorize each IRDC
dependently.FortheNH (2,2)lines,asinglegaussianwasfit based on its NH (1,1) emission morphology as either a
3 3
tothe lineindependentlyoftheresultsofthe (1,1)fit. From “sphere” for objects with an aspect ratio, r, close to one or
these fits, we extract the peak intensity, line-center velocity, a “filament” for objects with r much greater than one. The
andGaussianwidthofthelinesateachposition. axesusedtomakethisdistinctionareindicatedinFigures1–
6,andthemorphologicaltypeislistedinTable3.Forspheres,
3. RESULTS theaspectratioisnogreaterthan1.1,andtheelongatedstruc-
Figures1through6showtheNH (1,1)integratedintensity1 tures,or“filaments,”rangefrom1.6to2.9inr.
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and (2,2) integrated intensity plotted over the 8 and 24 µm G005.85−0.23: This source appears approximately round
Spitzer images of the regions (from Raganetal. 2009), re- (r ∼1.1)intheNH3 (1,1)and(2,2)integratedintensitymap.
spectively,andmapsofthelinecentervelocity(firstmoment) Thepeakatα(2000) = 17h59m51.4s,δ(2000) = −24◦01′10′′
and line width (second moment) for the central component correspondstothepositionofthepeakin8µmopticaldepth.
of the NH (1,1) signature. We also plot the physical scale Thereareno24µmsourcesinthemappedregion.
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assumingthedistancesinTable1. The smooth gradient in centroid velocity in this IRDC
permits us to straightforwardly quantify and distinguish
3.1. Propertiesofindividualsources the large-scale ordered motions and the remaining residual
motion on small scales. We show in the central panels of
Paper 1 demonstrates that the gas in these IRDCs exhibit
Figure 1 a clear velocity gradient oriented 30 degrees east
uniform temperatures, changing by only a few Kelvin in a
of north. The total gradient in the NH (1,1) emission is
givenobject,notsignificantlymorethantheerror.Incontrast, 3
1.2 km s−1 over 35 arcseconds, or 0.5 pc, resulting in a
their velocityfields– both the line-centervelocitiesand line
velocity gradient of 2.4 km s−1 pc−1. If this linear gradient
width measurements – show a connection between the
is subtracted, the residual values do no exceed 0.2 km s−1,
presence of embedded star formation activity and complex
indicating the bulk motion dominates the dynamics of the
kinematicsignatures. TheupperpanelsofFigures1–6show
cloud. The overall linewidth measured across the cloud is
the (zeroth, first, and second, from left to right) moment
verylow,between1.3and1.8kms−1,butitincreasessharply
mapsderivedfromtheNH (1,1)observationsforeachIRDC
3 at the edges(to ∼3 km s−1) where the centroidvelocityalso
in the sample with symbols indicating the locations of the
fallsoffquickly.
24 µm point sources and other young stars, and the lower
panels show the same for the NH (2,2) emission. Table 2
3 G009.28−0.15: In this “filament” (r ∼1.6), there are three
integrated intensity maxima: the central peak (α(2000) =
1 Since ourVLAbandpass included onlythe central 3.125MHzofthe 18h06m49.9s, δ(2000) = −20◦59′57′′, P2 in Table 2), which
∼3.5MHzhyperfinesignature,weobserveonlythecentralthreeofthefive
maincomponentsoftheNH3(1,1)hyperfinesignature.Tocomputetheinte- has a 24 µm source associated with it, P3 to the north (off-
gratedintensityoftheline,weassumethatthemissingouter-mostlinesare set 25 ′′), and the maximum (P1) to the south (offset 30 ′′).
0.22thestrengthofthemainlinesandthatthelinewidthsofallcomponents P2 is near the linewidth maximum (3.3 km s−1), and is also
areequal.
