Dense Molecular Gas Tracers in the Outflow of the Starburst Galaxy NGC 253

0.76 MB·

by  Fabian Walter

Draftversion January19,2017
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
DENSE MOLECULAR GAS TRACERS IN THE OUTFLOW OF THE STARBURST GALAXY NGC253
Fabian Walter1,2,3, Alberto D. Bolatto4,5, Adam K. Leroy6, Sylvain Veilleux4, Steven R. Warren7,4, Jacqueline
Hodge8, Rebecca C. Levy4, David S. Meier9,2, Eve C. Ostriker10, Ju¨rgen Ott2, Erik Rosolowsky11, Nick
Scoville3, Axel Weiss12, Laura Zschaechner1, Martin Zwaan13
Draft version January 19, 2017
ABSTRACT
7
1 We presenta detailed study of a molecularoutflow feature inthe nearby starburstgalaxyNGC253
0 using ALMA. We find that this feature is clearly associated with the edge of NGC253’s prominent
2 ionized outflow, has a projected length of ∼300pc, with a width of ∼50pc and a velocity dispersion
of ∼ 40kms−1, consistent with an ejection from the disk about 1Myr ago. The kinematics of the
n
molecular gas in this feature can be interpreted (albeit not uniquely) as accelerating at a rate of
a
J 1kms−1pc−1. In this scenario, the gas is approaching escape velocity at the last measured point.
8 Strikingly, bright tracers of dense molecular gas (HCN, CN, HCO+, CS) are also detected in the
1 molecular outflow: We measure an HCN(1–0)/CO(1–0) line ratio of ∼ 1/10 in the outflow, similar
to that in the central starburst region of NGC253 and other starburst galaxies. By contrast, the

] HCN/CO line ratio in the NGC253 disk is significantly lower (∼ 1/30), similar to other nearby
A galaxy disks. This strongly suggests that the streamer gas originates from the starburst, and that its
G physical state does not change significantly over timescales of ∼1Myr during its entrainment in the
outflow. Simple calculations indicate that radiation pressure is not the main mechanism for driving
.
h the outflow. The presence of such dense material in molecular outflows needs to be accounted for in
p simulations of galactic outflows.
- Subject headings: galaxies: individual (NGC 253) — infrared: galaxies — galaxies: evolution
o
r
t
as 1. INTRODUCTION galacticwindshavebeendetectedingalaxiesouttohigh
[ Galactic–scale winds are an ubiquitous phenomenon redshift, only in the nearby universe can they be ob-
served panchromatically in emission to understand the
in both starburst galaxies and galaxies that host ac-
1 mechanisms responsible for ejecting the gas. Steadily
tive galactic nuclei (e.g., Veilleux et al. 2013). They
v expandingobservationsandsimulationsshowthatwinds
are thought to be especially important at high red-
0 include cold (neutral atomic and molecular) gas compo-
shift, where strongly starforming galaxies on the so–
4
nents, both in starburst–driven as well as AGN–driven
0 called galaxy ‘main–sequence’ dominate the star forma-
outflows (e.g., in the case of M82: Walter et al. 2002,
5 tion budget, and the number density of active galactic
Engelbrachtet al. 2006,Roussel et al. 2010, Leroy et al.
0 nuclei is greater. Winds provide negative mechanical
2015a,other galaxies: e.g., Rupke et al. 2002,Alatalo et
. feedback, and have been invoked to resolve a number
1 al. 2011,Aalto et al. 2012, Meier & Turner 2012, Rupke
of important issues in cosmology and galaxy evolution
0 & Veilleux 2013), possibly through the entrainment of
(see e.g., Veilleux et al. 2005 for a review). Although
7 ambient material. This cold gas is very difficult to mea-
1 1MaxPlanckInstitutefu¨rAstronomie,K¨onigstuhl17,69117, sure, but likely constitutes the mass–dominant phase in
v: Heidelberg,Germany galactic winds. Mechanisms that have been proposed
i 2National Radio Astronomy Observatory, PO Box O, 1003 to drive cool winds include direct radiation forces (e.g.,
X Lo3peAzsvtirlolenoRmoyadD,Sepoacrotrmroe,nNt,ewCaMlifeoxrincioa8I7n8st0i1t,utUeSoAf Technology, Murray et al. 2011), cosmic ray pressure gradients (e.g.,
r MC105-24,Pasadena, California91125, USA Uhlig et al. 2012), pressure due to supernovae-driven
a 4Department of Astronomy, Laboratory for Millimeter-wave superbubbles (Fujita et al. 2009, Bolatto et al. 2013),
Astronomy, and Joint Space Institute, University of Maryland, and progressive entrainment into the ionized flow facili-
CollegePark,Maryland20742, USA
5Visiting, Max-Planck Institute for Astronomy, Heidelberg, tated by Kelvin–Helmholtz instabilities (Heckman et al.
Germany 2000). Recent models suggest that the cold gas could
6Department of Astronomy, Ohio State University, 100 W also be emerging from condensations of the hot ionized
18thAve,Columbus,OH43210, USA gas phase through runaway thermal instabilities (e.g.,
7Cray, Inc., 380 Jackson Street, Suite 210, St. Paul, MN
Faucher-Gigu`ere&Quataert2012,Zubovas&King2012,
55101, USA
8Leiden Observatory, Niels Bohrweg 2, 2333 CA Leiden, Nayakshin& Zubovas 2012,Thompson et al. 2016,Bus-
Netherlands tard et al. 2016). The latter predictions imply a small
9New Mexico Institute of Mining & Technology, 801 Leroy
spatialoffsetbetweenthehotburstandtheemergenceof
Place,Socorro,NM87801,USA
10Department of Astrophysical Sciences, Princeton Univer- coldmaterialcondensingoutofthe wind. Inmostcases,
sity,Princeton,NewJersey08544,USA observational constraints on the velocity of the outflow-
11Department of Physics, University of Alberta, Edmonton, ing gas, both neutral and ionized, indicate that the gas
AB,Canada
12Max-Planck-Institut fu¨r Radioastronomie, Auf dem Hu¨gel doesnotreachtheescapevelocity(abovereferences),i.e.
69,Bonn,Germany that the currently outflowing gas may be re–accreted at
13European Southern Observatory, Karl-Schwarzschild- later cosmic times.
Strasse2,85748Garching,Germany
2 Walter, Bolatto, Leroy, et al.
NGC253 is one of the best laboratories to study geting the CO(1–0) line to map the central 1kpc (∼1′)
starburst–drivengalactic–scalewinds indetail due to its of NGC253 in the CO(1–0) transition These observa-
proximity(D=3.5Mpc,Rekolaetal.2005). Itis known tionsusedthe followingcalibrators: J0038-2459(phase),
for the galactic wind emerging from its central ∼200pc J2357-5311 (bandpass) and Uranus (amplitude). Cycle
(e.g., Sharp& Bland–Hawthorn2010). The windis seen 1 observations were obtained on 19-Nov-2013, 01-Dec-
in Hα and X–ray emission (Strickland et al. 2000, 2002, 2013, and (twice) on 02-Dec-2013. The total on–source
Westmoquetteetal.2011),inneutralgas(Heckmanetal. integration time was 2.5h, using typically 36 antennas.
