A large CO and HCN line survey of Luminous Infrared Galaxies

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by  Padelis P. Papadopoulos

AstrophysicsandSpaceSciencemanuscriptNo.
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A large CO and HCN line survey of Luminous Infrared Galaxies
PadelisP.Papadopoulos · ThomasR.Greve · PaulvanderWerf · StefanieMu¨ehle ·
KateIsaak · YuGao
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Abstract A large CO, HCN multi-transition survey of 30 Keywords Galaxies:starbursts·Galaxies:ISM·Galaxies:

v1 LuminousInfraredGalaxies(LIR>1011L⊙)isnearingcom- active·ISM:molecules
pletion with the James Clerk Maxwell Telescope (JCMT)
9
2 onMaunaKea(Hawaii),andtheIRAM30-metertelescope
8 at Pico Veleta (Spain). The CO J=1–0, 2–1, 3–2, 4–3,6–5,
1 13CO J=2–1,HCN J=1–0,3–2,4–3observations,resulting
0 1 Introduction
from ∼250 hours of JCMT, ∼100 hours of 30-m observ-
7
0 ing time and data from the literature constitute the largest The importance of Luminous Infrared Galaxies (hereafter
h/ extragalacticmolecularlinesurveytodate,andcanbeused LIRGs) as the sites of the most extreme local star forma-
p to address a wide range of issues and eventually allow the tionevents([9])makesthemthebestavailabletemplatesfor
- constructionof reliable Spectral Line EnergyDistributions similareventsinthedistantUniverse.Moreovertheircom-
o
r (SLEDs) for the moleculargas in local starbursts. First re- pactCO-emittingregions([1])makesthemidealformulti-
st sultssuggestthat:a)HCNandHCO+ J=1–0lineluminosi- line studies of their global molecular gas reservoir up to
a ties can be poor mass estimators of dense molecular gas veryhighfrequencies(wherethehigh-excitationmolecular
v: (n≥104cm−3) unless their excitation is accounted for, b) lines lie) since the resulting narrow beams of today’s sin-
Xi CO coolingof such gas in ULIRGs may be comparableto gle dish radio telescopes can still measure their total line
that of the CII line at 158m m,and c) low excitationof the fluxeswith single pointings.Our sensitive CO J=2–1,3–2,
r
a globalmoleculargasreservoirremainspossibleinsuchsys- 4–3, 6–5, 13CO J=2–1 and HCN J=3–2 and 4–3 line ob-
tems.InsuchcasestheexpectedlowCOJ+1→Jlinelumi- servations with the JCMT and the IRAM 30-m telescope
nositiesforJ+1≥4canleadtoastrongbiasagainsttheir for ∼ 30 LIRGs, combined with available CO and HCN
detectionfromULIRGsathighredshifts. J=1–0datafromtheliterature,willallowforexcellentnew
constraintsonthe state oftheir globalmoleculargasreser-
PadelisP.Papadopoulos voirs. More specifically the high-J CO and the three HCN
Institutfu¨rAstronomie,ETHZurich,Zurich,Switzerland transitions can probe the excitation and mass of the dense
Tel.:[phone] (n ≥ 104cm−3) gas, considered as the immediate fuel of
Fax:[phone]
their prodigiousstar formation ([10], [3]). The completion
E-mail:[email protected]
ThomasR.Greve ofourCOJ=4–3andJ=6–5observationsandfollow-upob-
CaliforniaInstituteofTechnology,Pasadena,CA91125,USA servationsofstillhigherrotationalCO,13COandHCNtran-
PaulvanderWerf
sitionswithHerschelwillultimatelyallowthedeductionof
SterrewachtLeiden,2300RALeiden,TheNetherlands
robustSLEDsforthestar-formingmoleculargasofLIRGs
StefanieMu¨ehle
Department of Astronomy, Univ. of Toronto, 60 St. George Street, inthelocalUniverse.Thesecanthenbeusedtouncoverany
Toronto,ONM5S3H8,Canada universalaspectsof the star-formation/moleculargasinter-
KateIsaak
play in galaxies and find the best rest-frame mm/sub-mm
PhysicsandAstronomy,Univ.ofWales,CardiffCF243YB,UK
lines for detecting and imaging the numerous LIRGs that
YuGao
PurpleMountainObservatory,ChineseAcademyofSciences,Nanjing will be found at high redshifts in future deep mm/sub-mm
210008,People’sRepublicofChina continuumsurveys([5]).