VLAobservationsofNH inIRDCsII:Kinematics 3
3
Fig.1.—SpectralmomentsinG005.85−0.23. Topleft: Spitzer/IRAC8µmimagewithNH3(1,1)integratedintensitycontoursoverlaid. Contoursbeginat0.2
Jybeam−1 kms−1andincreasein0.1Jybeam−1 kms−1steps. Bottomleft: Spitzer/MIPS24µmimagewithNH3(2,2)integratedintensitycontoursoverlaid.
Contoursbeginat0.02Jybeam−1kms−1andincreasein0.01Jybeam−1kms−1steps.Topcenter:NH3(1,1)centroidvelocitymapinkms−1.Topright:FWHM
ofNH3(1,1)centrallineinkms−1. Bottomcenter: NH3(2,2)centroidvelocitymapinkms−1. Bottomright: FWHMofNH3(2,2)lineinkms−1. Theredline
representsthemajoraxisandthegreenlinerepresenttheminoraxis.Thestarsymbolrepresentsapointsourcewhichonlyappearsat24µm.TheVLAbeamis
shownatthelower-leftcornerofthefirstandsecondmomentpanels.
TABLE2
SummaryofNH3(1,1)and(2,2)peakcharacteristics
Peak TdvPositionofNH (1,1)transition NH (2,2)line
3 3
R
IRDC α δ v ∆v τ (1,1) v ∆v R Notes
lsr m lsr p
name (J2000) (J2000) (kms−1) (kms−1) (kms−1) (kms−1)
G005.85−0.23 17:59:51.4 −24:01:10 17.36±0.02 1.41±0.03 4.9 17.49±0.04 1.37±0.04 0.1 smoothv-grad.
G009.28−0.15 P1 18:06:50.8 −21:00:25 40.99±0.02 1.45±0.02 4.0 40.99±0.03 1.53±0.03 0.1 mainpeak
P2 18:06:49.9 −20:59:57 41.70±0.07 2.37±0.07 4.0 41.75±0.08 2.14±0.08 0.04 24µmsource
P3 18:06:49.8 −20:59:34 41.47±0.03 1.85±0.03 3.6 41.38±0.04 1.67±0.05 0.06 24µmsource
G009.86−0.04 18:07:35.1 −20:26:09 17.75±0.04 1.35±0.04 3.3 17.50±0.06 1.64±0.07 0.1 “quiescent”peak,2v-grad.
G023.37−0.29 18:34:54.1 −08:38:21 78.81±0.20 3.89±0.22 ··· a 78.16±0.15 3.66±0.19 24µmsource
G024.05−0.22 18:35:54.4 −07:59:51 81.65±0.03 1.96±0.03 2.6 81.62±0.05 2.14±0.07 0.05 N-Sv-grad.
G034.74−0.12 P1 18:55:09.5 +01:33:14 77.95±0.03 2.46±0.03 6.1 77.72±0.05 2.08±0.05 0.04 24µmsource
P2 18:55:11.0 +01:33:02 78.69±0.03 2.09±0.03 2.9 78.58±0.07 2.04±0.07 0.05 24µmsource
alinesaturated.
red-shiftedincentroidvelocity. P1,whilethestrongestinin- attheeasternedgeofthecloud,wherethelineisblue-shifted
tegrated intensity has the lowest linewidths detected in this by 1-1.5 km s−1 with respect to the bulk of the cloud at the
object(1.4kms−1),andthereisnoassociated24µmsource. southeast edge in both NH (1,1) and (2,2) emission. The
3
The northern integrated intensity peak is 10 ′′ away from a linewidths are also enhanced at this edge, though no YSOs
24µmpointsource,butthekinematicstructureisnotaltered aredetectedinthisregion.
byitspresence.
Apart from P2 and P3, the bulk of the cloud resides at a G009.86−0.04:Weapproximatethissourceasafilament,the
narrow range of line center velocities, between 41 and 41.5 most elongated structure (r ∼2.9) in our sample. The inte-
kms−1. Thesharpestchangesincentroidvelocityarelocated grated intensity peak (α(2000) = 18h07m35.0s, δ(2000) =
4 Raganetal.
Fig.2.—SameasFigure1butforG009.28−0.15.