2000), warm H (Veilleux et al. 2009), and in OH emis- These observations were complemented by a 2.6h, 7-
2
sion and absorption (Turner 1985, Sturm et al. 2011). pointing mosaic with the ALMA compact array (ACA)
HST imaging reveals the entrained dust emission in op- covering∼2′×2′ (2×2kpc) at 75pc resolution(cycle 1).
tical broad–band imaging, already suggesting that the These ACA observations used the following calibrators:
wind also carries significant amounts of molecular gas. J0038-2459(phase),J2258-2758(bandpass)andNeptune
ALMA cycle 0 imaging revealed the molecular wind (amplitude). The ACA observations were executed on
in NGC253 in CO(1–0) emission (Bolatto et al. 2013). 07-Oct-2013, twice on 01-Nov-2013, three times on 05-
Thismolecularwindcarriesenoughmasstosubstantially Nov-2013, three times on 06-Nov-2013, and on 14-Nov-
shortenthecurrentstarformationepisodeinthisgalaxy, 2013. The ACA observations were obtained using 9 an-
or to definitively quench star formation over a much tennas.
longer period if a substantial fraction of the gas were ALMA Low–frequency setup: A 3–pointing mosaic at
to reach escape velocity. In the interferometer imaging, 90GHz along the NGC253 bar (cycle 1), targeting the
thewindbreaksupinmolecularfilamentsor“streamers” high-dipole molecules CS(2–1), HCN(1–0), HCO+(1–0),
emerging from the central starburst area. The bright- and a host of other molecules contained in the band
est of these streamers is West of the central starburst (Meier et al. 2015). These observations used the follow-
and points to the South outlining an edge of the ap- ing calibrators: J0038-2459 (phase), J2258-2758 (band-
proaching side of the approximately conical ionized out- pass)andNeptune (amplitude). Observationsweredone
flow (Westmoquette et al. 2011); we hereafter refer to it on 31-Aug-2014, with a total of 27.5 minutes on–source
as the (south–west) SW streamer. In Meier et al. (2015) using 32 antennas.
we present a cartoon of the central starburst region of The cycle 1 data were combined with the cycle 0 data
NGC253, and discuss other tracers of the molecular gas publishedinBolattoetal.(2013)andMeieretal.(2015),
found in the central starburst region of NGC253, while andthecombineddatawereusedintheanalysisbyLeroy
in Leroy et al. (2015b) we focus on the properties of the et al. (2015). Because the continuum in the NGC253
giantmolecularcloudsandthestructureofthestarburst starburst is very bright it is possible to self-calibrate on
itself. it. We applied self-calibration to remove residual phase
In this study we present and discuss the properties andfluxcalibrationerrorsinherenttothedataandattain
of this brightest outflowing molecular gas streamer, as a dynamic range higher than otherwise possible. Phase
obtainedfromthe combined cycle 0 and1 ALMA obser- informationusingthecontinuumwereself-calibratedfirst
vations together with new IRAM single-dish data. This followed by amplitude self-calibration.
paper is structured as follows: §2 summarizes the inter- Natural and Briggs robust–weighted, continuum–
ferometric, single–dish, and Hubble Space Telescope ob- subtracted, cubes were created at 5kms−1 velocity res-
servations, and §3 shows the results. In §4 we present a olution. All values were primary beam corrected for all
discussionofourfindings,and§5summarizesourconclu- quantitative analysis. As a final step, for all compar-
sions. WeassumeadistancetoNGC253ofD =3.5Mpc isonsweconvolvethe low–andhigh–frequencysetups to
(Rekolaet al.2005),leadingto a linear (projected)scale a common beam size of 1.9′′ (32pc). The final robust-
of 1′′=17pc. weighedCO(1–0) cube has an rms of 1.6mJybeam−1 in
5 kms−1 channels and a beam size of 1.6”×1.2” with
2. OBSERVATIONS PA of 71.5◦. In velocity ranges where signal is present,
deconvolutionartefacts dominate the noise (despite self-
2.1. ALMA Observations
calibration and careful cleaning the data is dynamic–
The data presented here are based on ALMA cycle 0 range limited), and the practical noise floor is closer
and 1 observations and include IRAM 30m and Mopra to 6.5mJybeam−1. For the HCN, CN, CS, and HCO+
single dish observations to account for the missing short lines, the rms is 1 mJybeam−1 for a 1.9”×1.9”beam.
spacings in the interferometric ALMA imaging.
TwofrequencysetupswereobservedinALMAband3,
2.2. Single Dish Observations
one high–frequency setup (LSB: 99.8–103.7 GHz; USB:
111.8–115.7GHz), that covers the (redshifted) CO(1–0) In the absence of ALMA total power measurements,
and CN lines, and one low–frequency setup (LSB: 85.6– wehavecorrectedtheALMAinterferometricimagingfor
89.6 GHz; USB: 97.4–101.4 GHz) that covers key high– zero spacings using the following single dish telescopes:
densitytracermolecules,suchasHCN,HNCandHCO+ Mopra: We use measurements obtained by the Mo-
(seeMeieretal.2015forafulloverviewofalltransitions pratelescopetocorrecttheALMACO(1–0)observations
covered by the observations). Additional details about (high–frequency setup), as discussed in in Bolatto et al.
the ALMA observations and reduction can be found in (2013), Meier et al. (2015), and Leroy et al. (2015).
Bolatto et al. (2013), Leroy et al. (2015b), and Meier et IRAM 30m: We use new IRAM 30m telescope mea-
al. (2015). The ALMA band 3 interferometric observa- surements to correct the high–density tracer molecules
tions include: (low–frequency setup). These observations cover all rel-
ALMA high–frequency setup: A 7–point mosaic tar- evant tracer molecules, e.g., HCN, HCO+, CS (project
Dense Molecular Gas in the Outflow of NGC253 3
60 80
-25o16'30'' -25o16'30'' 70
50
60-1m)
Declination (J2000)--2255oo1177''0300'''' 234000T (K)B Declination (J2000)--2255oo1177''0300'''' 345000-10 Flux (Jy km s bea
20 O 1-
C
10
-25o18'00'' -25o18'00'' 10
0 0
00h47m36s00h47m34s 00h47m32s00h47m30s 00h47m36s00h47m34s 00h47m32s00h47m30s
Right Ascension (J2000) Right Ascension (J2000)
Fig.1.—ALMACO(1–0) observations ofNGC253. Thekinematiccenter determinedbyMu¨ller-Sa´nchezetal.(2010) at00h47m33s.14,
−25◦17′17′.′52(J2000)isindicatedwithamagentacross,withthe±1.2′′3σuncertaintyshownbythemagentacircle. Left: peakbrightness
mapoftheCO(1–0)emissioninNGC253coveredbyour7–pointALMAband3mosaic(wepresentheretheBriggs-weightedobservations
with a robust parameter of 0.5, which have a synthesized beam of 1.6′′×1.2′′ shown in the left bottom corner). Contours are drawn at
2.5,5,10,20,30,40,and50K.Thepeaksurfacebrightnessinthemapis60K.Theemissionisdominatedbythebrightcentralbar,but
emissionfromthespiralarmsinthediskisalsovisibletowardstheSEandNWedgesofthecoveredarea. Notethatthismapiscorrected
for the primary beam attenuation, so the noise increases at the edges of the mosaic. Right: velocity–integrated CO (1–0) flux density
Sν∆v over the velocity range 120–255 kms−1, highlightingthe SW streamer of the outflow. Thecontours correspond to 1.25, 2.5, 5, 10,
20,40Jybeam−1kms−1. Theredrectangle showstheorientationoftheposition-velocitydiagramshowninFigs.2–4,and6–7(centered
at00h47m32s.86, −25◦17′20′.′8withapositionangle of333◦ sothat offsets arepositiveN of thecentral bar). Theheliocentric velocity of
thecentral barmaterialinthisposition-velocitycutis280kms−1 (seeforexamplethetoppanelofFig. 7).
pointing, focus, and calibration. The map has a sen-
0.6 sitivity of 4mK in a 3kms−1 channel at 89GHz. The
400 disk cloud 00..45 -1m) frequency-dependent beam size ranges from 26′′ to 29′′.
