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Astronomy&Astrophysicsmanuscriptno.denseGasInM33-publisher c ESO2012 (cid:13) November7,2012 HerM33es ⋆,⋆⋆ Dense gas in M33 ( ) C.Buchbender1,C.Kramer1,M.Gonzalez-Garcia1,F.P.Israel2,S.Garc´ıa-Burillo3,P.vanderWerf4,J.Braine5, E.Rosolowsky6,B.Mookerjea7,S.Aalto8,M.Boquien9,P.Gratier10,C.Henkel1115,G.Quintana-Lacaci12,S.Verley13, andF.vanderTak14 1 InstitutoRadioastronom´ıaMilime´trica,Av.DivinaPastora7,NucleoCentral,E-18012Granada,Spain e-mail:[email protected] 2 SterrewachtLeiden,LeidenUniversity,POBox9513,2300RALeiden,TheNetherlands 3 ObservatorioAstrono´micoNacional(OAN)-ObservatoriodeMadrid,AlfonsoXII,3,28014-Madrid,Spain 2 4 LeidenObservatory,LeidenUniversity,P.O.Box9513,NL-2300RALeiden,TheNetherlands 1 5 Laboratoired’AstrophysiquedeBordeaux,Universite´deBordeaux,OASU,CNRS/INSU,33271Floirac,France 0 6 UniversityofBritishColumbiaOkanagan,3333UniversityWay,KelownaBCV1V1V7Canada 2 7 DepartmentofAstronomy&Astrophysics,TataInstituteofFundamentalResearch,HomiBhabhaRoad,Mumbai400005,India v 8 DepartmentofEarthandSpaceSciences,ChalmersUniversityofTechnology,OnsalaObservatory,43994Onsala,Sweden o 9 Laboratoired’Astrophysique deMarseille-LAM,Universie´ Aix-Marseille&CNRS,UMR7326, 38rueF.Joliot-Curie,13388 N MarseilleCEDEX13,France 10 IRAM,300ruedelaPiscine,38406St.Martind’He`res,France 6 11 Max-PlanckInstitutfu¨rRadioastronomie(MPIfR),AufdemHu¨gel69,53121Bonn,Germany 12 DepartamentodeAstrof´ısica,CentrodeAstrobiolog´ıa,CSIC-INTA,Ctra.deTorrejo´naAjalvirkm4,28850Madrid,Spain ] 13 Dept.F´ısicaTeo´ricaydelCosmos,UniversidaddeGranada,Spain A 14 SRONNetherlandsInstituteforSpaceResearch,Landleven12,9747ADGroningen,TheNetherlands G 15 Astron.Dept.,KingAbdulazizUniversity,P.O.Box80203,Jeddah,SaudiArabia . h p ABSTRACT - o Aims. Weaimtobetterunderstandtheemissionofmoleculartracersofthediffuseanddensegasingiantmolecularcloudsandthe r influencethatmetallicity,opticalextinction,density,far-UVfield,andstarformationratehaveonthesetracers. t s Methods. UsingtheIRAM30mtelescope,wedetectedHCN,HCO+,12CO,and13COinsixGMCsalongthemajoraxisofM33at a aresolutionof 114pcandouttoaradialdistanceof3.4kpc.Optical,far-infrared,andsubmillimeterdatafromHerschelandother [ observatoriesco∼mplementtheseobservations.Tointerprettheobservedmolecularlineemission,wecreatedtwogridsofmodelsof photon-dominatedregions,oneforsolarandoneforM33-typesubsolarmetallicity. 3 Results. Theobserved HCO+/HCN line ratiosrange between 1.1 and 2.5. Similarlyhigh ratioshave been observed inthe Large v MagellanicCloud.TheHCN/COratiovariesbetween0.4%and2.9%inthediskofM33.The12CO/13COlineratiovariesbetween 3 9and15similartovariationsfoundinthediffusegasandthecentersofGMCsoftheMilkyWay.Stackingofallspectraallowed 6 HNCandC Htobedetected.TheresultingHCO+/HNCandHCN/HNCratiosof 8and6,respectively,lieatthehighendofratios 2 2 ∼ observed in a large set of (ultra-)luminous infrared galaxies. HCN abundances are lower in the subsolar metallicity PDR models, 3 whileHCO+abundancesareenhanced.ForHCNthiseffectismorepronouncedatlowopticalextinctions.TheobservedHCO+/HCN . 0 andHCN/COlineratiosarenaturallyexplainedbysubsolar PDRmodels oflow opticalextinctions between4and10magandof 1 moderatedensitiesofn3103–3104cm 3,whiletheFUVfieldstrengthonlyhasasmalleffectonthemodeledlineratios.Theline − 2 ratiosarealmostequallywellreproducedbythesolar-metallicitymodels,indicatingthatvariationsinmetallicityonlyplayaminor 1 roleininfluencingtheselineratios. : v Keywords. Galaxies:individual:M33-Galaxies:ISM-ISM:molecules-ISM:clouds-ISM:photon-dominatedregion(PDR) i X r a 1. Introduction with densitiesin excess of 104cm 3. Because stars condense − ∼ out of dense cores of giant molecular clouds (GMCs), both Owingtotheirlargedipolemoments,eventherotationalground molecules are promisingtracers of star formation (SF) and the state transitions of HCN and HCO+ trace dense molecular gas star formation rate (SFR). A series of papers (Gao&Solomon 2004a,b; Wuetal. 2005; Gaoetal. 2007; Baanetal. 2008; Sendoffprintrequeststo:C.Buchbender,e-mail:[email protected] Gracia´-Carpioetal. 2008; Wuetal. 2010; Garc´ıa-Burilloetal. ⋆ Based on observations with the IRAM 30m telescope, Herschel, 2012; Liu&Gao 2012) have recently investigated the corre- andotherobservatories.IRAMissupportedbyCNRS/INSU(France), lation of HCN (and partly HCO+) with far-infrared (FIR) lu- the MPG (Germany), and the IGN (Spain). Herschel is an ESA minosities (L ) in galactic GMCs, centers of nearby galax- spaceobservatorywithscienceinstrumentsprovidedbyEuropean-led FIR ies, and (ultra-)luminous galaxies (LIRGs/ULIRGs), showing PrincipalInvestigator consortiaand withimportant participationfrom NASA. that HCN is indeed a good tracer of SF and tightly correlated ⋆⋆ FITS files of the presented spectra of the ground-state tran- withLFIR.Thereare,however,findingsthatcomplicatethispic- sitions of HCN, HCO+, 12CO and 13CO are available at the ture (see e.g.Costagliolaetal. 2011). In contrastto CO, which CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via traces the bulk of the molecular gas-phase carbon, HCN and http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/. 1 C.Buchbenderetal.:DensegasinM33(HerM33es), scope. M33 is a spiral galaxy with Hubble type SA(s)cd lo- cated at a distance of only 840kpc (Table 1 and Fig. 1). It is the third largest member of the Local Group (after M31 and theMilkyWay).Observationsofsmall-scalestructuresinM33 donotsufferfromdistanceambiguitiesasgalacticobservations do.Itssmalldistanceallowsustoobtainaspatialresolutionof 114pc(i.e.28 )atafrequencyof89GHz(i.e.3.3mm)with ′′ ∼ the 30m telescope. M33 is seen at an intermediate inclination of i = 56 , yielding a short line-of-sight depth, which allows ◦ us to study individualcloud complexes.It is roughlyten times less massive than the Milky Way, and its overall metallicity is 12+logO/H=8.27,subsolarbyaboutafactortwo(Magrinietal. 2010). Therefore M33 is particularly interesting to compare with theMilkyWay, butalso with the LargeMagellanicCloud thathasametallicitysimilartoM33(Hunteretal.2007). Using the IRAM 30m telescope, Rosolowskyetal. (2011) (hereafter RPG11) observed four massive GMCs of more than 3105M in M33, searching for the ground-state transition HCN. T⊙hey detected HCN in only two of the GMCs. The ob- served GMCs are under-luminous in HCN by factors between two to seven relative to their CO emission when compared to averagedvaluesintheMilkyWay. Here,wepresentnew,deepobservationsoftheground-state transitionsofHCN,HCO+,13CO,andCOtowardssevenGMCs in M33 includingthree of the clouds observedby RPG11. All fourtracersaredetectedinsixoftheGMCs.Therelativeweak- nessofHCNemissionisconfirmedandinterpretedusingmodels ofphoton-dominatedregions(PDRs).Tobettercharacterizethe observedGMCs,weestimatedtheirstarformationrate,totalin- Fig.1: SPIRE 250µm map of M33 (Xilourisetal. 2012). The fraredluminosities,andFUV fieldsusinga largeancillarydata rectangledelineatesthe2 40 widestripalongthemajoraxis ′ ′ set compiled in the framework of the Herschel open time key × shown in Fig.2. Crosses mark the positions of the observed projectHerM33es(Krameretal.2010).Figure2showsasubset GMCs. ofthisdataset. Table1: BasicpropertiesofM33 HCO+ are minor species. Their abundancesare thereforemore stronglyinfluencedbythedetailsofthechemicalnetwork(e.g. Lo´pez-Sepulcreetal. 2010). HCO+ is linked via ion-molecule M33 References reactions to the ionization equilibrium. Its collisional cross- sectionisclosetoafactor10largerthanthatofHCN,whichis RA(2000) 01:33:51.02 linkedtothehydrocarbonchemistryandtheamountofnitrogen DEC(2000) 30:39:36.7 Type SA(s)cd 1 inthegasphase.Elementaldepletioninlow-metallicityenviron- Distance[kpc] 840 2 mentsmaythereforehaveastrongeffectonitsabundance. 11 (30m@230GHz)equalto 45pc MostextragalacticobservationsofHCNandHCO+ haveso ′′ 21 (30m@115GHz)equalto 86pc ′′ far been restricted to the nuclei of galaxies or their integrated 28 (30m@89GHz)equalto 114pc fluxes. Exceptions are a study of HCN and HCO+ emission LS′R′ velocity[kms 1] 180 − in the disk of M31 by Brouilletetal. (2005), LMC observa- PositionAngle[deg] −22.5 3 tions(Chinetal.1996,1997, 1998;Heikkila¨etal.1999),HCN Inclination[deg] 56 4 mapsofsevenSeyfertgalaxiesbyCurranetal.(2001),andalso R25 30.8′or7.5kpc HCN mapping along the major axis in 12 nearby galaxies by Gao&Solomon(2004a,b)(hereafterGS04a,b).Wealsorecom- References. (1) deVaucouleursetal. (1991); (2) Galletietal. (2004); mend the studies of HCN and CO and their ratios in M51 by Freedmanetal. (1991); (3) Patureletal. (2003); (4) Regan&Vogel Kunoetal.(1995)andSchinnereretal.(2010).Theinterstellar (1994);Zaritskyetal.(1989). medium (ISM) of nuclear regions are often subject to particu- larly strong heating sources because they are often dominated by starbursts with intense UV fields heating the gas or active galacticnuclei(AGNs)withstrongX-rayemission.Indeed,the HCN to HCO+ line intensity ratios are found to be systemati- 2. IRAM30mobservations cally higher in AGN-dominatedregions, such as in the central part of NGC1068, and low in pure starburst environments, as WeusedtheIRAM30mtelescopetoperformsingle-pointedob- inM82(e.g.Kohnoetal.2003;Imanishietal.2009;Kripsetal. servationsoftheground-statetransitionsofHCN,HCO+,12CO, 2008,2011). and13COtowardssevenGMCsinM33.Observationswerecar- WehavetargetedsevenGMCsalongthemajoraxisofM33 ried out between December 2008 and July 2012, comprising a out to a radial distance of 4.6kpc, using the IRAM 30m tele- totalof109hoursofobservingtime.In2008,weusedthe now 2 C.Buchbenderetal.:DensegasinM33(HerM33es), Fig.2: Observed positions towards seven giant molecular clouds (GMCs) within a 2 40 strip along the major axis of M33. ′ ′ × Thestripextendsfrom10 southofthegalacticcenterto33.3 north.Thecenterofthestripisat01:34:11.8+30:50:23.4(J2000). ′ ′ Circles indicate the 30m beam size of 28 at 90GHz. Panels show from top to bottom: integrated intensities of Hα emission ′′ (Hoopes&Walterbos 2000) 24; 70µm emission observed with Spitzer (Tabatabaeietal. 