−20◦26′09′′) is dark at both 8 and 24 µm and corresponds The central region has very high linewidths (highest in the
towherethecentroidvelocityandlinewidthisthelowest,all sample, 4 km s−1, see Figure 4), although because of the
evidencefora quiescentregion. The NH (1,1)centroidve- high optical depth of the NH lines, these should be taken
3 3
locity field in this object is organized into two gradients in cautiously.
either directionfrom the centralpeak in integratedintensity.
The eastern (left-hand) gradient is of magnitude ∼1 km s−1 G024.05−0.22: This approximately round source (r ∼1.1)
oriented80◦ east of north, and the western (right-hand)gra- appears centrally-peaked in line intensity (left panels of
dientisofmagnitude∼1.5kms−1oriented65◦westofnorth. Figure5) at α(2000) = 18h35m54.1s, δ(2000) = −7◦59′51′′,
Thecentral“hinge”positionisindistinctinthelinewidthmea- which corresponds also to the peak in 8 µm optical depth.
surement. TheNH (1,1)mapsshowsa velocitygradientstartingatan
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Overall the velocity field in the filament is smooth, with east-westaligned“ridge”slightlyoffsettothenorthfromthe
several YSOs coincident with the ammonia emission: five peakofintegratedintensity. This“ridge”incentroidvelocity
east of the intensity peak, and one to the southwest. The also correspondswith enhancedlinewidths (∼3.6km s−1) in
NH (1,1)linewidthsare enhanced(> 2 km s−1) in the east. both the (1,1) and (2,2) lines, though with no distinction in
3
Curiously, the NH (2,2) emission, which appears to follow integratedintensitysimilartowhatweseeinG009.86−0.04.
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the locations of the YSOs in the eastern region, exhibits an From the center of the cloud across this ridge, the velocity
overall shift to lower line-center velocities (by 0.6 km s−1) changes by ∼1.1 km s−1, and corresponds to a gradient of
and higher linewidths(by 0.5 km s−1). The opticaldepth of 2.1kms−1 pc−1. Atthesoutherntipof(1,1)emission, there
theNH (1,1)mainline isbelow∼3inthe easternregion,so appears to be a clump with distinct red-shifted velocity but
3
it is unlikely that optical depth effects are the cause of the this is not detected in (2,2) emission. While there is one
increased linewidth. This object appears to be undergoing Class II source coincident with northern ridge, this IRDC
cluster formation, though the part of the cloud associated lacks24µmdetections(anindicatorofanembeddedsource)
withNH (1,1)emissionpeakremainsquiescent. anywhereinthecloud.
3
G023.37−0.29: The integrated intensity map of this G034.74−0.12: The overall velocity field of this filament
round IRDC (r ∼1.1) peaks at α(2000) = 18h34m54.1s, (r ∼2.0)appearsquitedisorganized,particularlyinlocations
δ(2000) = −8◦38′21′′, although throughout this cloud, where 24 µm point sources are detected. There are two in-
the lines are saturated and/or optically thick, making it tegrated intensity peaks in this IRDC, both in the vicinity
impossible to derive reliable optical depths or very accurate of 24 µm sources (star symbols in Figure 6), and both po-
lineproperties. Thereisa24µmpointsource(notpresentat sitions show high linewidth. The strong NH (1,1) peak in
3
8 µm) near the center of the region, slightly offset from the the northwestportionof the cloud (α(2000) = 18h55m09.5s,
intensity peak, which is likely a deeply embeddedprotostar. δ(2000) = +1◦33′14′′, P1 in Table 2) is directly coinci-
VLAobservationsofNH inIRDCsII:Kinematics 5
3
Fig.3.—SameasFigure1butforG009.86−0.04.NH3(2,2)integratedintensitycontours(overplottedinbottom-leftpanel)beginat0.03Jybeam−1kms−1and
increasein0.01Jybeam−1kms−1steps.