-1m s) 300 0.3 Jy bea wiItnhtthheecaMseoporfaCsOin(g1l–e0)d,iwshe cdoamtabiunseidngthtehAeLfeMatAhecruinbge
(kVLSR200 00..12 density ( Msuilrtiiandg tcausbkeiwmamserugseedwaisththaegmaiondoelf i1n6p.2utJiyn/Kth.eTChAeSreA-
x cleantaskontheALMA–onlydata. Wecombinedzero-
100 central bar disk cloud Flu spacings for the high–density tracer observations from
0 SW streamer 0.01 the IRAM 30m telescope with the ALMA cubes using
-30 -20 -10 0 10 20 the CASA task feather. For the ALMA–IRAM30m
Projected distance (")
combination,priortofeathering,weappliedagainfactor
Fig.2.—Position–velocityCO(1–0)diagramalongtheslitshown of6.15Jy/Konthe30-metercubes. AsintheCO(1−0)
inFig.1(equivalenttoalong-slitspectrum). Theemissionisdom-
combination,thefinalcubeswerecreatedusingthefeath-
inated by the intense emission from the central starburst region.
Thenarrowfeaturesatoffsets∼±20”areduetomolecularclouds eredcubesasmodelsfortheinversionoftheALMAdata.
in the galaxy’s rotating disk (labeled disk cloud). The CO (1–0) This was done by including the feathered cubes as mod-
outflow SW streamer (negative offsets) is alsolabeled. Thewhite elsintheALMAdatasetusingsetjy,followedbyclean
dashed line shows the adopted reference velocity for emission in
the central bar at this location in the galaxy, of 280kms−1. The using the same parameters as those used to create the
contoursindicateSν of0.09,0.25,0.49,0.81,and1.21Jybeam−1. original ALMA cubes. In both cases this zero spacing
The color scale uses a square root stretch, and the noise in the correction significantly improved the negative bowls in
data is 1.6mJybeam−1 in emission-free velocity ranges. A typ- the original (ALMA 12m–only) data cubes.
ical brightness for streamer material between the dashed lines is
40–60mJybeam−1
.
2.3. Hubble Space Telescope Observations
We compare our ALMA observations to the ionized
209–14 during 4–9-Mar-2015). NGC253 was above 20◦ gas outflow, traced by Hubble Space Telescope imaging
for ∼4.5 hours each day, resulting in a total of 12.6h of hydrogen recombination lines. We use a WFPC2 Hα
ofon–sourcetime withexceptionalobservingconditions. image from Proposal ID 5211, first presented in Wat-
Tomapa2′×2′ fieldaroundthecenterofNGC253with son et al. (1996). That project observed NGC253 in
uniform sensitivity, we obtained 36 back and forth on- the F656N (on) and F675W (off) filters. We pair this
the-flyscan–mapswith4′′spacing,alternatingbothright withPaschen–βimagingfromtwoprojects(ProposalIDs
ascension and declination. We used the E090 HIGH- 12206 and 13730), which observed the nuclear region in
DENS receiver setup spanning 81.335 to 89.335 GHz in theF128N(on)andF130N(off)filters. Inbothcaseswe
the LSB and 97.335 to 105.335 GHz in the USB, allow- use the Hubble legacy archive enhanced data products,
ing for simultaneous observationsof high-density tracers beginningwiththedrizzledimages. FortheHαwefound
including HCO+, HCN, and CS. We observed Mars for itnecessarytocombinethetwovisitstorejectcosmicray
4 Walter, Bolatto, Leroy, et al.
Offset (") highlights the SW streamer. The position and orienta-
0.6 −2
tion of the position–velocity cut used in our subsequent
analysisoftheSW streamerisillustratedbytheredbox.
−4
0.4 3.2. Spectra Along the SW Streamer
Theposition–velocity(pv)diagramoftheSWstreamer
−6
is shown in Fig. 2, where the prominent outflow is high-
lighted with dashed red lines; these lines represent the
0.2
−8 FWHM resulting from Gaussian fitting as a function of
position offset. As the orientation of this pv diagram is
essentially perpendicular to the major axis, we do not
−10
0 expect the velocities to be affected by the rotation of
Jy) the disk. A different view of the same data is shown in
S (ν −12 Fig.3,whichplotstheCO(1–0)spectraalongthepvdia-
gramforoffsets. Thespectralfeaturesidentifiedwiththe
−0.2 streamer and the results from the Gaussian fits (central
−14 velocity, peak, and velocity width) are also displayed in
Fig. 3.
The Gaussianfitting results are summarized in Fig. 4.
−0.4 −16
TheleftpanelshowsthecentralvelocityandFWHMve-
locitywidth(∆v)oftheemissionasafunctionofposition
−18 alongthe pv diagram. Afirstorderpolynomialfitto the
−0.6 central velocity as a function of position is overplotted
as a redline. The rightpanelshows the velocity andde-
−20
convolved (by subtracting the beam size in quadrature)
FWHM spatial width as a function of position: the out-
−100 0 100 V200 (km 3s0−01) 400 500 600 flow has a deconvolved FWHM width of approximately
LSR
40–50pc. Thereferencevelocityisindicatedbyadashed
Fig.3.—CO(1–0)spectraalongthesouthernpartoftheposition-
white line in Fig. 2. It corresponds to the approximate
velocitycutinFigs. 1and2,followingtheSWstreamer. Thecolors
indicateoffsetsfromtheplane,asindicatedbythecolorbartothe gas velocity of the bar at the base of the outflow.
right(spectraareoffsetverticallyby0.05Jyfordisplaypurposes).
Atsmalloffsets,theemissionisdominatedbythestarburstinthe 3.3. Is the Molecular Outflow Accelerating?
central bar, and for intermediate offsets (–4”to –10”) line split-
ting is observed. The diffuse molecular outflow is then picked up The SW streamer is blue–shifted with respect to the
atvlsr ∼200kms−1,andcanbetracedout tooffsets of–18′′ (the reference velocity, implying that the material is ap-
horizontalandverticalbarsindicatetheGaussian–fitFWHM,cen-
tralvelocity, andamplitudesinthestreamer). GiventhelowS/N proaching us. This blueshift gets larger with increasing
of the three spectra at the largest offsets, wedo not use their fits distancefromthegalaxyplane,whichcanbeinterpreted
inoursubsequentanalysis as an accelerating continuous outflow. This interpreta-
. ThenarrowCO(1–0)featurearoundvlsr∼350kms−1 isdueto tion of the data is, however, not unique. If the original
moleculargasemissionfromaGMCinthediskofNGC253,
labeledinFig. 2as“diskcloud”. ejection occurred over a short period of time and had a
distribution of velocities, the fast ejecta would have got-
artifacts. Then for both sets of images, we aligned the
ten farther away than the slower ejecta, giving rise to a
on– and off–line filters and fit a median scaling between
velocity gradient along the streamer. The apparent pro-
the two in a region near but not in the nucleus. The
jected velocity gradient could also be due to a gradient
scaling factor derived was 0.0241 and 0.95 for F675W
in the inclination of the outflow cone, with the (more or
and F130N, respectively. We used this scaling to sub-
less) constant velocity of CO gas becoming increasingly
tractthestellarcontaminationfromthenarrow-bandon-
aligned with the line of sight at farther distances from
line image. We then converted the resulting continuum-
subtracted image to have units of erg s−1 cm−2 sr−1 us- the central bar. Such geometry has been reported in
the ionizedgasofthe bubble ofNGC3079(e.g., Veilleux
ing the observatory supplied counts–to–flux conversion
et al. 1994, Cecil et al. 2001). The ionized outflow of
(PHOTFLAM keyword) and bandpass width (PHOTBW key-
NGC253,however,seems more like anopen–ended cone
word). ThevaluesemployedforHαandPaschen–β were
than a bubble (Westmoquette et al. 2011), suggesting
1.461×10−16and4.278×10−19forPHOTFLAM,and53.768
that this explanation is probably less likely.
and 357.438 for PHOTBW, respectively. Finally, both im-
Thetranslationofobservedline-of-sightvelocitiesinto
ageswereconvolvedto1.5′′ resolutionandalignedtothe
actualvelocitiesiscrucialto determine the massoutflow
ALMA astrometric grid.
rateandwhether thematerialescapesthe galaxyornot.