2007); continuum emission between 100µmand500µmobservedwithPACSandSPIREintheframeworkoftheHerM33esprogram(Krameretal.2010;Boquienetal. 2011;Xilourisetal.2012);12CO 2–130mobservationandHiVLAdata,bothtakenfromGratieretal.(2010).Alldataareshown attheiroriginalresolutions. decommissionedA100andB100receiversthathadabandwidth islessprobablefortheselines.Pleasenotethatthe12COdataof of500MHzandthe1MHzfilterbank,toobserveeachofthefour GMC1,GMC91,andGMC26aretakenfromRPG11 whoalso linesindividually. usedposition-switching. The bulk of the observations were carried out in 2009 em- All data were reduced using the GILDAS 1 software pack- ploying the new eight-mixer receiver EMIR and its instanta- age. Each scan was inspected and scans with poor baselines neous bandwidth of 16GHz in each polarization, connected or unreasonablyhigh rms values were rejected. Before averag- to the wide-band WILMA autocorrelator backend with 2MHz ing, linear baselines were fitted and removed. The data were spectralresolution.Thissetupallowedsimultaneousobservation regridded to a common velocity resolution. Spectra were con- ofHCNwithHCO+ and12COwith13CO.Oneadvantageofthe vertedfromtheTA∗ totheTmb scalebymultiplyingwiththera- simultaneous observations is that the relative intensity calibra- tio of forward efficiency(Feff = 95%) to main beam efficiency tionofthelinesisveryaccurate.Theobservationswerecarried (Beff =81%),takentobeconstantfortheobserved3mmlines. out in wobbler switching mode using the maximum available ThereducedspectraareshowninFig.3. throwof 120 andaswitchingfrequencyoftwoseconds.This Integrated intensities were extracted from the spectra on ′′ modeens±uresmorestablebaselinesthantheposition-switching a Tmb scale by summing all channels inside a velocity range mode.However,thevelocityresolutionofabout6kms 1 inthe aroundeachparticularline.Thevelocityrangewasdetermined − 3mmbandonlybarelyresolvesthespectrallinesofM33,which by eye for each position from the full width to zero intensity aretypically10–15kms 1 wide(Gratieretal.2010).Thebeam (FWZI)ofthe12CO 1–0lineandismarkedinFig.3.Wedeter- − sizes are 21 at 115GHz and 28 at 89.5GHz, corresponding minedσuncertaintiesoftheintegratedintensitiesbymeasuring ′′ ′′ toaspatialresolutionof86pcand114pc,respectively,inM33 thebaselinerms(Tmrmbs)ina300kms−1 windowcenteredonthe (cf.Table1). particularlineusingthecorresponding12CO 1–0FWZIasbase- The observationsof 12CO and 13CO were repeated in June line window and using σ = Trms √N∆v with the numberof mb res andJuly2012usingposition-switchingandtheFTSspectrome- channelsN andthechannelwidth∆v .Incasethe1σvalueis res ters with an off positionoutside of the disk of M33 to exclude higherthanthetypical10%calibrationerroroftheIRAM30m, thepossibilityofself-choppingeffectsinthespectra.Thelatter theformerisusedtoestimatetheobservationalerrorforthefol- werepresentinsomeoftheearlierwobbler-switched12COspec- lowinganalysis.Iftheintegratedintensitiesarelowerthan3σ, tra.DuetothehighcriticaldensityofthedensegastracersHCN weusethisvalueasanupperlimit.Table2liststheobservedin- and HCO+, as well as the observedvelocitygradientalong the majoraxisofM33(seeTable 2),we reckonthatself-chopping 1 http://www.iram.fr/IRAMFR/GILDAS 3 C.Buchbenderetal.:DensegasinM33(HerM33es), Fig.3: Spectra of the ground-statetransitions of HCN, HCO+, 12CO, and 13CO at the positions of seven GMCs along the major axis of M33 (cf. Fig.1,2). 12CO and 13CO have been observedin position switching;HCN and HCO+ with wobbler switching. The spectra are shown on a main beam brightnesstemperature scale (T ). The velocity resolution is given by the spectrometer mb withthelowestresolution,i.e. WILMA,andis5.4kms 1 incase of12CO and13CO, and6.7kms 1 forHCN andHCO+.Center − − velocitiesare listed in Table 2. The localstandardofrest (lsr)velocitydisplayedcovers300kms 1. The same velocityrangeis − usedtodeterminethebaseline(redlines),excludingthelinewindowsdeterminedfrom12CO 1–0(dottedlines),cf.Fig.A.1 tensities, intensity ratios, and further ancillary data. For details will yield additional insight into the properties of the ISM of onthelatterseeAppendixC.Errorestimatesaregiveninparen- M33.Inthefirstpaperson[Cii],wefocusedontheHiiregion thesesaftertheintegratedintensitiesinTable2. BCLMP302 (Mookerjeaetal. 2011) which is associated with GMCno3,andonBCLMP691(Braineetal., insubm.),which liesnearGMCno1. 3. GMCs:Selectionofpositionsandproperties Gratieretal.(2012)identifiedoverthreehundred12CO 2–1 Motivated by the HerM33es project, the GMCs were selected clumps in M33 and present a detailed view of each individual clump in Hα, 8µm, 24µm, and FUV, together with the corre- tolie withina2 40 widestripalongthemajoraxisofM33 ′× ′ spondingHI and12CO 2–1spectraand furthercomplementary showninFigs.1and2atarangeofgalacto-centricdistancesof data.ThesevenGMCsdiscussedhereareamongthe identified up to 4.6kpc. Three of the GMCs (GMC1, GMC26, GMC91) clumps.InTable2wegivethecorrespondingclumpnumbers. belongtothesampleofCO-brightcloudsstudiedbyRPG11in searchofHCNemission.WeaddedfourotherGMCs(no6,no3, no1,no2)to increase therangeof studiedgalacto-centricradii, 4. Observedlineratios aswellasphysicalconditions. Table 2 lists their observed propertiesand Appendix C de- 4.1.Spectraatindividualpositions:HCO+andHCN scribes in detail how they were derived. The masses of the molecular gas traced by CO, calculated using X -factors de- HCO+ is detected at six positions with 6 to 12mK peak tem- CO rived individually for every cloud as a function of integrated peraturesandwithgoodsignal-to-noiseratiosofatleastseven; CO1–0 intensities and total IR luminosity (cf. Appendix C.4), position no2 has not been detected. HCN emission is detected varybyafactor130between0.1105(GMCno2)and13105M atthesamesix positionswithsignal-to-noiseratiosoffourand (GMC1). The SFRs vary by more than a factor 50 and th⊙e better;positionno2isbuttentativelydetectedatasignal-to-noise far ultraviolet (FUV) field strengths by a factor larger than 20. ratioof3.5.TheHCO+/HCN ratiooflineintegratedintensities The GMC near the nucleus, GMC1, is the most massive and of positions where both moleculesare detected varies between showsthestrongestSFR,aswellasthehighestFUVflux,while 1.1and2.5(Table2).Below,wecomparetheobservedratiosin GMCno2at4.6kpcradialdistanceistheleastmassiveinmolec- detailwithratiosfoundintheMilkyWayandinothergalaxies. ularmassandshowsonlyweakactivity. AlthoughtheintegratedintensitieswefindforGMC26,GMC1, Individualareasofthestriphavebeenmappedin[Cii] and andGMC91differuptoafactoroftwofromthevaluesandup- otherFIRlinesintheframeworkoftheHerM33esproject,which perlimitsgiveninRPG11forthesamepositions,theyarecon- 4 C.Buchbenderetal.:DensegasinM33(HerM33es), Table2:Observedintensitiesandcomplementarydata no6 GMC1 GMC26 no3 GMC91 no1 no2 Clumpnumbera 42 108 128 256 245 300 320 RA[J2000] 01:33:33.77 01:33:52.40 01:33:55.80 01:34:07.00 01:34:09.20 01:34:16.40 01:34:21.77 DEC[J2000] +30:32:15.64 +30:39:18.00 +30:43:02.00 +30:47:52.00 +30:49:06.00 +30:52:19.52 +30:57:4.99 V [kms 1] -133.0 -168.0 -227.0 -257.0 -247.0 -266.0 -264.0 LSR − R[kpc] 2.01 0.11 0.873 2.18 2.51 3.38 4.56 I [Kkms 1] 7.1(10%) 7.2(10%)b 6.9(10%)b 9.4(10%) 21.6(10%)b 4.0(10%) 1.4(10%) 12CO(1 0) − FWHM12C−O(1-0)[kms 1]c 11(0.5) 8(0.2) 6(0.2) 8(0.1) 11(0.1) 9(0.2) 4(0.2) − I [Kkms 1]d 8.9(15%) 10.6(15%) 7.0(16%) 9.4(15%) 19.3(15%) 6.2(15%) 0.7(15%) 12CO(2 1) − I − [mKkms 1] 468(12%) 799(13%) 541(19%) 772(13%) 1690(10%) 369(12%) 132(15%) 13CO(1 0) − IHCO+(1−0)[mKkms−1] 205(10%) 182(10%) 66(10%) 119(10%) 97(15%) 77(10%) <12 I − [mKkms 1] 82(20%) 164(10%) 56(16%) 61(16%) 67(24%) 56(17%) 26(28%) HCN(1 0) − I − [mKkms 1] <43.9 <44.8 <15.6 <22.3 <47.5 <25.9 <20.4 HNC(1 0) − rm−s[mK]e 1.0 1.4 1.0 0.7 1.1 0.9 0.8 IHCO+(1 0)/IHCN(1 0) 2.5(0.2) 1.1(0.1) 1.2(0.1) 1.9(0.1) 1.4(0.3) 1.4(0.2) <0.5 I − /I − <0.5 <0.3 <0.3 <0.4 <0.7 <0.5 <0.8 HNC(1 0) HCN(1 0) I −/I −[%] 1.4(0.3) 2.9(0.4) 1.0(0.2) 0.8(0.2) 0.4(0.1) 1.7(0.4) 2.3(0.7) HCN(1 0) 12CO(1 0) IHCO+(1−0)/I12CO(1−0)[%] 3.5(0.5) 3.2(0.5) 1.1(0.2) 1.6(0.2) 0.6(0.1) 2.3(0.3) <1.0 I − /I − 15.1(2.4) 9.0(1.5) 12.8(2.8) 12.2(2.1) 12.8(1.8) 10.8(1.8) 10.6(1.9) 12CO(1 0) 13CO(1 0) L [103−Kkms 1−pc2] 87.7(8.8) 85.4(8.5) 87.9(8.8) 114.6(11.5) 248.9(24.9) 49.8(5.0) 17.3(1.7) ′CO − L [106L ] 4.4(0.1) 5.9(0.2) 1.4(0.0) 2.7(0.1) 1.3(0.0) 1.7(0.1) 0.3(0.0) TIR L /L [1⊙03] 3.5(0.7) 2.4(0.3) 1.6(0.3) 3.0(0.5) 1.3(0.3) 2.0(0.4) 0.8(0.2) TIR ′HCN L /L [103] 1.4(0.1) 2.2(0.2) 1.4(0.1) 1.5(0.2) 0.9(0.1) 1.5(0.2) >1.6 TIR ′HCO+ SFR[M Gyr 1pc 2] 35.9(4.3) 65.0(7.8) 6.6(0.8) 13.7(1.8) 4.0(0.6) 12.2(1.7) 1.2(0.1) − − ⊙X 5.1 6.9 1.6 3.2 1.5 2.0 0.3 CO M [105M ] 9.4(1.4) 5.8(0.9) 4.1(0.6) 8.0(1.2) 8.8(1.3) 5.2(0.8) 4.8(0.7) HI M [105M⊙] 9.6(1.0) 12.7(1.3) 3.0(0.3) 7.9(0.8) 8.2(0.8) 2.1(0.2) 0.1(0.0) H2 G ⊙ 37.3(1.2) 50.7(1.6) 11.6(0.4) 23.5(0.8) 11.3(0.4) 14.4(0.5) 2.5(0.1) 0 A 6.3(0.6) 6.1(0.5) 2.3(0.2) 5.3(0.5) 5.7(0.5) 2.4(0.3) 1.6(0.2) V Notes.Toppanel:lineintensitiesareontheT -scaleandontheiroriginalresolutions:12 for12CO 2–1,24 for12CO and13CO 1–0and28 mb ′′ ′′ ′′ for HCNand HCO+ 1–0.BottomPanel: lineratiosandcomplementary dataareon acommon resolution of 28 . SeeAppendix C for details. ′′ (a)12CO 2–1clumpnumbersfromGratieretal.(2012);(b)Rosolowskyetal.(2011);(c)FWHMsofGaussianfitstothehighresolution12CO 1–0 spectra(Fig.A.1);(d)Gratieretal.