dent with a 24 µm point source, enhanced linewidths and a itisunlikelythatopticaldeptheffectsarethecauseoftheen-
slightly blue-shifted centroid velocity. The optical depth of hancedlinewidth. Itmaybethecasethattheyoungstarsare
the NH (1,1) main line is 6.1. The centrally located peak havingadynamicaleffectontheslightlywarmergasprobed
3
(α(2000) = 18h55m11.0s, δ(2000) = +1◦33′02′′, P2 in Ta- bytheNH (2,2)emission.Thisdoesnotappeartobethecase
3
ble 2) is offset 10 ′′ from the position of the 24 µm source inG023.37−0.29,wherethereisonlyasingular24µmpoint
andoffset15′′fromthenearbypeakinlinewidth.Theoptical source and the lines are optically thick, which would likely
depth of the NH (1,1) line here is 2.9. These two locations limitourabilitytoprobenearanyembeddedsource(s).
3
arealsowheremostoftheappreciableNH (2,2)emissionis IRDC G034.74−0.12 also exhibits evidence that a cluster
3
found. is actively forming with the two 24 µm point sources cor-
responding to very strong NH emission and several other
3
3.2. Comparisonbetween(1,1)and(2,2)kinematics young stars in the vicinity. However, in these two locations
As is shown in the left panels of Figures 1 through 6 the of 24 µm point sources, which is also where most of the
NH3(1,1)emissiontendstobemorewidespreadthanthe(2,2) NH3(2,2) emission is detected, the linewidth appears more
emission.Inthissection,wecomparethevelocityfieldsofthe enhanced in the NH3(1,1) rather than (2,2). We note that
twostates.Thecentralpanelsshowtherangeinlinecenterve- thehighopticaldepthofNH3(1,1)atP1(6.1)maycontribute
locities, whichgenerallyencompassthesame range,andthe to the broadened line here. The observations of this object
rightpanelsshowsimilartrendsinlinewidth. Table2 shows were the noisiest of the sample, and it is also the most dis-
the velocity propertiesof the (1,1)and (2,2) at the locations tantIRDC,sowemaynotbesensitivetotheeffectweseein
oftheNH (1,1)intensitypeaks. G009.86−0.04(thenearestIRDC).
3
InG009.86−0.04and, to a lesser extentin G023.37−0.29,
wefindthatthemedianNH (2,2)linecentervelocityisoffset 3.3. Linewidth
3
tolowervelocitiesby∼0.6kms−1comparedtotheNH (1,1), The NH (1,1) linewidths in our sample of IRDCs are
3 3
andthemedianFWHMofthe(2,2)lineishigherthanthatof between 1.1 and 4 km s−1, occupying the high tail of the
the (1,1) line by ∼0.8 km s−1. Figure 7 shows the distribu- linewidthdistributionpresentedintheJijinaetal.(1999)sur-
tionsoflinecentervelocityandlinewidthfromboththe(1,1) veyof264densecores,butonparwithotherammoniastud-
and (2,2) maps. G009.86−0.04has several young stars and iesofIRDCs: Pillaietal.(2006),whofoundaslightlylower
24 µm point sources in the eastern part of the cloud, which rangeintheirsingle-dishstudy,andWangetal. (2008), who
correspondstothelocationsofthehighlinewidthsandblue- found that different cores within an IRDC exhibited differ-
shiftedlinecentervelocitiesintheNH (2,2)map.Theoptical entlinewidthsinthehigh-resolutionstudy: higherlinewidths
3
depthoftheNH (1,1)mainline islessthan3in thisregion, nearlocationsofembeddedstarformationactivityandlower
3
lowerthantypicalvaluesof4or5throughoutthesample,so linewidthsinquiescentregionsofthecloud. Foroursources,
6 Raganetal.
Fig.4.—SameasFigure1butforG023.37−0.29.
theenhancedlinewidthappearstocorrespondtothelocations Thenon-thermalvelocitydispersion
of24µmpointsources.