3. RESULTS&DISCUSSION Theinclinationofthegalaxyis78◦ (Westmoquetteetal.
2011),sotheprojectededgeofthesouthernoutflowcone
3.1. CO(1–0) map of the full FOV will have an inclination of 78◦−90◦ = −12◦, where the
For reference we show the CO(1–0) peak brightness negative sign indicates it is pointing towards us, result-
map of our ALMA mosaic in Fig. 1. The emission is ing in blue–shifted emission. If the emission originates
dominated by the centralbar in NGC253 that hosts the precisely from this projected edge, the sini correction
nuclear starburst. The second panel in Fig. 1 shows the would be very large (a factor of ∼5). Based on the me-
CO(1–0) emission integrated over a velocity range that dian of the distribution of tani for the surface of a cone
Dense Molecular Gas in the Outflow of NGC253 5
300 60 80
0
50
250 60
-1V (km s)LSR200 --15000 -1V-V (km s)ref-1∆ v (km s)234000 40Width (pc)
150 20
-150 10
100 0 0
-18 -16 -14 -12 -10 -8 -6 -4 -18 -16 -14 -12 -10 -8 -6 -4
Projected distance (") Projected distance (")
Fig. 4.— Fit results for the CO emission in the SW streamer. Left: Line-of-sight velocity versus projected distance. Note that we
estimatethattheprojectioncorrectionisafactorof∼3forvelocity(seetext), whileitisalmostnegligiblefordistance. Thepointsshow
thecentralvelocityasafunctionofpositionalongtheslit(originisthecenteroftheslitwhichcorrespondstothecenterofthebrightbar
emission; c.f., Fig. 1). The grey region indicates the FWHM of the CO line at the different positions. The red line is a linear fit to the
central velocitydata. Thelefty–axislabelsindicatethemeasuredvelocities,whereas thevelocitiesontherighty–axisareshownrelative
to a fiducial reference spectrum at 0 offset (Vref = 280 kms−1; Fig. 2). Right: Velocity and spatial width versus projected distance:
The black open squares show the FWHM of the spectral line (indicated as a grey region in the left panels) as a function of offset, with
correspondingscaleontheleftaxis. ThebluepointsshowthespatialFWHMoftheSWstreamer(deconvolved),measuredusingamoment
mapcreatedfortherelevantchannels,withcorrespondingscaleontherightaxis.
Fig. 5.—Comparisonofthemoleculargas(ALMA)andionizedgas(HST)emissionintheoutflow. Themolecularmapsshowthepeak
temperatureintheemissioninCO(1–0)andHCN(1-0)overavelocityrangeof120-250kms−1,chosentohighlighttheSWstreamerofthe
molecularoutflow. Theionizedgasoutflow isshowninHαwhitecontours intheleft,andinPaschen–β whitecontours intherightpanel
(contours are 1, 4, 16, 64×10−5 ergs−1cm−2sr−1; both panels show continuum-subtracted HST observations). The coordinate grid is
overlaidtofacilitatecomparison. TheSW streamerisalreadyclearlyapparentinHCNemission.
withtheopeningangleandorientationasdeterminedby ity by Hlavacek-Larrondo et al. 2011. This yields an es-
Westmoquette et al. (2011) for the ionized gas outflow, capevelocityof∼500kms−1(withsignificanterrorbars)
Bolattoetal.(2013)arguethatacorrectionfactorof∼3 , i.e. the measured outflow velocity is approaching the
is more likely, and we adopt that value here. Note that, escape velocity.
in practice, the actual factor can range from ∼ 1 to ∞ The velocity dispersion and width of the SW streamer
so the projection correction has large uncertainties. areconsistentwiththedynamicalageofthefeature. The
We measure a gradient of dv/dr = 6.1kms−1/[′′] or dynamical age of the streamer is ∼ 1Myr, correspond-
∼ 36kms−1/[100pc] from the polynomial fitting shown ing to the approximate ratio of its deprojected speed
in Fig. 4 (the symmetric ordinary-least-squares bisector of ∼300kms−1 and length of ∼ 300pc. The measured
yields a 25% higher value). Adopting the aforemen- velocity dispersion ∆v ∼ 35− 40kms−1, which would
tioned projection correction factor of 3 which includes naturally produce a structure with a width of ∼ 40pc
both the corrections for distances and velocities, we de- in 1Myr, very similar to the measured physical width
riveanactualvelocitygradientof∼100kms−1/[100pc], (Fig. 4). If the feature were significantly older than
or ∼1kms−1/[pc]. Towards the end of the detected 1Myr,itwouldhavetobeconfined(byexternalpressure
outflow we measure a line–of–sight outflow speed, rel- or magnetic fields) to stay as narrow in spatial extent
ative to our reference disk velocity, of 120kms−1. With as observed. This implies that, unless such confinement
the projection correction, this translates into an ac- is present, the outflow velocities cannot be much slower
tual outflow speed of ∼360kms−1. The escape velocity than 300kms−1.
of NGC253 can be approximated using equation 16 in
3.4. Deep Imaging of the Ionized Gas
Rupke,Veilleux, & Sanders2002,andthe circularveloc-
6 Walter, Bolatto, Leroy, et al.
Figure 5 compares the outflow observed in molecular AGN-driven outflow. In the starburst galaxy M82 the
gas (left: CO, right: HCN) to the well–known ionized detection of dense gas tracers at the base of its molecu-
gas outflow (left: Hα, right: Paβ). The tracers of the lar outflow (Walter et al. 2002, Leroy et al. 2015a)have
ionized outflow shown here (Hα and Paβ), are signifi- been reported by Salas et al. (2014).
cantly affected by extinction towards the Northern part The clear detection of dense gas tracers in the SW
of the galaxy. The ALMA images correspond to peak streamer of NGC253 provides an interesting new con-
intensityovertherangev =120−250kms−1,highlight- straint on small spatial scales of outflowing gas. Note
ing the disk, eastern shell, and western outflow. The that the CN emission suffers from artificial velocity
SW streamer aligns closely with the edge of the out- broadening due to blending of hyperfine components:
flow as seen in both Hα and the more extinction–robust the brightest CN hyperfine component (1,3/2,5/2 →
Paβ imaging(contours). Infactthestreameralignswith 0,1/2,3/2) is at 113490.9MHz, and the two bright-
brightfeaturesinbothrecombinationlines,furthersolid- est satellites (a factor of 3 fainter) are (1,3/2,1/2 →
ifying the associationofthis featurewith the ionizedgas 0,1/2,1/2)at113499.6MHzand(1,3/2,3/2→0,1/2,3/2)
outflow. Thehigherdensityofgasnearthecoldstreamer at 113508.9 MHz, corresponding to 24kms−1 and
presumably leads to a higher emission measure and cor- 50kms−1 bluewardof the maincomponent (see Skatrud
responding emissivity of ionized gas. We note that the et al. 1983 for CN transitions and rest frequencies).