(2010);(e)BaselinermsofHCO+spectraatavelocityresolutionof6.7kms 1. − sistent within 3σ of the baseline rms of the observations from The stacked spectrum centered on 112GHz (Fig.4b) does RPG11. We attribute the discrepanciesto baseline problemsof notshowadditionaldetectionsotherthan13COand12CO even theRPG11data. after additional smoothing of the velocity resolution. Table 3 lists the integrated intensities and upper limits of all detected transitionsinthestackedspectrum,aswellastheircorrespond- 4.2.Spectraatindividualpositions:12COand13CO ingLTEcolumndensitiesandabundances,derivedasexplained inAppendixD. Emissionfrom12COand13COisdetectedatallsevenpositions, thoughvaryingby a factor of more than15 between GMCno2 and GMC91. The ratio of line integrated12CO vs. 13CO inten- 4.4.Comparisonwithothersources sities varies between 9 for GMCno1 and 15 for GMCno6. In 4.4.1. HCO+/COvs.HCN/CO Fig. A.1 we show the 12CO and 13CO spectra at a high resolu- tionof1 kms 1.TheresolvedlineshapesareGaussianandthe Forthecomparisonofdifferenttracers,alldatawereconvolved − correspondingFWHMsaregiveninTable2. to the same resolution of 28 . We account for the different ′′ intrinsic beam sizes of the CO, HCN, and HCO+ 1–0 obser- vations by multiplying with beam filling factors determined 4.3.Stackedspectra fromthe12CO 2–1map(Gratieretal.2010).Figure5compares the HCO+/CO vs. HCN/CO line intensities ratios observed in Stacking of all spectra allows improvement on the signal-to- M33, with those observed at nine positions in the disk of the noise ratios and detection of faint lines. Figure4a shows the AndromedagalaxyM31(Brouilletetal.2005,hereafterBR05). stacked spectrum of all data taken near 89GHz. It was created M31liesatasimilardistanceasM33of780kpcandhadbeen byshiftingallspectrainfrequencysuchthattheemissionlines observedwiththe30mtelescopeaswell.Therefore,bothstudies aligninfrequency.Individualspectrawereweightedbyintegra- obtainaboutthesamespatialresolutionof 114pc. tion time. In addition to the lines of HCN and HCO+, the re- For M33, we find HCN/CO ratio∼s in the range of sulting spectrum shows detections of CCH and HNC1–0. The 0.4%–2.9% (mean: 1.5 0.8%) and HCO+/CO ratios in averagebaselinermsis0.27mKat5.4kms 1resolution.HOC+ 0.6%–3.5% (mean: 1.9 ± 1.0%). BR05 finds comparable − istentativelydetectedwithanupperlimitof19mKkms 1,re- values in the spiral arms ±of M31: HCN/CO 0.75%–2.8% − sultinginalowerlimittotheHCO+/HOC+ ratioof5.8. (mean: 1.7 0.5%) and HCO+/CO 1.1%–3.9% (mean: ± 5 C.Buchbenderetal.:DensegasinM33(HerM33es), Table3:LTEcolumndensitiesfromthestackedspectra. C H HCN HCO+ HOC+ HNC 13CO(1-0) 12CO(1-0) H a 2 2 I[mKkms 1] 25.1 81.0 110.0 <18.8 13.7 522.0 6280.0 - − N[x]b 5.10e+12 8.03e+11 2.11e+11 <7.10e+10 4.18e+10 1.80e+15 8.55e+15 8.47e+20 N[x]/N c -8.40 -9.20 -9.78 <-10.25 -10.48 -5.85 -5.17 1.00 H2 Notes. (a)deducedfrom13CO,seeAppendixD;(b)columndensity;(c)logarithmicvalues. 5 4 no6 %] GMC1 O)[3 C +/)I( no1 O (a) C2 H I( no3 no2 I(HCN)=I(HCO+) 1 GMC91 GMC26 FitM33 FitM31(BR05) GalaxiesGA04 MWDiskHE97 0 M31BR05 M33thiswork 0 1 2 3 4 5 I(HCN)/I(CO)[%] Fig.5: Ratios of integrated intensities HCO+/CO vs. HCN/CO for M33 (red points: this work) and for M31 (green points: BR05). Upper/lowerlimits are denoted by arrows. Linear least (b) squaresfitstodatafromM33(redsolidline)andM31(BR05, Fig.4: (a) Stacked spectrum of all data taken in the frequency black dashed) are shown. Both fits exclude points with upper rangebetween87.2and90.8GHz.Thereddashedlinedenotes limits. The solid black line shows the angle bisector where the 3σ value average over the entire baseline. The HCN and I(HCO+)=I(HCN). The gray shaded areas display the range HCO+ lines are not shown up to their maximum peak temper- of the HCN/CO found in the disk of the Milky Way (MW) by ature.(b)Stackedspectrumofthewobblerswitcheddatataken (Helfer&Blitz 1997, HE97) (darker gray) and in a sample of in the frequencyrange between 108.1 and 115.5GHz. The av- normalspiralgalaxies(GA04b)(lightergray). erage3σvalueisshownasreddashedline.Thebaselinenoise increaseswithfrequencybecauseoftheincreasingatmospheric opacity.C18O,C17O,andCN aremarkedbutnotdetected.The 12CO and13CO linesare notshownuptotheir maximumpeak (Kunoetal.1995).UnlikeGS04binasampleofnormalgalax- temperature. ies, we do not find a systematic change in the HCN/CO ratio between regions in the center of M33, i.e. the inner 1kpc ∼ (hereGMC1,GMC26)andregionsatgreatergalacto-centricdis- tances(cf. Table2). GS04b reportsthat HCN/CO dropsfrom 2.0 0.7%).AlinearleastsquaresfittotheM33dataresultsin ∼ HC±O+/CO = (1.14 0.15%)HCN/CO+(0.18 0.14%).This 10% in the centers of normalgalaxies to 1.5%–3% in their ∼ ± ± disks'4kpc.InULIRGsandAGNstheratiosmayreachglobal isconsistentwithinerrorstothefitresultsobtainedbyBR05for the M31 data: HCO+/CO = 1.07%HCN/CO + 0.23%. We HCN/COvaluesashighas25%(GS04bandreferencestherein). GS04b attribute these high ratios to the presence of starbursts excluded positions with upper limits from the fit, i.e. position andarguethatHCN/COmayserveasastarburstindicator. GMCno2. In the Milky Way in the solar neighborhood values of HCN/CO are found between 0.7% – 1.9% (mean: 1.4 4.4.2. HCO+/HCNvs.HNC/HCO+ ± 2%), while the Galactic plane hosts on average 2.6 0.8% (Helfer&Blitz 1997). HCO+/CO values in the Galacti±c center The HCO+/HCN ratios observed in M33 vary between 1.1 rangebetween0.9%and7.6%(Riquelmeetal.2010). and 2.5, while the upper limits derivedfor the HNC/HCO+ ra- TheHCN/COratiosfoundintheLMCbyChinetal.(1998) tios vary between 0.2 and 0.5 (Fig. 6). The upper limit of the andHeikkila¨etal.(1999)liebetween3%and6%,andarethus HNC/HCNratioisatmaximum0.7(GMC91),whilethestacked higher than any value found in M33, M31, and also M51 spectrumwhereHNChasbeendetectedshowsaHNC/HCNra- where HCN/CO=1.1%–2% in the spiral arms are reported tioof0.17(Fig.6). 6 C.Buchbenderetal.:DensegasinM33(HerM33es), galaxiesfoundbyHuettemeisteretal.(1995)of<4,higherthan thetypicalratiosof1observedinstarburstandSeyfertgalaxies (e.g. Aaltoetal. 2002), and also higher than ratios of 1-3 ob- no6 0.4 servedinGalacticmolecularcomplexes(Woottenetal.1978). no3 M82 This extraordinary high ratio indicates that the physics or chemistry in M33 may be different from that of AGNs and GMC91 starbursts. The dominance of strong X-ray radiation in the nu- 0.2 0) clei of AGNs or even of starbursts may be important for the N(1- NGC1068 differences in the line ratios, since it creates X-ray-dominated C /1-0)H 0.0 GMC26 NGCM2r5k3231 rMegeiiojenrsink(X&DSRpsa)anthsa2t0c0h5a).nge the chemical abundances (e.g. +O( no1 ThesubsolarmetallicityofM33mayalsoplayaroleincre- C H GMC1 ating such a high HCN/HNC ratio. However, the HCN/HNC g Arp220 lo −0.2 ratio obtained in M33 is significantly higher than those ob- served in similar low-metallicity environments, such as N159, LMC(CH97,CH98) 30Dor in the LMC, as well as LIRS36 in the SMC, which are LIRGS/ULIRGS(BA10) no higher than 3.6 (besides a lower limit of 4.7 in N27 in the −0.4 LRMMIa33Rn33gGeGs−St1MaM/U.c0k3CLe1sIdR(BGRS0(5C)O1−10).5 HNC/HCN=0.17 0.0HNC/HCN=0.4 HN0C./H5CN=1 tTgShuMheaeCrvraea)nfl(outCereeehs,fifnoaoruesvtnueadbrlsy.ion1hl9aGig9rah8ml;aHHceCtteiacNilklGi/kcHiMiltNay¨CCeastlroa(aHlnt.ieuo1es9d.t9ote9em)saenniosdttecrsoeemetmpaal.rtoa1b9bl9ee5t)oa. logHNC(1-0)/HCO+(1-0) Fig.6: Comparison of integrated intensities HCO+/HCN vs. 4.4.3. HCNvs.totalinfraredluminosity(LTIR) HNC/HCO+ in M33 (red filled circles and square; arrows TheL /L ratiosobservedinM33(Table2)rangebetween TIR HCN indicate upper/lower limits) with values found in the LMC 1300 and 3500 L /Kkms 1pc2. These ratios lie at the upper − (Chinetal. 1997, 1998) (CH97, CH98; blue diamonds) and endof the valuesf⊙oundin Milky Way clouds(Wuetal. 2010). in luminous infrared galaxies compiled by Baanetal. (2008) Normal galaxies show on average total L /L ratios of TIR HCN (BA08;opensymbols)andby Costagliolaetal. (2011) (CO11; 900 70L /Kkms 1pc2(Gracia´-Carpioetal.2008,GS04a,b). − crosses). The dotted, dashed, and dot-dashed lines shows H∼ighe±r valu⊙es, in the range of 1100–1700L /Kkms 1pc2, − HNC/HCN=1,0.4,and0.17 (stacked value of M33), respec- are foundfor(U-)LIRGs(Graci∼a´-Carpioetal. 20⊙08, GS04a,b), tively. The gray shaded area shows the range of the observed while the highest reported ratios are found at the extreme end HCO+/HCNratiosinM31byBR05. of the LIRG/ULIRGdistribution,as wellas in high-zgalaxies, and range up to 3900 L /Kkms 1pc2 (Solomonetal. 2003; − Gaoetal. 2007;∼Gracia´-C⊙arpioetal. 2008; Wuetal. 2010; These ratios are compared with the ratios found in lumi- Garc´ıa-Burilloetal. 2012). Thus the LTIR/LHCN ratios in M33 nousinfrarednucleibyBaanetal.(2008)andCostagliolaetal. areamongthehighestratiosobserved. (2011)andinHiiregionsoftheLMCbyChinetal.(1997,1998) InSect.6below,weattempttoshedmorelightontheweak (cf.Fig.3ainBaanetal.(2010)).TheHCO+/HCNratiosarealso HCN emissionandinvestigatethelineratiosofCO, HCN, and comparedto the range foundin the disk of M31 by BR05 (cf. HCO+ usingmodelsofphoton-dominatedregionsthattakenot Fig.6). only the chemical network into account, but also the detailed TheHCO+/HCNratios,foundinM33inthesixGMCswith heating and cooling processes of a cloud as well as radiative clear detections, lie at the upper end of the distribution of val- transfer. uesfoundinLIRGs.WhilethestarburstgalaxyM82exhibitsa higherratiothanthe AGNNGC1068(1.6vs.0.9),these ratios 4.4.4. 12CO/13COlineratio lie within the scatter of values and their errors observed in the diskofM33. InoursampleofGMCsinM33wefind12CO/13COlineinten- TheHCO+/HCNratiosinM33lieintheoverlapregionbe- sity ratios between 9 and 15. There is no obvious correlation tweentheonesfoundinM31andthosefoundintheLMC.We withgalacto-centricdistance,FUVstrength,orSFR.Inastudy find neitherratios as highas in the LMC, i.e. 3.5,nor ratiosas of eight GMCs in the outer disk of M33, Braineetal. (2010) lowasinM31,i.e.0.51.Interestingly,alldetectedregionsinthe foundsimilarratiosbetween8.9and15.7.IntheMilkyWay,the LMCaresituatedinthesameparameterspaceasthosedetected isotopicratioof12C/13Cvariesbetweenvaluesof80–90inthe inM33. solarneighborhoodand20intheGalacticcenter(Wilson1999). The detection of HNC in the stacked spectrum allows us Polketal.