We find that the linewidth increases with increasing dis- ∆v2 k T
tFaingcuereto8,IwReDCshso,wasthwatasthenomteadxibmyumPilllianieewtiadlt.h(d2e0t0e6c)t.edIinn σNT = s8ln2 − µmBH (1)
IRDCs varies directly with distance, which may be a result islargerthanthe soundspeed(see Paper1 fortemperatures,
of clumpingwithin the beam increasing with distance. Cer- ranging from 8 to 13 K) for all IRDCs – in fact, none of
tainly,withSpitzerwedoseeobjectsonthe2-3′′scalewhich the velocity dispersions are smaller than Mach 2. The dis-
wouldnotberesolvedwiththebeam(sometimes6-8′′inlow- tributionsare peaked, with tails to high Mach numbers. For
elevationsources). Atthesametime,theminimumlinewidth IRDCscontainingsources(seediscussionabove),thesetails,
doesnot show any trend with distance. As notedabove, the or “excesses” are correlated with locationsin the vicinity of
integrated intensity peaks with 24 µm point sources have a thesources.
higherlinewidththanthosepeakswithout,buttheapparently
starlesspeakstypicallyhaveverylowlinewidths,independent 4. DISCUSSION
of distance (∆v ∼ 1.4 km s−1). Therefore, in the following Judging from Figures 1 through 6, it is unlikely that one
discussion, we proceed with our analysis assuming that the model of IRDC kinematics can fully account for the range
trends are intrinsic features not determined by the distance. ofbehaviorsobserved. Thebroadcharacteristics– the over-
AsshowninthelowerpanelofFigure8,thecloudmassesin- all linewidths and ranges of centroid velocities, see Table 2
creaseslightlywithdistance,whichcouldsimplybeaselec- –areroughlyconsistentamongthecloudsinthesampleand
tioneffect. Highercloudmassessuggestdeepergravitational with previous molecular line studies (e.g. Pillaietal. 2006;
potentials,whichinturnwouldgiverisetolargerlinewidths. Raganetal.2006). However,oursampleexhibitsbothglobal
Yetthetotalkineticenergycalculatedfromtheaveragedline trends (e.g. smooth velocity gradients) and localized effects
widthsandthecentroidvelocitiesdoesnotchangeperceptibly (e.g. signatures of feedback from young embedded proto-
withdistance. stars) that can now be investigated with high angular reso-
ThedynamicalpropertiesofallIRDCsare summarizedin lutionobservations.
Figure9. Thecentroidvelocitydistributions(leftcolumn)are Asafirstapproximationofstability,wecomputethevirial
asymmetric, with tails to lower or higher velocities than the mass of the clouds (M = 5RV2 /(3G)), where R is the
Virial rms
systemicvelocity.Thiscouldindicateasubstantialelongation radiusof the cloud,G is the gravitationalconstant, and V
rms
alongtheline-of-sight,orhighlyasymmetricinfall. Therel- =321∆V/2.35where∆Vistheaveragelinewidthofthecloud.
ativemotionstothesystemicvelocityaremostlysupersonic. Virialmassesforthewholecloudsaretypically102−3M ,and
⊙
thevirialparameters,α= M /M,from0.1to0.7,suggest-
Virial
ing that IRDCs are bound structures prone to collapse. The
VLAobservationsofNH inIRDCsII:Kinematics 7
3
Fig.5.—SameasFigure1butforG024.05−0.22.NH3(2,2)integratedintensitycontours(overplottedinbottom-leftpanel)beginat0.04Jybeam−1kms−1and
increasein0.01Jybeam−1kms−1steps.