feature seen in dense gas emission on the other side of We quantify the relation between the different tran-
the disk (i.e. opposite to the SW streamer) is likely due sitions observed in the streamer using Fig. 7, in which
to peculiar motions in the NGC253’s disk. This region we present spectra in CO, HCN, HCO+, CS, and CN at
was already identified in early high-density tracer obser- different offsets from the center (as before, all data are
vations using OVRO (Knudsen et al. 2007). on a common resolution of 1.9′′). These spectra show
Themolecularcomponentseemstoconfinetheionized both emission from the molecular gas streamer (at ve-
features in the left panel of Fig. 5 (CO + Hα), with the locities∼200kms−1)aswellasemissionfromthediskof
ionized gas tapering to emerge from the center of the NGC253 (∼400kms−1) which allows us to immediately
circumnuclear starburst. As noted previously, both re-
compare their line ratios. In the central region we mea-
combinationlinesarebrightertothesouthofthegalaxy, surepeakfluxdensitiesforCO,CN,CS,HCNandHCO+
consistent with that being the approaching side of the of 1548.0, 139.8, 66.3, 112.6, and 123.5 mJy beam−1.
outflow. Strikingly, extinction due to the surrounding Consequently,we measurea fluxratioof∼13.8between
disk is strong enough that aside from one bright pillar,
CO and HCN, which corresponds to a Rayleigh-Jeans
even Paβ emission is not visible north of the starburst brightness temperature ratio of ∼ 8. Such ratios, indi-
region.
cating a high dense gas fraction, is typical of starburst
Perhaps surprisingly, there is little recombination line
environments (e.g., Gao & Solomon 2004) and also seen
emission (and thus little sign of very recent massive star
in some of the densest regions of nearby star forming
formation) in the far western part of the disk. Also,
disks (e.g., Usero et al. 2015).
we do not see strong emission centered on the putative At anoffset of −6.5′′, significantly off the centralstar-
expanding molecular shells identified in Sakamoto et al.
burst region and clearly corresponding to the kinemati-
(2006). Bolattoetal.(2013)arguedthatthese shells are
callydistinctSWstreamer,wemeasurethefollowingval-
associated with the molecular streamers in the outflow ues for the outflow feature at ∼200kms−1 (in the same
and are presumably driven by young stellar clusters of
order and units as above): 154.0, 9.8, 7.0, 12.8, 10.4.
M∗ ∼105M⊙. Iftheshellsaredrivenbystellarfeedback, These values yield a CO/HCN peak flux density ratio
the population driving them must be either older than ∼ 12. This is very similar to the value of the CO/HCN
∼ 5 Myr or so embedded as to remain inconspicuous in
ratioin the starburst,andthe same holds true for ratios
the Paβ image.
of CO to the other molecules. By contrast, the corre-
sponding values for the emission that we kinematically
3.5. Dense Gas in the Outflow identifywiththebackgroundrotatingdiskcomponentof
the galaxy (visible at a velocity of ∼400kms−1 in Fig.
FromFig.5,weconcludethatHCN(1–0)andCO(1–0)
7) are 193, 8.4, 1.9, 5.6, 6.4 (same order and units as
emission is present along the SW streamer in the rele-
vant velocity range 120–250kms−1. In Fig. 6 we show above). These yield a CO/HCN peak flux density ra-
tio ∼ 35, a factor of 3 higher than the starburst or the
the position velocity diagrams for high–dipole molecules
streamer. Such a value for the CO/HCN ratio is in ex-
(HCN, HCO+, CS, and CN) associated with dense gas
cellentagreementwithtypicalmeasurementsinthedisks
andcomparethemtotheCOemission(CNisusuallythe
of nearby galaxies (Usero et al. 2015). We conclude that
chemicalbyproductofthe photodissociationofHCN).It
thefractionofdensemoleculargasintheSWstreamerof
isclearfromFigs.5and6thatthedensegastracersalso
the outflow is high and stays high throughout the region
showthesameoutflowcharacteristicsapparentinCO(1–
0), at position offsets <−5′′ and line–of–sight velocities where we detect emission. Moreover,the material in the
molecular streamer displays a dense gas fraction that is
of<300kms−1. HCNandHCO+ emissioninanoutflow
similar to that of the starburst region and significantly
haveto date only beenreportedina few systems: These
higher than that of the main disk of NGC253, strongly
include the ULIRG QSO Mrk231 (Aalto et al. 2012,
suggestinga starburstoriginfor the gasin the streamer.
Lindberg et al. 2016),a system with SFR∼200M⊙yr−1,
70 times larger than that of NGC253. In NGC1266,
3.6. Mass and Density of the SW Streamer
velocity wings are detected in CS(2–1) and HCN(1–0)
(Alatalo et al. 2015). In NGC1068, Garc´ıa-Burillo et A key result in the study of Meier et al. (2015) is
al. (2014) report dense molecular gas detections in an that the CO(1–0), HCN(1–0), and HCO+(1–0) transi-
Dense Molecular Gas in the Outflow of NGC253 7
500
CO HCO+ HCN
400
)
1
-s
m 300
k
(
R
S200
L
V
100
500
CN CS CO
HCN
) 400 CN
1
-s
m 300
k
(
R
S200
L
V
100
-10 -5 0 5 -10 -5 0 5 -10 -5 0 5
Projected distance (") Projected distance (") Projected distance (")
Fig.6.—Position–velocitydiagramsforarangeofgastracers,showingdense-gasemissionintheSWstreameratoffsets-5′′to-12′′ (see
Fig. 2). The panels show emissionfor individual lines convolved to a common resolution of 1.9′′ (left-to-right, top-to-bottom): CO(1–0),
HCO+(1–0), HCN(1–0), CN (including blended hyperfine structure, see §4.1.3), and CS(1–0). The white contours are at 50, 90, 160,
290, 530, 950mJybeam−1 in all panels the noise is ∼1 mJybeam−1 for the high density tracers, and ∼1.5 mJybeam−1 for the CO at
1.9′′ resolution. The color scale is 0–250mJybeam−1 for CO and 0–22.5mJybeam−1 for the other species with a square root stretch.
The bottom right panel shows the CO spectrum with contours of CN (yellow contour at 50mJybeam−1) and HCN (magenta contour
100mJybeam−1) overlaid. Note that all data cubes except for the CN incorporate total power information from single-dish (Mopra for
CO,IRAM30mfortherest,see§2.1.2).
tions areoptically thick (and in fact have similar optical an optically thick environment (e.g., Stacey et al. 2011,
depths, τ ∼ 5) in the central region of NGC253. The Shirley et al. 2015). The effective excitation density can
fact that the line ratios we measure do not change from be 1–2 orders of magnitude lower than the critical den-
thoseinthecentralregionsuggeststhattheiremissionis sity even for HCN, particularly in environments of high
alsoopticallythick inthe SW streamer. This hasimpor- optical depth (Shirley et al. 2015). Nonetheless, even
tant potential consequences for the molecular gas mass after taking these effects into account our derived aver-
in the outflow. The CO(1–0) luminosity of the streamer age gas density would be still 2–3 orders of magnitudes
is L ∼ 2.8×106Kkms−1pc2, with an approximate too low to explain the presence of bright emission from
CO
areaof ∼1.1×104pc2 and an integratedsurface bright- HCN and other high-dipole molecules. Note also that
nessofT ∆v ∼250Kkms−1. Inordertoobtainafirm theline-widthsarelarge(Fig. 7),whichlowerstheeffect
CO
lower limit for its mass Bolatto et al. (2013) used a CO- of radiative trapping.
to-H2conversionfactorαCO =0.34M⊙(Kkms−1pc2)−1 There are a number of factors that could bring these
based on optically thin CO(1–0) emission. This leads to numbers in agreement. First of all is the clumping – the
a “minimum mass” of the streamer of ∼106M⊙ (∼ 13 moleculargasislikelynotdistributedsmoothly. Theob-
served surface brightness in the SW streamer at 32 pc
times lower than what one would derive with the typi-
resolution is Tobs ∼ 1.5 − 2 K. Assuming an intrinsic
cally assumed Galactic conversionfactor). b
brightnessofT ∼100K(notunreasonablesincethegas
b
3.6.1. Inferred volume densities and implications for dense inthestreamerislikelywarm),theresultingareaclump-
gas excitation ing factor would be ∼T /Tobs ≈50−70, corresponding
b b
The projected area of the streamer is ∼240×60pc2 to a clumping factor in volume of 503/2 ∼ 350− 600.