2
Arp 220 Arp 193
CO J=3−2 CO J=3−2
5x(HCN J=4−3) 100x(HCN J=4−3)
Fig.1 HCNJ=4–3(scaledbythenumbersintheupperrightofeachpanel)andCOJ=3–2spectraoffourLIRGs.TheHCN(4–3)/(1–0)brightness
temperatureratior43(HCN)isfoundtovaryfromr43(HCN)≤0.1(Arp193)tor43(HCN)∼0.8(Arp 220).
Table1 HCNvsHCO+lineratiosinArp220andNGC6240
Galaxy HCN(4−3) HCN(3−2) HCO+(4−3) HCO+(3−2)
(1−0) (1−0) (1−0) (1−0)
Arp220 0.8±0.2 1.0±0.3 0.33±0.10 0.27±0.10
NGC6240 0.6±0.2 1.0±0.3 0.21±0.06 0.24±0.08
2 HCNversusHCO+linesasdensegasmasstracers
TherotationaltransitionsoftheHCNandHCO+molecules
with their high dipole moments are currently the best (i.e.
mostluminous)densegastracersinLIRGs.AnHCNJ=1–0
linesurveyofsuchgalaxies([3])hasevenrevealedapoten-
tiallyconstantstarformationefficiency((cid:181) L /M (H ))
IR dense 2
for molecular gas at densities n≥104cm−3, attributed to
such a gas phase being the direct fuel of star formation in
molecularclouds([13]).Recentstudieshaveinsertedsome
doubtastowhetherHCNlinesaregoodtracersofsuchagas
phaseandsuggestedthoseofHCO+ instead([2]).Oursur-
vey revealsa large rangeof HCN line excitation in LIRGs
with otherwise similar IR, CO and HCN J=1–0 luminosi- Fig.2 CO,CI,CIIcoolingcontributionsintheULIRG/QSOMrk231
ties (Figure 1). In the case of Arp193 the observed very
lowHCN(4–3)/(1–0)lineratio(itsHCN4–3is>100times
weakerthanitsCOJ=3–2line!)iscompatiblewiththecom-
has been completed, allowing for a unique view into the
pleteabsenceofamassiveanddensemoleculargasphase.
global excitation conditions of its molecular gas. Analysis
On theotherhandforthe two archetypalULIRGsArp220
of the relative strengths of the CO J=1–0, 2–1, 3–2, 4–3,
andNGC6240the HCN ratiosaresignificantlylargerthan
6–5and HCN J=1–0, 4–3 lines (and 13CO J=2–1 from the
thecorrespondingones(inrotationallevel)ofHCO+(Table
literature,[4]) finds the low-J CO lines dominated by dif-
1). This, along with the fact that HCO+ rotational transi-
fuse (n ∼ few×102cm−3), and warm (T ∼ 90−100K)
tions also have ∼5−7 times lower critical densities than k
gas,whilethehigh-excitationCOJ=6–5,4–3andtheHCN
thoseofHCN,signifiesthatthelatteristracingadensergas
transitionstraceadenser(∼(1−3)×104cm−3)phasewith
phase and may thus may remain as the most suitable gas
somewhatlower temperatures(T ∼50−70K). The latter
mass tracer in galaxies once the excitation of its rotational k
dominatesthetotalmoleculargasmassinMrk231,andits
linesisproperlyaccountedfor.
total CO line cooling is similar to that of the CII line at
158m m(Figure2).Thissuggestsadifferentthermalbalance
3 ThedensemoleculargasinMrk231 tothatoflowerIR-luminositygalaxies,andmayexplainthe
relativeweaknessoftheCIIcoolinglineinsuchsystemsas
The prototypical ULIRG/QSO Mrk231 is the first object a result of the dominanceof dense Photon Dominated Re-
in our sample for which the full set of line observations gions(PDRs)formostoftheirmoleculargas([6]).
3
Table2 COlineratiosofa“warm”anda“cold”ULIRG
Galaxy CO(2−1) CO(3−2) CO(4−3) 12CO
(1−0) (1−0) (1−0) 13CO
17208-0014 1.02±0.22 0.72±0.17 0.74±0.23 ≥35
05189-2524 0.32±0.08 0.24±0.07 <0.32 6±2
4 A“cold”anda“warm”ULIRG
Moleculargasexcitationat levelsfar below those foundin
Mrk231 or other such galaxies used as templates for high
redshift starbursts (e.g. Arp220), have been uncovered by
ourongoingsurvey,furtherunderlyingtheneedforlocalCO
SLEDstobeestablishedforalargesampleoflocalLIRGs.