cloud masses M are taken from the dust extinction maps by linewidthatthepositionsofyoungstarsand(sometimes)dis-
Raganetal.(2009). tinctcentroidvelocitycomponents.Weseenostrongtrendfor
Thelinewidthsobservedinoursample(1.1-4.0kms−1)are thepositionsofthe24µmpointsourcestohaveexceptionally
inexcessofthethermallinewidth,∼0.18kms−1. Theratioof highopticaldepthsin NH (1,1),thuswe donotexpectopti-
3
thermaltonon-thermalpressureinthecloud,asexpressedby caldeptheffectstocontributestronglytolinewidthenhance-
Ladaetal.(2003),isR ≡c2/σ2 ,wherec istheisothermal ments. Furthermore,starlesspeaksinNH intensity(existing
p s NT s 3
sound speed, and σ is the three-dimensionalnon-thermal inallcloudsexceptG023.37−0.29anG034.74−0.12)tendto
NT
velocitydispersion. WecalculatevaluesforR ateachofthe havethelowestlinewidths.
p
intensitypeaksintheIRDCs(seeTable2)andfindanaverage Broadened linewidths are an indicator of increased inter-
value of 0.07, indicating that non-thermal pressure is dom- nal motions which accompany the onset of star formation
inant. “Non-thermal effects” encompass many things, such (Beutheretal. 2005). In each IRDC (except G005.85−0.23
asinfall,outflows,orsystematiccloudmotions(i.e. rotation) and G024.05−0.22) of our sample, we see that the sites of
andpossible“support”fromturbulentmotions(Arons&Max 24 µm point sources are accompanied by an enhancement
1975) or magnetic fields (Mouschovias&Spitzer 1976). In inNH (1,1)linewidth(inG009.86−0.04,theenhancementis
3
the followingsections, we explorethese effectsindividually, ratherseen in the NH (2,2)linewidth). Beutheretal. (2005)
3
sotodeterminethedominantprocesses. found that in a molecular core with an infrared counterpart,
the N H+(1-0)emission (knownto trace dense gas similarly
2
4.1. ConnectingstarformationandIRDCkinematics toNH ,e.g.Johnstoneetal.2010)exhibitedbroadlineemis-
3
TheIRDCsinoursamplespanarangeofstagesofstarfor- sion,whereaslinewidthsininfrared-darkcores–presumably
mation,somedevoidofembeddedsourcesandsomehosting atanearlierevolutionarystage–weresignificantlynarrower
several. In Paper 1, we showed that the presence of young (∆v ∼ 1 kms−1). Theincreasedinternalmotionsgivingrise
stars does not significantly affect the kinetic temperature of to broadenedlines can be attributedto orderedmotion, such
the gas traced by ammonia, at least beyond the ∼1K errors asinfall,outfloworrotation.
weestimate2.However,asweshowedinSection3.1,thepres-
ence of youngstars, particularly24 µm point sources, has a 4.2. Kinematicsof“starless”IRDCs
localizedeffectontheIRDCdynamics,namelyanincreasein Because the feedback from embeddedprotostars can con-
fuse the kinematic signatures in the IRDCs, here we take
2WenotedinPaper1thattheseobservationsofthe(1,1)and(2,2)tran- a closer look at the IRDCs which show little or no evi-
sitionsofammoniaarenotnecessarilysensitivetohottergasonthesmall-
dence of actively forming stars. Our sample includes one
estscales,which(forexample)mayarisefromayoungembeddedprotostar
heatingacompactcore.Gasinwarm,compactregionsarebetterprobedwith IRDC, G005.85−0.23, which lacks any coincident Spitzer
higher-Jtransitionsofammoniaorothermolecules. point sources, thus does not appear to host embedded star
8 Raganetal.
Fig.6.—SameasFigure1butforG034.74−0.12.NH3(2,2)integratedintensitycontours(overplottedinbottom-leftpanel)beginat0.04Jybeam−1kms−1and
increasein0.01Jybeam−1kms−1steps.