(or ∼ 1041cm2), leading to an average H column den- This implies that our bulk density of 40cm−3 is physi-
2
sity of N(H ) ∼ 4×1021cm−2 in the plane of the sky. callycloserto2×104cm−3,withtypicalcolumndensities
If we assum2e that the feature has cylindrical geometry of N(H2)∼ 1022cm−2. Note that these estimates come
(depth∼width) this results in an average volume den- from the “minimum mass” yielded by the assumption of
sity of 40cm−3. This is completely insufficient to excite an optically thin CO–to–H2 conversion factor. Depend-
ing on the optical depth of the CO(1–0) transition, the
the detected dense gas tracers through collisions: for
actual conversion factor (and consequently the derived
example, the critical density (defined as the density at
molecularmassandresultingmassoutflowrate)couldbe
which the rate of the collisionaldepopulation of a quan-
factorsoffew to severaltimes higher. To produce bright
tum level equals the spontaneous radiative decay rate)
for the HCN(1–0) emission is 2.6×106cm−3. As is fre- HCNemissionit wouldbe more comfortableto raise the
mass in the streamer by a factor of a few, which would
quently pointed out, however, what is more relevant to
raise the physical volume density to 105cm−3 and reach
the excitation is the ‘effective excitation density’, which
conditions where HCN(1–0) would be efficiently excited.
includes the summation of all collisional transitions to
Followingthislineofreasoning,thebrightHCNemission
the lower level and the effect of radiative trapping in
8 Walter, Bolatto, Leroy, et al.
3.6.3. Constraints to mass and surface density from
CO × 0.1 −1.5"
150 CN extinction
CS
100 HCN
HCO+ WecanusetheextinctioninferredfortheSWstreamer
50 fromthe observedHαtoPaβ lineratiotoindependently
constrain its molecular mass. The typical Hα/Paβ ratio
0
50 −4.0" in the streamer is Robs = 4.5 −5, while the expected
40 intrinsic line ratio in a 104 K plasma is Rint ≈ 17.5
30 for case B recombination (Osterbrock & Ferland 2006).
20 For a Cardelli, Clayton, & Mathis (1989) extinction
mJy) 100 curve, the ratio of extinction at 1.28µm to extinction
S (ν20 −6.5" at 0.66µm is rPβHα = APaβ/AHα ≃ 1/3. Thus ex-
plaining the observed line ratio with a single screen of
15
extinction in front of the ionized gas requires A =
10 Hα
−2.5log[R /R ]/(1− r ) ≈ 2− 2.2, which re-
5 obs int PβHα
0 sults in AV ∼ 2.5. For the Milky Way, this implies
a molecular column density in the plane of the sky of
−9.0"
10 N(H ) ∼ 2.5 × 1021cm−2 (Bohlin, Savage, & Drake
2
1978). The screen geometry is a lower limit to the col-
5
umn density, since any unextincted ionized gas emission
in front of the screen leads to an increase in R , while
0 obs
converselyany extinction onthe back side of the ionized
−100 0 100 200 300 400 500 600
V (km s−1) emissionwould remain undetected. To accountfor some
LSR
of these effects a “double screen” geometry is usually
Fig.7.—Spectra along the southernoutflow atdifferent offsets
from the centre (shown in the top right of each panel in arcsec, preferred,leading to N(H2)∼5×1021cm−2. We would
‘0’ offset corresponds to the plane of the disk). The black lines reach a 20% larger higher value if we assumed that the
indicatetheCOemission(scaleddownbyafactorof10fordisplay dust was mixed with the gas. These numbers are con-
purposes), and the CN, CS, HCN, HCO+ spectra are shown in
sistent with the “lower limit” mass estimate from CO
blue, red, green, and magenta, respectively (all data have been
convolved to the same beam of 1.9”). Note that the CN line is inferred above. Optically thin CO emission, however, is
artificiallybroadenedbytheblendedhyperfinestructure(see§3.5). somewhat surprising given the relatively low CO/HCN
The feature at ∼200kms−1 is associated with the SW streamer, ratio in the streamer, reminiscent of the central star-
and the emissionat ∼400kms−1 is from the disk. Note how the
burst region. Thus it is likely that there is significant
ratioofCO/(densegastracer)ismuchlowerintheoutflow(and
similarthere to the central starburst regionat offsets –1.5”) than ionizedgasinfrontofthestreamer. ALMAobservations
in the disk, suggesting that dense clumps with properties similar ofhigher–JCOlineswillhelptoconstraintheproperties
tothedensegasinthestarburst(andofmuchlargerdensitythan of the SW streamer and the conditions in the molecular
gasintypicaldiskGMCs)surviveintheoutflow. .
outflow further.
in the streamer suggests that the minimum molecular
outflow rate of 3M⊙yr−1 is probably an underestimate 3.7. The driving of the SW Streamer
by factors of a few (Bolatto et al. 2013).
The mechanisms to impart momentum and accelerate
cold gas in a galactic outflow are not well understood.
3.6.2. Excitation by electrons?
The presence of molecular material &300pc away from
Collisions with electrons are also an effective way of the central bar, particularly in the form of dense gas as
exciting polar molecules (Dickinson et al. 1977). In traced by HCN, HCO+ and CS, already provides strong
fact they are the main mechanism for HCN excitation constraints on the stability of wind-entrained clouds
in comets (Lovell et al. 2004). Calculations show that againstphotoandthermalevaporation,Kelvin–Helmoltz
the rate coefficient for excitation of HCN(1–0) by colli- instabilities, and shedding due to ablation (e.g., Mar-
sions with electrons is > 105 times larger than for col- colini et al. 2005). Simulations of radiative clouds em-
lisions with H (Saha et al. 1981; Faure et al. 2007). bedded in a supersonic flow show that radiative cool-
2
The reason why electrons are usually not considered as ing stabilizes clouds against destruction (Cooper et al.