ThetwoULIRGs(LIR>1012L⊙)IRAS17208-0014and
IRAS05189-2524 have been found to have very different
CO line excitation(Figure4)despite similarIRcontinuum
andCO J=1–0line luminositiesandsimilar S60m m/S100m m
ratios.ForIRAS17208-0014theCObrightnesstemperature
Fig.3 COlineluminositiesofvarioushigh-redshiftobjectscompared
ratiosmeasuredaretypicalforthehigh-excitationstarform-
totheMrk231COdensegasSLEDtemplate(bars).Luminositiesare
ing molecular gas found in many such galaxies, but those
normalized to those of Mrk231 at either CO J=3–2 or J=2–1. Line
fluxesforthevariousobjectsaretakenfromtheliterature([11],[8]). foundforthe“cold”IRAS05189-2524aresurprisinglytyp-
ical of the quiescent molecular clouds found in the Milky
Waydisk(Table2).One-phaseradiativetransfermodelsus-
ing the Large Velocity Gradient (LVG) approximation and
3.1 Acomparisonwithhigh-zstarbursts constrainedbythelineratiosofIRAS17208-0014arecom-
patible with densities of n ∼ 103cm−3 and mostly warm
T ∼(70−100)K.The high12CO/13CO J=2–1 ratio mea-
k
Thehigh-JCOandtheHCNlinesdetectedinMrk231pro-
sured for this merger system (Table 2) is responsible for
vide good constraints on radiative transfer models which
the low CO J=1–0 optical depths (t ≤ 0.5) of this gas
10
thenallowagoodCOSLEDtobeconstructed.Itscompari-
phase,aresultofitshightemperaturesandlargeaverageve-
sonwithobjectsathighredshiftsshowsthatdespitethehigh locitygradientsdV/dR∼30kms−1pc−1(foranabundance
excitation of its dense molecular gas its CO SLED stands of [CO/H ]=10−4). The latter corresponds to a parame-
2
as rather average (Figure 3). However, strong lensing (af-
terK = (dV/dR)obs ∼30−50,typicalofanunboundgas
fecting severalobjectsin Figure 3) can selectively amplify vir (dV/dR)virial
phase([7]).Itisalsoworthnotingthatthehigh(4–3)/(1–0)
thehighlyexcitedCOlineemissionofcompactstarburstre-
CO ratio (comparableto that of CO(3–2)/(1–0))cannotbe
gionsattheexpenceofamuchmoreextendedlow-excitation
accountedbyasinglegasphasemodelandprobablysigni-
non-star forming molecular gas phase (e.g. [12]). Further-
fiestheemergenceofasecondwarmeranddenserphase.In
more, the high-J CO observations usually conducted after
severalLIRGs (e.g.Mrk231)such a phasehasbeen found
spectroscopic redshifts of high-z objects become available
to dominate CO J+1→J, J+1≥4 and HCN line emis-
mayselectmostlyfromasubsetofstarburstswherethe(star-
sionandcontainingmostoftheirmoleculargasmass,andis
forming)/(nonstar-forming)H gasmassratio(expectedto
2
mostlikelythedirectfueloftheirprodigiousstarformation.
varystronglyintheseveralmergersystemsamongLIRGs)
TheCOemissioninIRAS05189-2524ontheotherhand
is particularly large. The limited sensitivity of today’s cm
is compatible with gas of n ∼ 100cm−3, T ∼ 15K, and
andmm/sub-mmsingledishtelescopesandinterferometers k
K ∼1,typicalofthequiescentandmostlyself-gravitating
willfurtheraccentuatetheaforementionedbiases,whichmay vir
molecularcloudsfoundin the Galactic disk immersedin a
thusproduceCO SLEDsathighredshiftswhicharebiased
towardsthehighexcitationregime,andarethusunrepresen- FUVradiationfieldofG◦∼1(Habingunits).Insuchcon-
ditions high-J CO lines are expected to be very faint with
tativeofthefullexcitationrangeinstar-forminggalaxies.