TABLE3
Summaryoffitresults
IRDC geometrya n0b T0c ξnd σrese ∆f
name (cm−3) (K) (kms−1)
G005.85−0.23 sphere 1.1×106 5.0×103 1.1 2.4 0.18
G009.28−0.15a filament 4.0×105 5.4×103 2.3 2.5 0.23
G009.28−0.15b filament 8.9×105 1.2×104 5.5 3.8 0.27
G009.86−0.04 filament 6.8×105 2.0×103 6.2 1.6 0.28
G023.37−0.29 sphere 6.0×105 6.0×103 1.7 2.6 0.31
G024.05−0.22 sphere 5.5×105 6.0×103 1.7 2.7 0.4
G034.74−0.12 filament 3.4×105 2.3×104 2.0 5.3 0.18
aFittinggeometry(BE-sphereorisothermalcylinder).
bFittedcentraldensity.
cFittedisothermaltemperature.
dNormalizedstabilityparameter.ξn=ξmlforcylinders,andξn=ξBE/6.5forBE-spheres.ξn>1indicatesinstability.
eResidualvelocitydispersionneededatminimumtosupportthecloudenergetically.
fNormalizedrmserroroffit.
formation. G024.05−0.22, has one Class II object (with no tonortheast(33◦ eastofnorth)centeredontheintegratedin-
24 µm counterpart, see Figure 5) near the edge of the NH tensitypeak. InG024.05−0.22thegradientisfromnortheast
3
(1,1)emittingregion,thusthe bulkof the IRDCappearsde- tosouthwest(8◦eastornorth)butisnotsymmetricaboutthe
voidofYSOs. Itispossibleinbothcasesthatembeddedpro- peakinintegratedintensity.Inthiscase,thevelocitygradient
tostarsintheIRDCareheavilyextinctedbydustbeyondour isnotsmooth,buthasasharpridgestructureintheeast-west
detectionlimit,butwecontinueourdiscussionassumingthat direction, which also corresponds with enhanced linewidths
thekinematicsaredominatedbytheglobalforcesratherthan (∼3.6kms−1 comparedto<2kms−1 throughouttherestof
protostellarfeedback. thecloud).
Both of these IRDCs have nearly round projected mor- Smooth velocity gradients are often interpreted as
phologies. In G005.85−0.23,the gradientis fromsouthwest signatures or rotation (e.g. Arquilla&Goldsmith 1986;
VLAobservationsofNH inIRDCsII:Kinematics 9
3
Fig.8.— Top: Linewidth vs. IRDC distance. The linewidth at the in-
tegrated intensity peaks (marked with asterisks where there 24 µm point
sourcespresentand+signsforthosewithout24µmpointsources)andthe
maximumdetected linewidth overall inthecloud(markedwithdiamonds)
bothincreasewithincreasingdistance. Bottom: Totalmass(from8µmab-
sorption,Raganetal.2009)vs.distance.
cal IRDCs is a tempting interpretation, but one that should
be made cautiously. Burkert&Bodenheimer (2000) show
that large-scale turbulent motions (i.e. turbulence with a
steep power spectrum) can lead to centroid velocity gradi-
ents that look like shear or rotation. Indeed, even in these
IRDCswhichappeartobethemostquiescent,thelinewidths
Fig.7.—Distribution ofvelocity fitparameters forIRDCG009.86−0.04
far exceed the thermal sound speed in the typical IRDC en-
aNnHd3G(20,223).d3i7s−tr0ib.2u9tiownitphlothtteedNiHn3t(h1e,1r)edplhoitstteodgrinamth.eBbolathckhihsitsotgorgarmamsaarnedntohre- vironment (about 0.2 km s−1). Such linewidths could be
malizedtothenumberofpositionswitha(1,1)measurement. Theleftpanel causedbyoutflowsfromalow-massstellarcomponentnotde-
showsthelinecentervelocitiesandthebottompanelshowstheFWHM,both tectableintheRaganetal.(2009)Spitzerobservations. Even
inkms−1.
in the absence of stellar feedback, non-thermal linewidths
would be expected as consequence of the cloud formation
Goodmanetal.1993). Sincetheprojectedgeometryofboth
(e.g.Va´zquez-Semadenietal.2007;Heitschetal.2008b),or
oftheseIRDCsisroughlycircular,wecanreasonablyapprox-
ifglobalgravitationalcollapsedominatesthecloudevolution
imatethecloudsasspheres.Ifweassumesolid-bodyrotation,
(e.g.Burkert&Hartmann2004;Fieldetal.2008). Below,we
theresultingvelocitygradientsare2.4and2.1kms−1pc−1for
examinethesepossibilitiesingreaterdetail.