important collisional partners for HCN is that the elec- 2009). Radiativecloudsundergoaloweraccelerationand
tron fraction, x , in dark molecular clouds is thought to have a higher Mach number relative to the flow than
e
be x ∼ 10−8 or lower, controlled by cosmic ray ion- adiabatic (energy–conserving) clouds. They do experi-
e
ization (e.g., Flower et al. 2007). But for x & 10−6 ence fragmentation due to Kelvin–Helmholtz instability,
e
excitation by collisions with electrons will be important but the resulting cloudlets are denser and are more re-
and likely dominant. The SW streamer is close to and sistent to destruction than adiabatic clouds, and they
probably embedded in the ionized flow, so it is possi- are drawn into the flow creating filaments. Addition-
ble that it possesses a higher ionization fraction than a ally, magnetized clouds entrained in a hot wind have
typical dark cloud, and HCN(1–0) is mostly excited by also been shown to be more stable than unmagnetized
electrons. Thiscouldalsobethecaseif,forexample,the ones, even for very moderate initial internal magnetic
outflowiscosmic-raydriven. Wenote,however,thatthe fields, while also leading to filamentary structures (Mc-
lineratiosintheSW streamerareverysimilartotheline Court et al. 2015). In a set of recent calculations, Scan-
ratios observedin the starburst (Fig. 7), which makes it napieco & Bru¨ggen (2015) show that supersonic flows
unlikely that the excitation mechanisms that dominate suppress the Kelvin-Helmholtz instability thus allowing
in one region do not also dominate in the other. cooling clouds to survive longer, but the compression of
Dense Molecular Gas in the Outflow of NGC253 9
thecloudalsomakesitsaccelerationlessefficientsothat 1031dyn,resultinginanestimatedaccelerationofa ∼
rad
they only reach ∼ 15% of the hot flow velocity before 4×10−8cms−2. In the absence of gravity, the resulting
being disrupted. The inclusion of cloud evaporation in- velocityatr ∼100pcwouldbev ∼60kms−1,lower
cloud
ducesfurthercompressionandnaturallyproduceshighly than the observed velocity before projection correction
elongated,filamentarycloudssimilartothestreamerswe (Fig. 4). If we instead assume that the radiation is
observe,butalthoughitcanhelpthecloudssurviveeven exerting pressure on the long side of the cylinder (the
longeritalsomakesthemomentumtransferlessefficient, geometrywiththemaximumarea),f wouldbe5times
rad
resulting in even lower final velocities before disruption larger, with a corresponding v ∼ 135kms−1. Note,
cloud
(Bru¨ggen & Scannapieco 2016). Very recent simulations however, that the ratio of gravity to radiation pressure
by McCourtet al.(2016),however,suggestthat crushed for stars is f /f = GΣ (4πc)/Ψ, where ψ ≈
grav rad cloud
clouds shatter into tiny, dense, cloudlets that do no dis- 2000ergs−1g for a fully sampled IMF containing young
appear and can be much more easily accelerated by the stars, and Σ is the surface density of the cloud (Ψ
cloud
hot wind, making the entrainment process efficient. In is the light–to–mass ratio of the stellar population, e.g.,
this scenario the SW streamer could be composed of a equation33inKimetal.2016). Fortheend-ongeometry
collection of dense molecular clumps embedded in the Σ ∼ 0.075gcm−2, while for the side-on geometry
cloud
much hotter outflow. Σ ∼ 0.015gcm−2, resulting in 0.94 ≥ f /f ≥
cloud grav rad
Notably, a feature corresponding to the molecular gas
0.19. So for our assumed geometry and mass the net
streamer is also seen in radio continuum maps obtained
acceleration a = a (1−a /a ) is between 6%
net rad grav rad
at 20cm (Fig. 9 in Ulvestad et al. 1997, see also Heesen
and 81% of a , which would result in 15 ≤ v ≤
rad cloud
et al. 2009), implying that synchrotron emission is asso- 120kms−1.
ciated with the outflowing molecular gas. This suggests
Fromtheaboveestimatesitappearsthatradiationcan
that magnetic fields, and possibly cosmic rays, are as-
contribute maybe up to a few tens of percent of the
sociated with the outflow. Cosmic rays have been sug-
momentum, but it is extremely unlikely to completely
gested as an important mechanism for driving outflows
explain the observations. Doing so would require using
ingalaxies(e.g.,Boothetal.2013,Salem&Bryan2014,
themostfavorablegeometricassumption(maximalcloud
Girichidisetal.2016,Simpsonetal.2016),althoughob-
area, which maximizes f and minimizes f /f )
rad grav rad
servationally constraining their importance remains elu-
and also assuming that there is almost no projection
sive.
correction to the measured velocity (which requires a
To impart momentum to the cold gas, an alternative
contrived geometry). This suggests that radiation pres-
to entrainment in a hot flow is acceleration by radia-
sure is not the dominant mechanismfor accelerating the
tion pressure. In order to explore the viability of ra-
streamer, although it is non-negligible. Note also that
diation pressure to explain the observations, we per-
any increase in the mass of the streamer over its mini-
form a rough calculation. The radiation flux at a dis-
mum mass (see discussion in §3.6.1), or accounting for
tance r along its axis from the center of a uniform-
the old stars in the galaxy disk outside the starburst
brightness disk of radius R and luminosity L is
src src (whichwouldincreasegravity),wouldalsoreducetheim-
F = L /[2π(R2 +r2)]. Only the photons absorbed
src src portance of radiation pressure. Much higher resolution
by the dusty cold gas can impart momentum, so the
observations of the SW streamer may help further eluci-
force due to radiation pressure experienced by a cloud
datethesequestions,inparticularwhichmechanismsare
that subtends a solid angle Ω and has an optical depth
driving the molecular gas in the outflow.
τ is f = F Ωr2(1−e−τ)/c. The resulting velocity is
then sriamdply v2 = 2R f /M dr. For the pur- 4. SUMMARY
cloud rad cloud
poses of the calculations below we will assume that the Our new ALMA band 3 observations (CO, and dense
SW streamer is exposed to radiation from the “naked” gas tracers) of the central starburst region of NGC253
starburst,whichwouldbemostlyfar-ultravioletandeas- givenew insights onthe propertiesofthe molecularout-
ily absorbed (τ ≫ 1). There is the potential for the flowing gas originally discussed in Bolatto et al. (2013).
momentum imparted by the radiation to be boosted by The most prominent outflow feature towards the south,
a factor of order τ , the mean infrared optical depth whichisthemainsubjectofthispaper,possessesalarge
IR
of the streamer (e.g., Thompson et al. 2015; Zhang & intrinsicvelocitydispersion. Itsextentanddispersionare
Davis 2016). Given our column density estimates, the consistent with an ejection from the disk starting about
SW streamer does not have a large enough column to ∼1Myr ago.
possess substantial optical depth to its own reradiated It is currently unclear whether or not the molecular
IR photons (τ ∼0.2 for the most favorable geometry), mass entrained in the outflow will escape the galaxy, or
IR
so we assume that this boosting is unimportant. be recycled to fuel later episodes of star formation. For
NGC253 has a total infrared luminosity of L = most of the gas in the observed outflow to escape the
TIR
1.4×1044ergs−1,abouthalfofwhichoriginateswithina galaxy it would need to be accelerated as it moves away
diameter of ∼200pc from its center (Leroy et al. 2015), from the disk. The kinematics of the molecular gas are
i.e. R =100pc. The SW streamer has a “minimum consistent with accelerating with a velocity gradient of
src
mass”Mcloud ≈106M⊙,aprojectedsizeontheskyof∼ 1kms−1pc−1, and at its last measurable point it ap-
60×240pc2,andatypicaldistancetothecenteroforder proachesthe escapevelocity. As discussed,this interpre-
r ∼100pc. If we assume the geometry corresponds to a tation is not unique: the kinematics could be also con-
cylinder60pc indiameter(Ωr2 =π(30pc)2)illuminated sistentwithanoutflowwitharangeofspeeds,wherethe
byL =1044ergs−1,theforceduetoradiationpressure material farther from the disk is there because it is the
src
appliedatthebaseofthestreamerwouldbef ∼7.5× fastest. In that scenario, only the fastest fraction of the