(1988)studiedthe12CO/13COlineratiointheMilky to derive an average HCO+/HNC ratio of 7.8 (Table3) for the Way, for several large regions of the plane in comparison with GMCsobservedinM33.Thisratioliesattheveryhighendof the averageemissionfromGMCs andofthe centersofGMCs. therangeofvaluesfoundinanyoftheothersamplesplottedin Theyfindthattheaveragevaluerisesfromthreeinthecentersof Fig.6. GMCs,to4.5averagedoverGMCs,to6.7fortheGalacticplane, More remarkably, the HCN/HNC value of 5.9 from the withpeaksof 15.Theirinterpretationisthatthehigherratios ∼ stacked spectra is higher than any from the other samples we observedin theplane arecaused bydiffusegasofonlymoder- compare with in Fig.6. Furthermore, it is higher than ratios ateopticalthicknessin12CO.Asimilarinterpretationmayhold observed over the surface of IC342 which are only 1 2 inM33.Thefractionofdensegaswithinthebeamandoptical ∼ − (Meier&Turner 2005), higher than the ratios in a range of deptheffectsmayaffecttheratiosobservedinM33. 7 C.Buchbenderetal.:DensegasinM33(HerM33es), Table4:MolecularabundancesinM33andtypicalexamplesof HCO+/HCN abundance ratio of 0.5 in the diffuse ISM of the galacticandextra-galacticsources. MilkyWay,similartotheratiofoundinthesolarneighborhood, andsimilartothehigherratiosfoundinM33. Source HCO+ HCN HNC C H References 2 M33min 10.3 9.8 - - − − M33stacked 9.8 9.2 10.5 8.4 6. PDRmodels − − − − M33max 9.5 9.1 - - LMCN159 −9.7 −9.7 10.2 - 1,2 6.1.Setup − − − NGC253 8.8 8.3 9.0 7.7 1,3 − − − − To improve on the LTE analysis and to better understand why M82 8.4 8.4 8.8 7.6 1,3 − − − − HCNislessluminousthanHCO+ inM33,wecomparetheob- IC342GMC-A 8.7 7.5 5 − − served HCO+/HCN, HCN/CO, and HCO+/CO line ratios with OrionBar 8.5 8.3 9.0 8.7 1 TMC-1 −8.4 −7.7 −7.7 −7.1 1,6 modelsofphoton-dominatedregions(PDRs)usingtheMeudon Transl.Cl. −8.7 −7.4 −8.6 −- 1,4 PDR code (LePetitetal. 2006; GonzalezGarciaetal. 2008). − − − The line intensities of the molecules13CO, HNC, and C H are 2 Notes.Entriesshowlog(N(X)/N(H2)). notmodeled. Weranagridofmodelsfordifferentdensitiesn =0.1,0.5, References. (1) Omont (2007); (2) Johanssonetal. (1994); H (3) Mart´ınetal. (2006); (4) Turner (2000); (5) Meier&Turner 1, 5, 10, 50, 102 104cm−3, FUV fields G0 = 10,50,100 (2005)andreferencestherein;(6)Hogerheijde&Sandell(2000) in Habing units2,×and optical extinctions Av=2–50mag, i.e. in stepsof logA 0.2. We calculatedthisgridofmodelsfor a v ∼ solar and a subsolarmetallicity. The subsolarone is tailored to Ratios in the Magellanic clouds are observed by describethemetallicityinthediskofM33.SeeAppendixAfor Heikkila¨etal. (1999) to cover values between 5 and 18, a adetaileddescriptionofthemodelsetup. somewhat wider range than found in M33. Unlike in M33, a gradient is seen on larger scales in a set of IR-bright nearby galaxies, dropping from values of about ten in the center to 6.2.Results valuesas low as two at larger radii(Tanetal. 2011). Although The modeled HCN/CO line ratio (Fig.7, TablesE.1 and E.2) Aaltoetal. (1995) finds variations with galacto-centric radius hardly varies with metallicity, n or FUV. It varies, however H in somegalaxiesof theirIR-brightsample,othergalaxiesexist strongly,withA .Afteraninitialdropofuptoanorderofmag- where the 12CO/13CO stays constant with radius. They report v nitude for extinctions less than about 4 mag, it rises until A v a mean value of 12 for the centers of most galaxies in their 20magandthensaturatesatavaluethatisnearlyindependent ∼ sampleexceptforthemostluminousmergerswithratiosof'20 ∼ ofanyoftheinputparameters. (seealsoCasolietal.1992). Incontrast,themodeledHCO+/HCNratioshowsdifferences between the two metallicities for A .10mag. The subsolar v modelshowshigherratiosthanthe solar modelfora given A , 5. Molecularabundances v FUVfield,anddensity.StrongFUVfieldsandlowdensitiesin- WeusetheobservedlineintensitiesofHCO+,HCN,HNC,and creasetheratio.Athigheropticalextinctionstheratiosarehardly C Htoestimatethemolecularcolumndensitiesandabundances, influenced by changes in metallicity, A or FUV. This reflects 2 v assuminglocalthermodynamicequilibrium(LTE)andoptically thevariationinHCNandHCO+ abundances.AtlowA ,subso- v thinemission.DetailsonthecalculationsaregivenAppendixD, lar HCN abundancesare lower than solar ones by up to a fac- withresultsshowninTableD.1.Theabundancesareonlylower torof 0.6dex,while HCO+ isenhancedin the subsolarmodels limitsincasetheemissionofHCNandHCO+isopticallythick. byup to 0.7dex.Also the CO abundancesshow a cleardepen- In comparison with our PDR-model analysis below we find, denceonmetallicity.Forallinputparameters,itsabundancein however,thatforthebest-fittingmodelstoourobservationsthe thesubsolarmodelsis 0.6dexlowerthaninthesolarmodels. opticaldepths(τ)inthecentersofthelinesofHCNandHCO+ Thisdirectlyreflectsthe∼underabundanceofcarbonof0.6dexin areτ<0.1,suggestingthatemissionislikelytobeopticallythin thesubsolarmodels.Asaresult,theHCN/COlineratioisfairly (cf.Sect.7).InTable4,wecomparetheabundanceswiththose independentofmetallicity. found in other galaxies (LMC, NGC253, M82, IC342), in se- For optical extinctions in the range of A 8 mag, where v ≤ lectedGalacticsources,thephoton-dominatedregionOrionBar, thebulkofmoleculargasingalaxiesresides(Tielens2005),the the dark cloud TMC-1, and a translucent cloud. The estimated HCN/COratioincreaseswithincreasingdensities.Ingeneralin columndensitiesforthestackedvaluesaregiveninTable3. LIRGs/ULIRGs where most of gas has higher density than in The abundances of HCO+, HCN, and HNC found in M33 normalgalaxies,theHCN/COisalsohigher(GS04a,b). are very similar to those found in the LMC cloud N159. The abundancesderivedfromthestackedspectrumofM33agreeto within 0.5dex with those of N159. Galactic sources have val- 7. ComparisonwithPDRmodels ues that are higher by more than an order of magnitude. The Figure7 shows the range in observed intensity ratios Orion Bar, for example, shows 1.8dex to 0.8dex higher abun- (cf.Table2), where the plot shows that the optical extinc- dances. The good agreement with the LMC may be driven by tions play a decisive role in determining the line ratios. High its similar metallicity of 0.3–0.5 relative to the solar metallic- extinctions of A >16mag are inconsistent with the observed ity (Hunteretal. 2007), which is only slightly lower on aver- v HCN/CO ratios. Similarly, high densities of 105cm 3 cannot agethaninM33(Magrinietal.2007,2010).However,theC H − 2 abundanceobservedinM33andintheOrionBaragreewithin 2 Habing units correspond to an average interstellar radiation field 0.3dex. (ISRF)between6eV hν 13.6eVof1.610 3ergcm 2s 1 (Habing − − − The LTE HCO+/HCN abundance ratios in M33 range be- 1968).Anotherunittha≤tisfr≤equentlyusedistheDraineunitofthelocal tween0.2and0.5(TableD.1).Godardetal.(2010)measuredan averageISRF,whichis1.7 G inHabingunits. × 0 8 C.Buchbenderetal.:DensegasinM33(HerM33es), 7 6 nH=1×103cm−3 nH=5×103cm−3 nH=1×104cm−3 nH=5×104cm−3 5 N C4 H +/O3 C H2 1 0 7 0.4 0.6 0.8 1.0 1.2 1.4 1.6 6 nH=1×105cm−3 nH=5×105cm−3 nH=1×106cm−3 logAv(mag.) 5 subsolar N C4 solar H +/O3 G0=10 C H2 G0=50 1 G0=100 0 observedM33 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.4 0.6 0.8 1.0 1.2 1.4 1.6 logAv(mag.) logAv(mag.) logAv(mag.) 1.5 nH=1×103 nH=5×103 nH=1×104 nH=5×104 1.0 O) /NC 0.5 C 0H 0.0 0 1 g(−0.5 o l −1.0 1.5 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.0 nH=1×105 nH=5×105 nH=1×106 logAv(mag.) O) subsolar /NC 0.5 solar C 0H 0.0 G0=10 0 g(1−0.5 G0=50 lo G0=100 −1.0 observedM33 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.4 0.6 0.8 1.0 1.2 1.4 1.6 logAv(mag.) logAv(mag.) logAv(mag.) Fig.7:PDRmodellineratiosforsubsolar(solidlines)andsolarmetallicity(dashedlines):HCO+/HCN(top)andHCN/CO(bottom). DifferentcolorsindicatedifferentFUVfieldstrengthsG =10(red),50(green),and100(blue).Everypanelofasubfigureshows 0 theresultsforonedensity;fromlefttorightandtoptobottomn =0.1,0.5,1,5,10,50,and102 104cm 3.Grayareasmarkthe H − × rangeofobservedratiosinM33.Thedashedhorizontallinesshowthevaluesfromthestackedspectra. reproduce the measured HCO+/HCN ratios. Interestingly, withaverageline-of-sightcolumndensitiesof8mag.Thebeam- metallicityonlyplaysa minorrole.Ingeneral,bothmetallicity fillingfactorΦ derivedfromratioofthebeam-averagedTIR FUV modelsallowtheobservedrangeoflineratiostobereproduced. intensity to the fitted local FUV field is 30%; i.e., the fitted ∼ Toquantifytheagreementbetweenthelineratiosofthedif- FUVfieldstrengthsaresignificantlyhigherthanexpectedfrom ferentmodelsand the individualobservedcloudsincludingthe theobservations.Thesameholdsforthebeamfillingfactorsde- stacked spectrum,we use a χ2 fit routine.To geta handleonto ducedfromtheratiosofextinctionsderivedfromCOandtheA v the errors of the best-fitting models, a Monte Carlo analysis is of the best-fitting models, which are about Φ =50%. This is Av employed.The details on the fitting and Monte Carlo methods notsurprising,however,andindicatesthatemissionisclumped are given in Appendix B. Table 5 shows the input parameters withinthe114pcbeam.Fromthemodelsofthecalculatedgrid A ,n ,andFUVofthebest-fittingsubsolarandsolarmetallic- closest to the best fit, i.e., Av = 6 and 10mag, n = 1104, v H H itymodels,i.e.thosehavingthelowestχ2,forthestackedvalues and FUV = 50G , we find optical depths (τ) in the centers of 0 andeachobservedcloud. the lines of HCN and HCO+ that lie between 0.02–0.1 and 0.07–0.1,respectively.Thusbothlinesareopticallythin.12CO ismoderatelyopticallythickwithopticaldepthsofτ4–25.The 7.1.Stackedratios linewidthassumedintheMeudonPDRcodeis 3kms 1 (cf. − ∼ Thebest-fittingmodelsforreproducingthestackedHCO+/HCN AppendixA). and HCN/CO ratios of 1.4 and 1%, respectively, that describe theaveragedGMCpropertiesare A =8mag,n =3104cm 3, v H − and FUV=68G . Emission stems from moderately dense gas 0 9 C.Buchbenderetal.:DensegasinM33(HerM33es), Table5:Best-fittingPDRmodels HCO+/HCN HCN/CO A n FUV Φ a Φ b bestχ2c v H Av FUV [%] [mag.] [cm 3] [G ] − 0 subsolarmetallicityModels Stacked 1.4 0.2 1.0 0.1 8 3 (3 4)104 68 24 0.5 0.2 0.3 0.1 0.1 0.1 NO6 2.5±0.2 1.4±0.3 4±2 (6±4)103 41±16 1.7±1.1 0.9±0.4 1.4±0.5 NO3 1.9±0.1 0.8±0.2 8±2 (3±2)103 27±20 0.7±0.2 0.8±0.6 0.6±0.3 GMC91 1.4±0.3 0.4±0.1 6±2 (1±5)104 54±44 0.9±0.3 0.2±0.2 0.1±0.1 NO1 1.4±0.2 1.7±0.4 7±4 (3±4)104 42±40 0.3±0.2 0.3±0.3 0.2±0.1 GMC1 1.1±0.1 2.9±0.4 10± 3 (1±2)104 30±37 0.6±0.2 1.7±2.1 0.7±0.5 GMC26 1.2±0.1 1.0±0.2 9 ±2 (1±3)104 67±24 0.3±0.1 0.2±0.1 0.1±0.1 ± ± ± ± ± ± ± ± solarmetallicitymodels Stacked 1.4 0.2 1.0 0.1 10 1 (5 1)103 47 12 0.4 0.0 0.5 0.1 0.6 0.4 NO6 2.5±0.2 1.4±0.3 2 ±0 (5±1)103 100± 4 3.1±0.8 0.4±0.0 3.4±1.2 NO3 1.9±0.1 0.8±0.2 6±3 (2±2)103 81 ±32 0.9±0.5 0.3±0.1 3.4±0.9 GMC91 1.4±0.3 0.4±0.1 4±1 (1±1)103 98±11 1.4±0.3 0.1±0.0 0.5±0.4 NO1 1.4±0.2 1.7±0.4 7±4 (5±3)103 50±43 0.4±0.2 0.3±0.2 0.2±0.2 GMC1 1.1±0.1 2.9±0.4 11± 2 (3±6)103 26±34 0.6±0.1 1.9±2.6 0.5±0.2 GMC26 1.2±0.1 1.0±0.2 9 ±2 (5±1)103 52±17 0.2±0.1 0.2±0.1 0.4±0.3 ± ± ± ± ± ± ± ± Notes.(a)beam-fillingfactorderivedfromA (12CO)/A (Model);(b)beam-fillingfactorderivedfromFUV(TIR)/FUV(Model);(c)averageχ2ofthe v v best-fittingmodels. 7.2.Individualregions GMC1andGMC26 Thesetwocloudshostthelowestobserved ratiosofHCO+/HCNof1.1–1.2.Here,thesolarmodelsprovide Here,wefocusonindividualregionsgroupedbytheirparticular slightly better or equalfits than the subsolar models. However, HCO+/HCN ratios and thus their best-fit Av values:no6 show- sincetheISMofM33issubsolar,herewediscussonlythebest ingahighratioof2.5,GMC91,no3,andno1haveintermediate fitstothesubsolarmodels.TheseGMCshavesimilarbest-fitting ratiosof1.4-1.9andGMC26,GMC1havingratiosof1.1-1.2. inputparametersof A 9–10magandn 104cm 3,while the v H − ∼ best-fitting FUV field strengths are 30G and 70G , respec- 0 0 ∼ tively.Howevercomparingtheirphysicalpropertiesin Table2, GMCno6 This cloud shows the highest HCO+/HCN ratio of again, these two clouds are actually not at all alike. GMC1 is 2.5,while atthe same time the HCN/CO ratiois relativelylow located in the very center of M33 and is by far the most mas- with1.4andweakerthanexpectedfromthelinearfittotheM33 sivecloudinoursample.Itisactivelyformingstarsatarateof data(cf.Sect.4.4.1andFig.5).GMCno6isbestfittedbysubso- 65M Gyr 1pc 2,thehighestinoursample,andhasancorre- − − larmodelsthatyieldalowbest-fittingvalueforAv of 4mag, spond⊙inglyhighFUVfieldof50.7G .GMC26hasmuchlower while the best-fitting density and FUV strength are 6∼103cm−3 HCN/COratios,anditsSFRisonly60.6M Gyr 1pc 2thesec- − − and40G0,respectively.Thebeam-fillingfactorderivedfromAv ondlowestinthesamplewithexceptionof⊙no2. is 1.7, indicating that emission completely fills the beam with For all best-fitting solutions of the six individual positions severalcloudsalongtheline-of-sight.Thiscloudhasthesecond wefindthatthemodeledopticaldepthsofHCNandHCO+ are higheststarformationrateofoursampleof35.9M Gyr 1pc 2, − − τ 0.1,whichrendersemissionoftheselinestobelikelyopti- andthesameholdsfortheFUVfieldstrengthof37⊙.3G . ≤ 0 callythin.Thisalsojustifiestheassumptionofopticalthinemis- sion for the LTE analysis in Sect5. Indeed, the PDR modeled abundances of both molecules are comparable to the ones de- GMC91, no3, and no1 The line ratios of these three clouds rivedfromLTE(cf.Table4andE.2). are best described by subsolar models. The best-fitting A are v In conclusion,it is noteworthyto repeat that the line ratios similar with 6–8mag. So are the FUV 30–50G and the den- 0 studiedherearefairlyindependentofthemetallicity,SFactivity, sities 3103cm 3 – 3104cm 3, and no1 and no3 have similar − − and FUV field strength of the parent GMC, while the optical SFRratesof 13M Gyr 1pc 2, whileno3isa factoroffour more massive∼than n⊙o1 wit−h M− =8105M . GMC91 lies at extinctionhasamajorinfluenceonthemodeledlineratios. only 320pc distance in close viHc2inity of GM⊙Cno3 and is only slightly moremassive than the same. Itis the mostCO intense 8. Summary cloud in our sample while its HCN and HCO+ emission is rel- atively weak. This renders GMC91 somewhat peculiar and re- We present IRAM 30m observations of the ground-state tran- sults in a low HCO+/CO ratio of 0.6% and, as already found sitions of HCN, HCO+, 12CO and 13CO of seven GMCs dis- byRPG11,a particularlylow HCN/CO intensityratioof 0.4% tributed along the major axis in the disk of the nearby spiral (Table2).TheHCN/COratioofGMC91ismuchlowerthanthe galaxy M33. We achieve a spatial resolution of 114pc at a ∼ ratiosobservedin thedisk ofthe MilkyWay byHelfer&Blitz frequencyof89GHz. (1997), who find 2.6% 0.8% and also in the inner disks (5– The molecular gas masses of the target GMCs vary by a 10kpc) of normalgalax±iesby GS04b who find 4% 2%. The factorof 130between0.1105M (GMCno2)and13105M relatively weak HCN and HCO+ emission may indi±cate a low (GMC1)a∼ndthestarformationrate⊙sderivedfromHαand24µm⊙ fraction of dense mass in GMC91, which thusmay be a rather imagesvarybyafactorofmorethan50.TheFUVfieldstrengths quiescentGMCwith onlya low SFR. Indeed,ithasthe lowest showavariationofmorethanafactor20.Below,wesummarize starformationrateof4M Gyr 1pc 2ofallobservedclouds. themainresults. − − ⊙ 10

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Astronomy&Astrophysicsmanuscriptno.denseGasInM33-publisher c ESO2012 (cid:13) November7,2012 HerM33es ⋆,⋆⋆ Dense gas in M33 ( ) C.Buchbender1,C.Kramer1,M.Gonzalez-Garcia1,F.P.Israel2,S.Garc´ıa-Burillo3,P.vanderWerf4,J.Braine5, E.Rosolowsky6,B.Mookerjea7,S.Aalto8,M.Boquien9,P.Gratier10,C.Henkel1115,G.Quintana-Lacaci12,S.Verley13, andF.vanderTak14 1 InstitutoRadioastronom´ıaMilime´trica,Av.DivinaPastora7,NucleoCentral,E-18012Granada,Spain e-mail:[email protected] 2 SterrewachtLeiden,LeidenUniversity,POBox9513,2300RALeiden,TheNetherlands 3 ObservatorioAstrono´micoNacional(OAN)-ObservatoriodeMadrid,AlfonsoXII,3,28014-Madrid,Spain 2 4 LeidenObservatory,LeidenUniversity,P.O.Box9513,NL-2300RALeiden,TheNetherlands 1 5 Laboratoired’AstrophysiquedeBordeaux,Universite´deBordeaux,OASU,CNRS/INSU,33271Floirac,France 0 6 UniversityofBritishColumbiaOkanagan,3333UniversityWay,KelownaBCV1V1V7Canada 2 7 DepartmentofAstronomy&Astrophysics,TataInstituteofFundamentalResearch,HomiBhabhaRoad,Mumbai400005,India v 8 DepartmentofEarthandSpaceSciences,ChalmersUniversityofTechnology,OnsalaObservatory,43994Onsala,Sweden o 9 Laboratoired’Astrophysique deMarseille-LAM,Universie´ Aix-Marseille&CNRS,UMR7326, 38rueF.Joliot-Curie,13388 N MarseilleCEDEX13,France 10 IRAM,300ruedelaPiscine,38406St.Martind’He`res,France 6 11 Max-PlanckInstitutfu¨rRadioastronomie(MPIfR),AufdemHu¨gel69,53121Bonn,Germany 12 DepartamentodeAstrof´ısica,CentrodeAstrobiolog´ıa,CSIC-INTA,Ctra.deTorrejo´naAjalvirkm4,28850Madrid,Spain ] 13 Dept.F´ısicaTeo´ricaydelCosmos,UniversidaddeGranada,Spain A 14 SRONNetherlandsInstituteforSpaceResearch,Landleven12,9747ADGroningen,TheNetherlands G 15 Astron.Dept.,KingAbdulazizUniversity,P.O.Box80203,Jeddah,SaudiArabia . h p ABSTRACT - o Aims. Weaimtobetterunderstandtheemissionofmoleculartracersofthediffuseanddensegasingiantmolecularcloudsandthe r influencethatmetallicity,opticalextinction,density,far-UVfield,andstarformationratehaveonthesetracers. t s Methods. UsingtheIRAM30mtelescope,wedetectedHCN,HCO+,12CO,and13COinsixGMCsalongthemajoraxisofM33at a aresolutionof 114pcandouttoaradialdistanceof3.4kpc.Optical,far-infrared,andsubmillimeterdatafromHerschelandother [ observatoriesco∼mplementtheseobservations.Tointerprettheobservedmolecularlineemission,wecreatedtwogridsofmodelsof photon-dominatedregions,oneforsolarandoneforM33-typesubsolarmetallicity. 3 Results. Theobserved HCO+/HCN line ratiosrange between 1.1 and 2.5. Similarlyhigh ratioshave been observed inthe Large v MagellanicCloud.TheHCN/COratiovariesbetween0.4%and2.9%inthediskofM33.The12CO/13COlineratiovariesbetween 3 9and15similartovariationsfoundinthediffusegasandthecentersofGMCsoftheMilkyWay.Stackingofallspectraallowed 6 HNCandC Htobedetected.TheresultingHCO+/HNCandHCN/HNCratiosof 8and6,respectively,lieatthehighendofratios 2 2 ∼ observed in a large set of (ultra-)luminous infrared galaxies. HCN abundances are lower in the subsolar metallicity PDR models, 3 whileHCO+abundancesareenhanced.ForHCNthiseffectismorepronouncedatlowopticalextinctions.TheobservedHCO+/HCN . 0 andHCN/COlineratiosarenaturallyexplainedbysubsolar PDRmodels oflow opticalextinctions between4and10magandof 1 moderatedensitiesofn3103–3104cm 3,whiletheFUVfieldstrengthonlyhasasmalleffectonthemodeledlineratios.Theline − 2 ratiosarealmostequallywellreproducedbythesolar-metallicitymodels,indicatingthatvariationsinmetallicityonlyplayaminor 1 roleininfluencingtheselineratios. : v Keywords. Galaxies:individual:M33-Galaxies:ISM-ISM:molecules-ISM:clouds-ISM:photon-dominatedregion(PDR) i X r a 1. Introduction with densitiesin excess of 104cm 3. Because stars condense − ∼ out of dense cores of giant molecular clouds (GMCs), both Owingtotheirlargedipolemoments,eventherotationalground molecules are promisingtracers of star formation (SF) and the state transitions of HCN and HCO+ trace dense molecular gas star formation rate (SFR). A series of papers (Gao&Solomon 2004a,b; Wuetal. 2005; Gaoetal. 