CO(J+1−J)/(1−0)≤0.01forJ+1≥4forthebulkofthe
InthatrespectCO(andHCN)SLEDsoflocalULIRGs, moleculargas.Thisismuchlowerthanthatexpectedfrom
establishedfromobservationallywell-sampledJ-laddersof CO SLEDs of other ULIRGs such as Mrk231 or Arp220
rotationaltransitions,areindispensibleasbenchmarksunaf- orindeedmosthighredshiftobjects([8]),withonlytheex-
fectedbytheaforementionedbiases. tremelyredobjectHR10comingclose(Figure3).
4
Fig.4 COJ=4–3,3–2and2–1spectrallines(plottedwithdeclininglinethickness)forthe“cold”IRAS05189-2524(leftpanel)andthe“warm”
IRAS17208-0014(rightpanel)galaxies.
Thus starburst galaxies at high resdshifts with similar Acknowledgements Wewouldliketothankthetelescopeoperators
COSLEDstothatofIRAS05189-2524wouldremainunde- andstaffoftheJCMTandIRAM30-mtelescopefortheirassistance
duringtheextensiveobservationsthatmadethisprojectpossible.Spe-
tectedbytypicalhigh-JCOobservationswithtoday’sradio
cialthankstoJimHoge,PerFribergandIainCoulsonattheJCMTfor
telescopes, even after scaling their CO line fluxes by their
expertadviceandassistancewiththeobservationsduringmanyyears.
∼10 timeshigherIR luminosities. One of the goalsof the
CO and HCN multi-transition survey of LIRGs is to find
how often such low-excitation molecular SLEDs occur in References
starbursts,andseekoutcorrelationswithothergalaxychar-
acteristics (e.g. merger status, starburst age, dust tempera- 1. DownesD.,&SolomonP.M.1998,ApJ,507,615
2. Gracia´-Carpio J., Garcia´-Burillo S. & Colina L. 2006, ApJ, 640,
tures).Thismayinturnshedsomelightintohowotherwise
L135
vigorously star-forming galaxies can harbor large amounts 3. GaoY.,&SolomonP.M.2004,ApJ,606,271
oflow-excitationmoleculargas. 4. GlennJ.,&HunterT.R.2001,ApJS,135,177
5. HughesD.H.inTheNeutralISMinStarburstGalaxies,ASPCon-
ferenceSeries,Vol.320,2004,pg.317
6. KaufmanM.J.,WolfireM.G.,HollenbachD.J.,&LuhmanM.J.
1999,ApJ,527,795
5 Conclusions 7. PapadopoulosP.P.,&SeaquistE.R.1999,ApJ,516,114
8. Riechers,D.A.et.al.2006,ApJ,650,604
WereportpreliminaryresultsfromourongoingCOandHCN 9. SandersD.B.,&IshidaC.M.2004,inTheNeutralISMinStarburst
Galaxies,ASPConferenceSeries,Vol.320,2004,pg.230
multi-transitionof 30 local LIRGs, the largest such survey
10. SolomonP.M.,DownesD.,&RadfordS.J.E.1992,ApJ,387,
todate.Thesecanbesummarizedasfollows, L55
11. SolomonP.M.,&VandenBoutP.A.2005,ARA&A,43,677
12. Weiß,A.,Walter,F.,&Scoville,N.Z.2004,inTheNeutralISM
– AlargerangeofHCNlineexcitationprecludesthesim-
inStarburstGalaxiesASPConferenceSeriesVol.320,p.142
pleuseoftheHCNorHCO+J=1–0lineluminosityasa
13. Wu J., Evans II N.J., Gao Y.,Solomon P. M., ShirleyY. L., &
densegasmasstracer. VandenBoutP.A.2005,ApJ,635,L173
– In the two cases of Arp220 and NGC6240, the HCN
versustheHCO+lineexcitationsuggeststheformertrac-
ingthedensestgasphase.
– In the ULIRG/QSO Mrk231 the luminous CO J=4–3
and J=6–5 line emission emanates from a differentgas
phasethantheonedominatingthelow-JCOtransitions.
InthatphasetotalCOlinecoolingiscomparabletothat
of the CII line at 158m m. If confirmed for more such
galaxiesthisraisesthepossibilityofaverydifferentther-
mal balance than that seen in lower IR-luminosity sys-
temswheretheCIIlineisthedominantcoolant.
– ThediscoveryofverylowCOexcitationintheULIRG
IRAS05189-2524raises the possibility of a large exci-
tationbiasagainstdetectingsimilarobjectsathighred-
shiftsviatheirhigh-JCOlineemission.