rad
outflowing gas may escape the galaxy. Approved, more
10 Walter, Bolatto, Leroy, et al.
sensitive ALMA observationswill trace the outflow even port by the National Science Foundation through grant
further out, and will shed light on whether or not part AST-1009620. S.V. acknowledges NSF grant AST-
of the ejected molecular material will escape the galaxy. 1009583. A.D.B. acknowledges visiting support by the
Strikingly,tracersofthe dense gasphaseofthe molec- Alexander von Humboldt Foundation, and support by
ular medium (HCN, HCO+, CS, CN) are also spatially the National Science Foundation through a CAREER
coincident with the SW streamer of the molecular out- grant AST-0955836 and AST-1412419. E.C.O. is sup-
flow. The line ratios HCN/CO of ∼ 1/10 measured in portedbytheNationalScienceFoundationthroughgrant
the outflow are high and consistent with ratios observed AST-1312006. This paper makes use of the follow-
in the central starburst region of NGC253 and in other ing ALMA data: ADS/JAO.ALMA #2011.0.00172.S,
starbursts. The HCN/CO line ratio in the disk, on the #2012.1.00108.S. ALMA is a partnership of ESO (rep-
otherhand,issignificantlylower(∼1/30),typicalofgas resenting its member states), NSF (USA), and NINS
in the disks of nearby galaxies. In principle, this is in- (Japan), together with NRC (Canada), and NSC and
dicative the dense molecular gas being ejected from the ASIAA (Taiwan), in cooperation with the Republic of
centralregionsintothe outflow,while retainingitsprop- Chile. The Joint ALMA Observatory is operated by
erties in this process. It also suggests that the CO(1–0) ESO, AUI/ NRAO and NAOJ. The NRAO is a facil-
and HCN(1–0) emission are optically thick also in the ity of the National Science Foundation operated under
streamer, implying that the estimated mass loading pa- cooperative agreement by Associated Universities, Inc.
rameter η & 3 (Bolatto et al. 2013) is likely a lower Based on observations made with the NASA/ESA Hub-
limit. Note that electron excitation of polar molecules ble Space Telescope, obtained from the data archive at
isanoften-ignoredmechanismthatcouldplayanimpor- the Space Telescope Science Institute (STScI). Some of
tant role at exciting HCN, HCO+, and CS emission in the HST data presented in this paper in this paper were
outflows, although the fact that we see the same ratios obtainedfromtheMikulskiArchiveforSpaceTelescopes
of these transitions to CO(1–0) in the starburst and the (MAST), others where acquired under program HST-
streamersuggestssimilar excitationmechanisms in both GO-13730 with support provided by NASA through a
regions,implyingdensegasisthemostlikelycause. Sim- grant from the STScI. STScI is operated by the Asso-
plecalculationsindicatethatradiationpressureisnotthe ciation of Universities for Research in Astronomy, Inc.,
main mechanism for driving the outflow. The presence under NASA contract NAS5-26555. The Mopra radio
of a dense gas phase in molecular outflows (with volume telescope is part of the Australia Telescope NationalFa-
densities>104cm−3 andprobably∼105cm−3)willhave cility which is funded by the Australian Governmentfor
to be accounted for in numerical simulations of galactic operation as a National Facility managed by CSIRO.
winds, both at low and high redshift. Basedonobservationscarriedoutunder projectnumber
209-14 with the IRAM 30m Telescope. IRAM is sup-
ported by INSU/CNRS (France), MPG (Germany) and
We thank the referee for excellent comments that im-
IGN (Spain).
proved the paper. D.S.M. acknowledges partial sup-
REFERENCES
Aalto,S.,Garcia-Burillo,S.,Muller,S.,etal.2012, A&A,537, Heckman,T.M.,Lehnert, M.D.,Strickland,D.K.,&Armus,L.
A44 2000,ApJS,129,493
Alatalo,K.,Blitz,L.,Young,L.M.,etal.2011,ApJ,735,88 Heesen,V.,Beck,R.,Krause,M.,&Dettmar,R.-J.2009,A&A,
Alatalo,K.,Lacy,M.,Lanz,L.,etal.2015,ApJ,798,31 494,563
Bohlin,R.C.,Savage,B.D.,&Drake,J.F.1978,ApJ,224,132 Hlavacek-Larrondo,J.,Carignan,C.,Daigle,O.,etal.2011,
Bolatto,A.D.,Warren,S.R.,Leroy,A.K.,etal.2013,Nature, MNRAS,411,71
499,450 Knudsen,K.K.,Walter,F.,Weiss,A.,etal.2007,ApJ,666,156
Booth,C.M.,Agertz,O.,Kravtsov,A.V.,&Gnedin,N.Y.2013, Kim,J.-G.,Kim,W.-T.,&Ostriker,E.C.2016,ApJ,819,137
ApJ,777,L16 Leroy,A.K.,Walter,F.,Martini,P.,etal.2015a, ApJ,814,83
Bru¨ggen,M.,&Scannapieco, E.2016,ApJ,822,31 Leroy,A.K.,Bolatto,A.D.,Ostriker,E.C.,etal.2015b,ApJ,
Bustard,C.,Zweibel,E.G.,&D’Onghia,E.2016, ApJ,819,29 801,25
Cardelli,J.A.,Clayton,G.C.,&Mathis,J.S.1989,ApJ,345, Lindberg,J.E.,Aalto,S.,Muller,S.,etal.2016,A&A,587,A15
245 Lovell,A.J.,Kallivayalil,N.,Schloerb,F.P.,etal.2004,ApJ,
Cecil,G.,Bland-Hawthorn,J.,Veilleux,S.,&Filippenko,A.V. 613,615
2001, ApJ,555,338 Nayakshin,S.,&Zubovas,K.2012, MNRAS,427,372
Cooper,J.L.,Bicknell,G.V.,Sutherland,R.S.,& Marcolini,A.,Strickland,D.K.,D’Ercole,A.,Heckman,T.M.,
Bland-Hawthorn,J.2009, ApJ,703,330 &Hoopes,C.G.2005,MNRAS,362,626
Dickinson,A.S.,Phillips,T.G.,Goldsmith,P.F.,Percival,I.C., McCourt,M.,O’Leary,R.M.,Madigan,A.-M.,&Quataert, E.
&Richards,D.1977,A&A,54,645 2015,MNRAS,449,2
Engelbracht,C.W.,Kundurthy,P.,Gordon,K.D.,etal.2006, McCourt,M.,Oh,S.P.,OLeary,R.M.,&Madigan,A.-M.2016,
ApJ,642,L127 ArXive-prints,arXiv:1610.01164
Faucher-Gigu`ere,C.-A.,&Quataert, E.2012,MNRAS,425,605 Meier,D.S.,&Turner,J.L.2012,ApJ,755,104
Faure,A.,Varambhia,H.N.,Stoecklin,T.,&Tennyson, J.2007, Meier,D.S.,Walter,F.,Bolatto,A.D.,etal.2015,ApJ,801,63
MNRAS,382,840 Mu¨ller-Sa´nchez,F.,Gonz´alez-Mart´ın,O.,Fern´andez-Ontiveros,
Flower,D.R.,PineauDesForˆets,G.,&Walmsley,C.M.2007, J.A.,Acosta-Pulido,J.A.,&Prieto,M.A.2010,ApJ,716,
A&A,474,923 1166
Fujita,A.,Martin,C.L.,MacLow,M.-M.,New,K.C.B.,& Murray,N.,M´enard,B.,&Thompson,T.A.2011,ApJ,735,66
Weaver, R.2009,ApJ,698,693 Osterbrock,D.E.,&Ferland,G.J.2006,Astrophysicsofgaseous
Gao,Y.,&Solomon,P.M.2004,ApJS,152,63 nebulaeandactivegalacticnuclei,2nd.ed.byD.E.Osterbrock
Garc´ıa-Burillo,S.,Combes,F.,Usero,A.,etal.2014,A&A,567, andG.J.Ferland.Sausalito,CA:UniversityScienceBooks,
A125 2006,
Girichidis,P.,Naab,T.,Walch,S.,etal.2016,ApJ,816,L19