2007; Baanetal. 2008; Sendoffprintrequeststo:C.Buchbender,e-mail:[email protected] Gracia´-Carpioetal. 2008; Wuetal. 2010; Garc´ıa-Burilloetal. ⋆ Based on observations with the IRAM 30m telescope, Herschel, 2012; Liu&Gao 2012) have recently investigated the corre- andotherobservatories.IRAMissupportedbyCNRS/INSU(France), lation of HCN (and partly HCO+) with far-infrared (FIR) lu- the MPG (Germany), and the IGN (Spain). Herschel is an ESA minosities (L ) in galactic GMCs, centers of nearby galax- spaceobservatorywithscienceinstrumentsprovidedbyEuropean-led FIR ies, and (ultra-)luminous galaxies (LIRGs/ULIRGs), showing PrincipalInvestigator consortiaand withimportant participationfrom NASA. that HCN is indeed a good tracer of SF and tightly correlated ⋆⋆ FITS files of the presented spectra of the ground-state tran- withLFIR.Thereare,however,findingsthatcomplicatethispic- sitions of HCN, HCO+, 12CO and 13CO are available at the ture (see e.g.Costagliolaetal. 2011). In contrastto CO, which CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via traces the bulk of the molecular gas-phase carbon, HCN and http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/. 1 C.Buchbenderetal.:DensegasinM33(HerM33es), scope. M33 is a spiral galaxy with Hubble type SA(s)cd lo- cated at a distance of only 840kpc (Table 1 and Fig. 1). It is the third largest member of the Local Group (after M31 and theMilkyWay).Observationsofsmall-scalestructuresinM33 donotsufferfromdistanceambiguitiesasgalacticobservations do.Itssmalldistanceallowsustoobtainaspatialresolutionof 114pc(i.e.28 )atafrequencyof89GHz(i.e.3.3mm)with ′′ ∼ the 30m telescope. M33 is seen at an intermediate inclination of i = 56 , yielding a short line-of-sight depth, which allows ◦ us to study individualcloud complexes.It is roughlyten times less massive than the Milky Way, and its overall metallicity is 12+logO/H=8.27,subsolarbyaboutafactortwo(Magrinietal. 2010). Therefore M33 is particularly interesting to compare with theMilkyWay, butalso with the LargeMagellanicCloud thathasametallicitysimilartoM33(Hunteretal.2007). Using the IRAM 30m telescope, Rosolowskyetal. (2011) (hereafter RPG11) observed four massive GMCs of more than 3105M in M33, searching for the ground-state transition HCN. T⊙hey detected HCN in only two of the GMCs. The ob- served GMCs are under-luminous in HCN by factors between two to seven relative to their CO emission when compared to averagedvaluesintheMilkyWay. Here,wepresentnew,deepobservationsoftheground-state transitionsofHCN,HCO+,13CO,andCOtowardssevenGMCs in M33 includingthree of the clouds observedby RPG11. All fourtracersaredetectedinsixoftheGMCs.Therelativeweak- nessofHCNemissionisconfirmedandinterpretedusingmodels ofphoton-dominatedregions(PDRs).Tobettercharacterizethe observedGMCs,weestimatedtheirstarformationrate,totalin- Fig.1: SPIRE 250µm map of M33 (Xilourisetal. 2012). The fraredluminosities,andFUV fieldsusinga largeancillarydata rectangledelineatesthe2 40 widestripalongthemajoraxis ′ ′ set compiled in the framework of the Herschel open time key × shown in Fig.2. Crosses mark the positions of the observed projectHerM33es(Krameretal.2010).Figure2showsasubset GMCs. ofthisdataset. Table1: BasicpropertiesofM33 HCO+ are minor species. Their abundancesare thereforemore stronglyinfluencedbythedetailsofthechemicalnetwork(e.g. Lo´pez-Sepulcreetal. 2010). HCO+ is linked via ion-molecule M33 References reactions to the ionization equilibrium. Its collisional cross- sectionisclosetoafactor10largerthanthatofHCN,whichis RA(2000) 01:33:51.02 linkedtothehydrocarbonchemistryandtheamountofnitrogen DEC(2000) 30:39:36.7 Type SA(s)cd 1 inthegasphase.Elementaldepletioninlow-metallicityenviron- Distance[kpc] 840 2 mentsmaythereforehaveastrongeffectonitsabundance. 11 (30m@230GHz)equalto 45pc MostextragalacticobservationsofHCNandHCO+ haveso ′′ 21 (30m@115GHz)equalto 86pc ′′ far been restricted to the nuclei of galaxies or their integrated 28 (30m@89GHz)equalto 114pc fluxes. Exceptions are a study of HCN and HCO+ emission LS′R′ velocity[kms 1] 180 − in the disk of M31 by Brouilletetal. (2005), LMC observa- PositionAngle[deg] −22.5 3 tions(Chinetal.1996,1997, 1998;Heikkila¨etal.1999),HCN Inclination[deg] 56 4 mapsofsevenSeyfertgalaxiesbyCurranetal.(2001),andalso R25 30.8′or7.5kpc HCN mapping along the major axis in 12 nearby galaxies by Gao&Solomon(2004a,b)(hereafterGS04a,b).Wealsorecom- References. (1) deVaucouleursetal. (1991); (2) Galletietal. (2004); mend the studies of HCN and CO and their ratios in M51 by Freedmanetal. (1991); (3) Patureletal. (2003); (4) Regan&Vogel Kunoetal.(1995)andSchinnereretal.(2010).Theinterstellar (1994);Zaritskyetal.(1989). medium (ISM) of nuclear regions are often subject to particu- larly strong heating sources because they are often dominated by starbursts with intense UV fields heating the gas or active galacticnuclei(AGNs)withstrongX-rayemission.Indeed,the HCN to HCO+ line intensity ratios are found to be systemati- 2. IRAM30mobservations cally higher in AGN-dominatedregions, such as in the central part of NGC1068, and low in pure starburst environments, as WeusedtheIRAM30mtelescopetoperformsingle-pointedob- inM82(e.g.Kohnoetal.2003;Imanishietal.2009;Kripsetal. servationsoftheground-statetransitionsofHCN,HCO+,12CO, 2008,2011). and13COtowardssevenGMCsinM33.Observationswerecar- WehavetargetedsevenGMCsalongthemajoraxisofM33 ried out between December 2008 and July 2012, comprising a out to a radial distance of 4.6kpc, using the IRAM 30m tele- totalof109hoursofobservingtime.In2008,weusedthe now 2 C.Buchbenderetal.:DensegasinM33(HerM33es), Fig.2: Observed positions towards seven giant molecular clouds (GMCs) within a 2 40 strip along the major axis of M33. ′ ′ × Thestripextendsfrom10 southofthegalacticcenterto33.3 north.Thecenterofthestripisat01:34:11.8+30:50:23.4(J2000). ′ ′ Circles indicate the 30m beam size of 28 at 90GHz. Panels show from top to bottom: integrated intensities of Hα emission ′′ (Hoopes&Walterbos 2000) 24; 70µm emission observed with Spitzer (Tabatabaeietal. 2007); continuum emission between 100µmand500µmobservedwithPACSandSPIREintheframeworkoftheHerM33esprogram(Krameretal.2010;Boquienetal. 2011;Xilourisetal.2012);12CO 2–130mobservationandHiVLAdata,bothtakenfromGratieretal.(2010).Alldataareshown attheiroriginalresolutions. decommissionedA100andB100receiversthathadabandwidth islessprobablefortheselines.Pleasenotethatthe12COdataof of500MHzandthe1MHzfilterbank,toobserveeachofthefour GMC1,GMC91,andGMC26aretakenfromRPG11 whoalso linesindividually. usedposition-switching. The bulk of the observations were carried out in 2009 em- All data were reduced using the GILDAS 1 software pack- ploying the new eight-mixer receiver EMIR and its instanta- age. Each scan was inspected and scans with poor baselines neous bandwidth of 16GHz in each polarization, connected or unreasonablyhigh rms values were rejected. Before averag- to the wide-band WILMA autocorrelator backend with 2MHz ing, linear baselines were fitted and removed. The data were spectralresolution.Thissetupallowedsimultaneousobservation regridded to a common velocity resolution. Spectra were con- ofHCNwithHCO+ and12COwith13CO.Oneadvantageofthe vertedfromtheTA∗ totheTmb scalebymultiplyingwiththera- simultaneous observations is that the relative intensity calibra- tio of forward efficiency(Feff = 95%) to main beam efficiency tionofthelinesisveryaccurate.Theobservationswerecarried (Beff =81%),takentobeconstantfortheobserved3mmlines. out in wobbler switching mode using the maximum available ThereducedspectraareshowninFig.3. throwof 120 andaswitchingfrequencyoftwoseconds.This Integrated intensities were extracted from the spectra on ′′ modeens±uresmorestablebaselinesthantheposition-switching a Tmb scale by summing all channels inside a velocity range mode.However,thevelocityresolutionofabout6kms 1 inthe aroundeachparticularline.Thevelocityrangewasdetermined − 3mmbandonlybarelyresolvesthespectrallinesofM33,which by eye for each position from the full width to zero intensity aretypically10–15kms 1 wide(Gratieretal.2010).Thebeam (FWZI)ofthe12CO 1–0lineandismarkedinFig.3.Wedeter- − sizes are 21 at 115GHz and 28 at 89.5GHz, corresponding minedσuncertaintiesoftheintegratedintensitiesbymeasuring ′′ ′′ toaspatialresolutionof86pcand114pc,respectively,inM33 thebaselinerms(Tmrmbs)ina300kms−1 windowcenteredonthe (cf.Table1). particularlineusingthecorresponding12CO 1–0FWZIasbase- The observationsof 12CO and 13CO were repeated in June line window and using σ = Trms √N∆v with the numberof mb res andJuly2012usingposition-switchingandtheFTSspectrome- channelsN andthechannelwidth∆v .Incasethe1σvalueis res ters with an off positionoutside of the disk of M33 to exclude higherthanthetypical10%calibrationerroroftheIRAM30m, thepossibilityofself-choppingeffectsinthespectra.Thelatter theformerisusedtoestimatetheobservationalerrorforthefol- werepresentinsomeoftheearlierwobbler-switched12COspec- lowinganalysis.Iftheintegratedintensitiesarelowerthan3σ, tra.DuetothehighcriticaldensityofthedensegastracersHCN weusethisvalueasanupperlimit.Table2liststheobservedin- and HCO+, as well as the observedvelocitygradientalong the majoraxisofM33(seeTable 2),we reckonthatself-chopping 1 http://www.iram.fr/IRAMFR/GILDAS 3 C.Buchbenderetal.:DensegasinM33(HerM33es), Fig.3: Spectra of the ground-statetransitions of HCN, HCO+, 12CO, and 13CO at the positions of seven GMCs along the major axis of M33 (cf. Fig.1,2). 