G005.85−0.23andG024.05−0.22,respectively. Ifthisorga-
nized motion is linearly fit and subtracted from the centroid
4.3. DynamicalconditionsofIRDCs
velocityfield,theresidualsarelessthan0.2kms−1.
We can then compare the importance of rotational kinetic To refine our energetics estimates of the previous sec-
energy to gravitational energy, parameterized by the ratio, tion, we analyzed the spatial energy distribution within the
β (in this case for a uniform density sphere), defined as clouds. Todate,dynamicalstudiesofIRDCshavebeenlim-
β = Ω2R3/(3GM), where Ω is the angular velocity, R is the ited mainly to single-dish surveys with resolution elements
cloudradius,andMisthecloudmass(Goodmanetal.1993). of ∼30′′, which is insufficient to resolve the relevant (sub-
For the mass, we take valuesfromRaganetal. (2009) using parsec) scales. With our VLA dataset, we have mapped the
8 µm absorptionas a mass-tracer overthe regionmappedin velocityfield acrossan IRDCs, so we are nowable to quan-
ammonia. For reference, a value of β = 1 is equivalent to tifytheenergydistributioninIRDCs.Todeterminethedegree
3 ofstability, we will fit(idealized)geometries, guidedbyour
breakupspeedforasphericalcloud,andlowervaluessignify
classificationoftheIRDCsin”spheres”and”filaments”(see
a lessening role of rotation in cloudenergetics. Underthese
assumptions,wefindβvaluesof2×10−4and5×10−4,lower Tab.3). Thefitsresultindensityprofiles,isothermaltemper-
atures, and a criticality parameter (see below). We use the
thantheextremelylowendoftheβrangeseenindarkcores
fittedtemperaturesasameasureoftheenergyrequiredtobal-
(Goodmanetal. 1993) because the masses are much higher
ance gravitational and kinetic energy content, which can be
(500 and 2500 M respectively). Adopting a more realistic
⊙
centrallypeakeddensityprofile,ρ∝r−2forexample,reduces comparedtothemeasuredtemperaturesfromPaper1.Thefit
resultsaresummarizedinTable3.
β by a factor of 3. In this simplistic picture, rotation plays
onlyasmallroleinthedynamicsofthecloud.
4.3.1. “Filaments”
Such a simple solid-body rotation model for such spheri-
10 Raganetal.
Fig.9.—SummaryofIRDCkinematics.Leftcolumn:Histogramofthecentroidvelocityinunitsofthesoundspeed.Verticaldashedlinesindicatethetransition
betweensub-andsuper-sonic.Rightcolumn:Histogramsofthenon-thermalvelocitydispersion(eq.1).Allvaluesaresupersonicbyatleastafactorof2.
Forthefilament-likeclouds,weconstructradialmassden- withthecylinderscaleheight
sity profiles from the dust column densities assuming an
isothermalcylinderastheunderlyingmodel.Thedensitypro- 2c2
H2 ≡ s , (3)
fileforanisothermalcylinderisgivenby
πGρ
0
(e.g.Ostriker1964)withthesoundspeedc ≡ k T /(µm ).
s B 0 H
Thefittingisdoneintwosteps. First,weconstructafilament
following the mass distribution as traced by dpust extinction
ρ
ρ(r)= 0 , (2) withSpitzer(Raganetal.2009)bycalculatingthecenter-of-
(1+(r/H)2)2