12CO and 13CO have been observedin position switching;HCN and HCO+ with wobbler switching. The spectra are shown on a main beam brightnesstemperature scale (T ). The velocity resolution is given by the spectrometer mb withthelowestresolution,i.e. WILMA,andis5.4kms 1 incase of12CO and13CO, and6.7kms 1 forHCN andHCO+.Center − − velocitiesare listed in Table 2. The localstandardofrest (lsr)velocitydisplayedcovers300kms 1. The same velocityrangeis − usedtodeterminethebaseline(redlines),excludingthelinewindowsdeterminedfrom12CO 1–0(dottedlines),cf.Fig.A.1 tensities, intensity ratios, and further ancillary data. For details will yield additional insight into the properties of the ISM of onthelatterseeAppendixC.Errorestimatesaregiveninparen- M33.Inthefirstpaperson[Cii],wefocusedontheHiiregion thesesaftertheintegratedintensitiesinTable2. BCLMP302 (Mookerjeaetal. 2011) which is associated with GMCno3,andonBCLMP691(Braineetal., insubm.),which liesnearGMCno1. 3. GMCs:Selectionofpositionsandproperties Gratieretal.(2012)identifiedoverthreehundred12CO 2–1 Motivated by the HerM33es project, the GMCs were selected clumps in M33 and present a detailed view of each individual clump in Hα, 8µm, 24µm, and FUV, together with the corre- tolie withina2 40 widestripalongthemajoraxisofM33 ′× ′ spondingHI and12CO 2–1spectraand furthercomplementary showninFigs.1and2atarangeofgalacto-centricdistancesof data.ThesevenGMCsdiscussedhereareamongthe identified up to 4.6kpc. Three of the GMCs (GMC1, GMC26, GMC91) clumps.InTable2wegivethecorrespondingclumpnumbers. belongtothesampleofCO-brightcloudsstudiedbyRPG11in searchofHCNemission.WeaddedfourotherGMCs(no6,no3, no1,no2)to increase therangeof studiedgalacto-centricradii, 4. Observedlineratios aswellasphysicalconditions. Table 2 lists their observed propertiesand Appendix C de- 4.1.Spectraatindividualpositions:HCO+andHCN scribes in detail how they were derived. The masses of the molecular gas traced by CO, calculated using X -factors de- HCO+ is detected at six positions with 6 to 12mK peak tem- CO rived individually for every cloud as a function of integrated peraturesandwithgoodsignal-to-noiseratiosofatleastseven; CO1–0 intensities and total IR luminosity (cf. Appendix C.4), position no2 has not been detected. HCN emission is detected varybyafactor130between0.1105(GMCno2)and13105M atthesamesix positionswithsignal-to-noiseratiosoffourand (GMC1). The SFRs vary by more than a factor 50 and th⊙e better;positionno2isbuttentativelydetectedatasignal-to-noise far ultraviolet (FUV) field strengths by a factor larger than 20. ratioof3.5.TheHCO+/HCN ratiooflineintegratedintensities The GMC near the nucleus, GMC1, is the most massive and of positions where both moleculesare detected varies between showsthestrongestSFR,aswellasthehighestFUVflux,while 1.1and2.5(Table2).Below,wecomparetheobservedratiosin GMCno2at4.6kpcradialdistanceistheleastmassiveinmolec- detailwithratiosfoundintheMilkyWayandinothergalaxies. ularmassandshowsonlyweakactivity. AlthoughtheintegratedintensitieswefindforGMC26,GMC1, Individualareasofthestriphavebeenmappedin[Cii] and andGMC91differuptoafactoroftwofromthevaluesandup- otherFIRlinesintheframeworkoftheHerM33esproject,which perlimitsgiveninRPG11forthesamepositions,theyarecon- 4 C.Buchbenderetal.:DensegasinM33(HerM33es), Table2:Observedintensitiesandcomplementarydata no6 GMC1 GMC26 no3 GMC91 no1 no2 Clumpnumbera 42 108 128 256 245 300 320 RA[J2000] 01:33:33.77 01:33:52.40 01:33:55.80 01:34:07.00 01:34:09.20 01:34:16.40 01:34:21.77 DEC[J2000] +30:32:15.64 +30:39:18.00 +30:43:02.00 +30:47:52.00 +30:49:06.00 +30:52:19.52 +30:57:4.99 V [kms 1] -133.0 -168.0 -227.0 -257.0 -247.0 -266.0 -264.0 LSR − R[kpc] 2.01 0.11 0.873 2.18 2.51 3.38 4.56 I [Kkms 1] 7.1(10%) 7.2(10%)b 6.9(10%)b 9.4(10%) 21.6(10%)b 4.0(10%) 1.4(10%) 12CO(1 0) − FWHM12C−O(1-0)[kms 1]c 11(0.5) 8(0.2) 6(0.2) 8(0.1) 11(0.1) 9(0.2) 4(0.2) − I [Kkms 1]d 8.9(15%) 10.6(15%) 7.0(16%) 9.4(15%) 19.3(15%) 6.2(15%) 0.7(15%) 12CO(2 1) − I − [mKkms 1] 468(12%) 799(13%) 541(19%) 772(13%) 1690(10%) 369(12%) 132(15%) 13CO(1 0) − IHCO+(1−0)[mKkms−1] 205(10%) 182(10%) 66(10%) 119(10%) 97(15%) 77(10%) <12 I − [mKkms 1] 82(20%) 164(10%) 56(16%) 61(16%) 67(24%) 56(17%) 26(28%) HCN(1 0) − I − [mKkms 1] <43.9 <44.8 <15.6 <22.3 <47.5 <25.9 <20.4 HNC(1 0) − rm−s[mK]e 1.0 1.4 1.0 0.7 1.1 0.9 0.8 IHCO+(1 0)/IHCN(1 0) 2.5(0.2) 1.1(0.1) 1.2(0.1) 1.9(0.1) 1.4(0.3) 1.4(0.2) <0.5 I − /I − <0.5 <0.3 <0.3 <0.4 <0.7 <0.5 <0.8 HNC(1 0) HCN(1 0) I −/I −[%] 1.4(0.3) 2.9(0.4) 1.0(0.2) 0.8(0.2) 0.4(0.1) 1.7(0.4) 2.3(0.7) HCN(1 0) 12CO(1 0) IHCO+(1−0)/I12CO(1−0)[%] 3.5(0.5) 3.2(0.5) 1.1(0.2) 1.6(0.2) 0.6(0.1) 2.3(0.3) <1.0 I − /I − 15.1(2.4) 9.0(1.5) 12.8(2.8) 12.2(2.1) 12.8(1.8) 10.8(1.8) 10.6(1.9) 12CO(1 0) 13CO(1 0) L [103−Kkms 1−pc2] 87.7(8.8) 85.4(8.5) 87.9(8.8) 114.6(11.5) 248.9(24.9) 49.8(5.0) 17.3(1.7) ′CO − L [106L ] 4.4(0.1) 5.9(0.2) 1.4(0.0) 2.7(0.1) 1.3(0.0) 1.7(0.1) 0.3(0.0) TIR L /L [1⊙03] 3.5(0.7) 2.4(0.3) 1.6(0.3) 3.0(0.5) 1.3(0.3) 2.0(0.4) 0.8(0.2) TIR ′HCN L /L [103] 1.4(0.1) 2.2(0.2) 1.4(0.1) 1.5(0.2) 0.9(0.1) 1.5(0.2) >1.6 TIR ′HCO+ SFR[M Gyr 1pc 2] 35.9(4.3) 65.0(7.8) 6.6(0.8) 13.7(1.8) 4.0(0.6) 12.2(1.7) 1.2(0.1) − − ⊙X 5.1 6.9 1.6 3.2 1.5 2.0 0.3 CO M [105M ] 9.4(1.4) 5.8(0.9) 4.1(0.6) 8.0(1.2) 8.8(1.3) 5.2(0.8) 4.8(0.7) HI M [105M⊙] 9.6(1.0) 12.7(1.3) 3.0(0.3) 7.9(0.8) 8.2(0.8) 2.1(0.2) 0.1(0.0) H2 G ⊙ 37.3(1.2) 50.7(1.6) 11.6(0.4) 23.5(0.8) 11.3(0.4) 14.4(0.5) 2.5(0.1) 0 A 6.3(0.6) 6.1(0.5) 2.3(0.2) 5.3(0.5) 5.7(0.5) 2.4(0.3) 1.6(0.2) V Notes.Toppanel:lineintensitiesareontheT -scaleandontheiroriginalresolutions:12 for12CO 2–1,24 for12CO and13CO 1–0and28 mb ′′ ′′ ′′ for HCNand HCO+ 1–0.BottomPanel: lineratiosandcomplementary dataareon acommon resolution of 28 . SeeAppendix C for details. ′′ (a)12CO 2–1clumpnumbersfromGratieretal.(2012);(b)Rosolowskyetal.(2011);(c)FWHMsofGaussianfitstothehighresolution12CO 1–0 spectra(Fig.A.1);(d)Gratieretal.(2010);(e)BaselinermsofHCO+spectraatavelocityresolutionof6.7kms 1. − sistent within 3σ of the baseline rms of the observations from The stacked spectrum centered on 112GHz (Fig.4b) does RPG11. We attribute the discrepanciesto baseline problemsof notshowadditionaldetectionsotherthan13COand12CO even theRPG11data. after additional smoothing of the velocity resolution. Table 3 lists the integrated intensities and upper limits of all detected transitionsinthestackedspectrum,aswellastheircorrespond- 4.2.Spectraatindividualpositions:12COand13CO ingLTEcolumndensitiesandabundances,derivedasexplained inAppendixD. Emissionfrom12COand13COisdetectedatallsevenpositions, thoughvaryingby a factor of more than15 between GMCno2 and GMC91. The ratio of line integrated12CO vs. 13CO inten- 4.4.Comparisonwithothersources sities varies between 9 for GMCno1 and 15 for GMCno6. In 4.4.1. HCO+/COvs.HCN/CO Fig. A.1 we show the 12CO and 13CO spectra at a high resolu- tionof1 kms 1.TheresolvedlineshapesareGaussianandthe Forthecomparisonofdifferenttracers,alldatawereconvolved − correspondingFWHMsaregiveninTable2. to the same resolution of 28 . We account for the different ′′ intrinsic beam sizes of the CO, HCN, and HCO+ 1–0 obser- vations by multiplying with beam filling factors determined 4.3.Stackedspectra fromthe12CO 2–1map(Gratieretal.2010).Figure5compares the HCO+/CO vs. HCN/CO line intensities ratios observed in Stacking of all spectra allows improvement on the signal-to- M33, with those observed at nine positions in the disk of the noise ratios and detection of faint lines. Figure4a shows the AndromedagalaxyM31(Brouilletetal.2005,hereafterBR05). stacked spectrum of all data taken near 89GHz. It was created M31liesatasimilardistanceasM33of780kpcandhadbeen byshiftingallspectrainfrequencysuchthattheemissionlines observedwiththe30mtelescopeaswell.Therefore,bothstudies aligninfrequency.Individualspectrawereweightedbyintegra- obtainaboutthesamespatialresolutionof 114pc. tion time. In addition to the lines of HCN and HCO+, the re- For M33, we find HCN/CO ratio∼s in the range of sulting spectrum shows detections of CCH and HNC1–0. The 0.4%–2.9% (mean: 1.5 0.8%) and HCO+/CO ratios in averagebaselinermsis0.27mKat5.4kms 1resolution.HOC+ 0.6%–3.5% (mean: 1.9 ± 1.0%). BR05 finds comparable − istentativelydetectedwithanupperlimitof19mKkms 1,re- values in the spiral arms ±of M31: HCN/CO 0.75%–2.8% − sultinginalowerlimittotheHCO+/HOC+ ratioof5.8. (mean: 1.7 0.5%) and HCO+/CO 1.1%–3.9% (mean: ± 5 C.Buchbenderetal.:DensegasinM33(HerM33es), Table3:LTEcolumndensitiesfromthestackedspectra. C H HCN HCO+ HOC+ HNC 13CO(1-0) 12CO(1-0) H a 2 2 I[mKkms 1] 25.1 81.0 110.0 <18.8 13.7 522.0 6280.0 - − N[x]b 5.10e+12 8.03e+11 2.11e+11 <7.10e+10 4.18e+10 1.80e+15 8.55e+15 8.47e+20 N[x]/N c -8.40 -9.20 -9.78 <-10.25 -10.48 -5.85 -5.17 1.00 H2 Notes. (a)deducedfrom13CO,seeAppendixD;(b)columndensity;(c)logarithmicvalues. 5 4 no6 %] GMC1 O)[3 C +/)I( no1 O (a) C2 H I( no3 no2 I(HCN)=I(HCO+) 1 GMC91 GMC26 FitM33 FitM31(BR05) GalaxiesGA04 MWDiskHE97 0 M31BR05 M33thiswork 0 1 2 3 4 5 I(HCN)/I(CO)[%] Fig.5: Ratios of integrated intensities HCO+/CO vs. HCN/CO for M33 (red points: this work) and for M31 (green points: BR05). Upper/lowerlimits are denoted by arrows. Linear least (b) squaresfitstodatafromM33(redsolidline)andM31(BR05, Fig.4: (a) Stacked spectrum of all data taken in the frequency black dashed) are shown. Both fits exclude points with upper rangebetween87.2and90.8GHz.Thereddashedlinedenotes limits. The solid black line shows the angle bisector where the 3σ value average over the entire baseline. The HCN and I(HCO+)=I(HCN). The gray shaded areas display the range HCO+ lines are not shown up to their maximum peak temper- of the HCN/CO found in the disk of the Milky Way (MW) by ature.(b)Stackedspectrumofthewobblerswitcheddatataken (Helfer&Blitz 1997, HE97) (darker gray) and in a sample of in the frequencyrange between 108.1 and 115.5GHz. The av- normalspiralgalaxies(GA04b)(lightergray). erage3σvalueisshownasreddashedline.Thebaselinenoise increaseswithfrequencybecauseoftheincreasingatmospheric opacity.C18O,C17O,andCN aremarkedbutnotdetected.The 12CO and13CO linesare notshownuptotheir maximumpeak (Kunoetal.1995).UnlikeGS04binasampleofnormalgalax- temperature. ies, we do not find a systematic change in the HCN/CO ratio between regions in the center of M33, i.e. the inner 1kpc ∼ (hereGMC1,GMC26)andregionsatgreatergalacto-centricdis- tances(cf. Table2). GS04b reportsthat HCN/CO dropsfrom 2.0 0.7%).AlinearleastsquaresfittotheM33dataresultsin ∼ HC±O+/CO = (1.14 0.15%)HCN/CO+(0.18 0.14%).This 10% in the centers of normalgalaxies to 1.5%–3% in their ∼ ± ± disks'4kpc.InULIRGsandAGNstheratiosmayreachglobal isconsistentwithinerrorstothefitresultsobtainedbyBR05for the M31 data: HCO+/CO = 1.07%HCN/CO + 0.23%. We HCN/COvaluesashighas25%(GS04bandreferencestherein). GS04b attribute these high ratios to the presence of starbursts excluded positions with upper limits from the fit, i.e. position andarguethatHCN/COmayserveasastarburstindicator. GMCno2. In the Milky Way in the solar neighborhood values of HCN/CO are found between 0.7% – 1.9% (mean: 1.4 4.4.2. HCO+/HCNvs.HNC/HCO+ ± 2%), while the Galactic plane hosts on average 2.6 0.8% (Helfer&Blitz 1997). HCO+/CO values in the Galacti±c center The HCO+/HCN ratios observed in M33 vary between 1.1 rangebetween0.9%and7.6%(Riquelmeetal.2010). and 2.5, while the upper limits derivedfor the HNC/HCO+ ra- TheHCN/COratiosfoundintheLMCbyChinetal.(1998) tios vary between 0.2 and 0.5 (Fig. 6). The upper limit of the andHeikkila¨etal.(1999)liebetween3%and6%,andarethus HNC/HCNratioisatmaximum0.7(GMC91),whilethestacked higher than any value found in M33, M31, and also M51 spectrumwhereHNChasbeendetectedshowsaHNC/HCNra- where HCN/CO=1.1%–2% in the spiral arms are reported tioof0.17(Fig.6). 6 C.Buchbenderetal.:DensegasinM33(HerM33es), galaxiesfoundbyHuettemeisteretal.(1995)of<4,higherthan thetypicalratiosof1observedinstarburstandSeyfertgalaxies (e.g. Aaltoetal. 2002), and also higher than ratios of 1-3 ob- no6 0.4 servedinGalacticmolecularcomplexes(Woottenetal.1978). no3 M82 This extraordinary high ratio indicates that the physics or chemistry in M33 may be different from that of AGNs and GMC91 starbursts. The dominance of strong X-ray radiation in the nu- 0.2 0) clei of AGNs or even of starbursts may be important for the N(1- NGC1068 differences in the line ratios, since it creates X-ray-dominated C /1-0)H 0.0 GMC26 NGCM2r5k3231 rMegeiiojenrsink(X&DSRpsa)anthsa2t0c0h5a).nge the chemical abundances (e.g. +O( no1 ThesubsolarmetallicityofM33mayalsoplayaroleincre- C H GMC1 ating such a high HCN/HNC ratio. However, the HCN/HNC g Arp220 lo −0.2 ratio obtained in M33 is significantly higher than those ob- served in similar low-metallicity environments, such as N159, LMC(CH97,CH98) 30Dor in the LMC, as well as LIRS36 in the SMC, which are LIRGS/ULIRGS(BA10) no higher than 3.6 (besides a lower limit of 4.7 in N27 in the −0.4 LRMMIa33Rn33gGeGs−St1MaM/U.c0k3CLe1sIdR(BGRS0(5C)O1−10).5 HNC/HCN=0.17 0.0HNC/HCN=0.4 HN0C./H5CN=1 tTgShuMheaeCrvraea)nfl(outCereeehs,fifnoaoruesvtnueadbrlsy.ion1hl9aGig9rah8ml;aHHceCtteiacNilklGi/kcHiMiltNay¨CCeastlroa(aHlnt.ieuo1es9d.t9ote9em)saenniosdttecrsoeemetmpaal.rtoa1b9bl9ee5t)oa. logHNC(1-0)/HCO+(1-0) Fig.6: Comparison of integrated intensities HCO+/HCN vs. 4.4.3. HCNvs.totalinfraredluminosity(LTIR) HNC/HCO+ in M33 (red filled circles and square; arrows TheL /L ratiosobservedinM33(Table2)rangebetween TIR HCN indicate upper/lower limits) with values found in the LMC 1300 and 3500 L /Kkms 1pc2. These ratios lie at the upper − (Chinetal. 1997, 1998) (CH97, CH98; blue diamonds) and endof the valuesf⊙oundin Milky Way clouds(Wuetal. 2010). in luminous infrared galaxies compiled by Baanetal. (2008) Normal galaxies show on average total L /L ratios of TIR HCN (BA08;opensymbols)andby Costagliolaetal. (2011) (CO11; 900 70L /Kkms 1pc2(Gracia´-Carpioetal.2008,GS04a,b). − crosses). The dotted, dashed, and dot-dashed lines shows H∼ighe±r valu⊙es, in the range of 1100–1700L /Kkms 1pc2, − HNC/HCN=1,0.4,and0.17 (stacked value of M33), respec- are foundfor(U-)LIRGs(Graci∼a´-Carpioetal. 20⊙08, GS04a,b), tively. The gray shaded area shows the range of the observed while the highest reported ratios are found at the extreme end HCO+/HCNratiosinM31byBR05. of the LIRG/ULIRGdistribution,as wellas in high-zgalaxies, and range up to 3900 L /Kkms 1pc2 (Solomonetal. 2003; − Gaoetal. 2007;∼Gracia´-C⊙arpioetal. 2008; Wuetal. 2010; These ratios are compared with the ratios found in lumi- Garc´ıa-Burilloetal. 2012). Thus the LTIR/LHCN ratios in M33 nousinfrarednucleibyBaanetal.(2008)andCostagliolaetal. areamongthehighestratiosobserved. (2011)andinHiiregionsoftheLMCbyChinetal.(1997,1998) InSect.6below,weattempttoshedmorelightontheweak (cf.Fig.3ainBaanetal.(2010)).TheHCO+/HCNratiosarealso HCN emissionandinvestigatethelineratiosofCO, HCN, and comparedto the range foundin the disk of M31 by BR05 (cf. HCO+ usingmodelsofphoton-dominatedregionsthattakenot Fig.6). only the chemical network into account, but also the detailed TheHCO+/HCNratios,foundinM33inthesixGMCswith heating and cooling processes of a cloud as well as radiative clear detections, lie at the upper end of the distribution of val- transfer. uesfoundinLIRGs.WhilethestarburstgalaxyM82exhibitsa higherratiothanthe AGNNGC1068(1.6vs.0.9),these ratios 4.4.4. 12CO/13COlineratio lie within the scatter of values and their errors observed in the diskofM33. InoursampleofGMCsinM33wefind12CO/13COlineinten- TheHCO+/HCNratiosinM33lieintheoverlapregionbe- sity ratios between 9 and 15. There is no obvious correlation tweentheonesfoundinM31andthosefoundintheLMC.We withgalacto-centricdistance,FUVstrength,orSFR.Inastudy find neitherratios as highas in the LMC, i.e. 3.5,nor ratiosas of eight GMCs in the outer disk of M33, Braineetal. (2010) lowasinM31,i.e.0.51.Interestingly,alldetectedregionsinthe foundsimilarratiosbetween8.9and15.7.IntheMilkyWay,the LMCaresituatedinthesameparameterspaceasthosedetected isotopicratioof12C/13Cvariesbetweenvaluesof80–90inthe inM33. solarneighborhoodand20intheGalacticcenter(Wilson1999). The detection of HNC in the stacked spectrum allows us Polketal.(1988)studiedthe12CO/13COlineratiointheMilky to derive an average HCO+/HNC ratio of 7.8 (Table3) for the Way, for several large regions of the plane in comparison with GMCsobservedinM33.Thisratioliesattheveryhighendof the averageemissionfromGMCs andofthe centersofGMCs. therangeofvaluesfoundinanyoftheothersamplesplottedin Theyfindthattheaveragevaluerisesfromthreeinthecentersof Fig.6. GMCs,to4.5averagedoverGMCs,to6.7fortheGalacticplane, More remarkably, the HCN/HNC value of 5.9 from the withpeaksof 15.Theirinterpretationisthatthehigherratios ∼ stacked spectra is higher than any from the other samples we observedin theplane arecaused bydiffusegasofonlymoder- compare with in Fig.6. Furthermore, it is higher than ratios ateopticalthicknessin12CO.Asimilarinterpretationmayhold observed over the surface of IC342 which are only 1 2 inM33.Thefractionofdensegaswithinthebeamandoptical ∼ − (Meier&Turner 2005), higher than the ratios in a range of deptheffectsmayaffecttheratiosobservedinM33. 7 C.Buchbenderetal.:DensegasinM33(HerM33es), Table4:MolecularabundancesinM33andtypicalexamplesof HCO+/HCN abundance ratio of 0.5 in the diffuse ISM of the galacticandextra-galacticsources. MilkyWay,similartotheratiofoundinthesolarneighborhood, andsimilartothehigherratiosfoundinM33. Source HCO+ HCN HNC C H References 2 M33min 10.3 9.8 - - − − M33stacked 9.8 9.2 10.5 8.4 6. PDRmodels − − − − M33max 9.5 9.1 - - LMCN159 −9.7 −9.7 10.2 - 1,2 6.1.Setup − − − NGC253 8.8 8.3 9.0 7.7 1,3 − − − − To improve on the LTE analysis and to better understand why M82 8.4 8.4 8.8 7.6 1,3 − − − − HCNislessluminousthanHCO+ inM33,wecomparetheob- IC342GMC-A 8.7 7.5 5 − − served HCO+/HCN, HCN/CO, and HCO+/CO line ratios with OrionBar 8.5 8.3 9.0 8.7 1 TMC-1 −8.4 −7.7 −7.7 −7.1 1,6 modelsofphoton-dominatedregions(PDRs)usingtheMeudon Transl.Cl. −8.7 −7.4 −8.6 −- 1,4 PDR code (LePetitetal. 2006; GonzalezGarciaetal. 2008). − − − The line intensities of the molecules13CO, HNC, and C H are 2 Notes.Entriesshowlog(N(X)/N(H2)). notmodeled. Weranagridofmodelsfordifferentdensitiesn =0.1,0.5, References. (1) Omont (2007); (2) Johanssonetal. (1994); H (3) Mart´ınetal. (2006); (4) Turner (2000); (5) Meier&Turner 1, 5, 10, 50, 102 104cm−3, FUV fields G0 = 10,50,100 (2005)andreferencestherein;(6)Hogerheijde&Sandell(2000) in Habing units2,×and optical extinctions Av=2–50mag, i.e. in stepsof logA 0.2. We calculatedthisgridofmodelsfor a v ∼ solar and a subsolarmetallicity. The subsolarone is tailored to Ratios in the Magellanic clouds are observed by describethemetallicityinthediskofM33.SeeAppendixAfor Heikkila¨etal. (1999) to cover values between 5 and 18, a adetaileddescriptionofthemodelsetup. somewhat wider range than found in M33. Unlike in M33, a gradient is seen on larger scales in a set of IR-bright nearby galaxies, dropping from values of about ten in the center to 6.2.Results valuesas low as two at larger radii(Tanetal. 2011). Although The modeled HCN/CO line ratio (Fig.7, TablesE.1 and E.2) Aaltoetal. (1995) finds variations with galacto-centric radius hardly varies with metallicity, n or FUV. It varies, however H in somegalaxiesof theirIR-brightsample,othergalaxiesexist strongly,withA .Afteraninitialdropofuptoanorderofmag- where the 12CO/13CO stays constant with radius. They report v nitude for extinctions less than about 4 mag, it rises until A v a mean value of 12 for the centers of most galaxies in their 20magandthensaturatesatavaluethatisnearlyindependent ∼ sampleexceptforthemostluminousmergerswithratiosof'20 ∼ ofanyoftheinputparameters. (seealsoCasolietal.1992). Incontrast,themodeledHCO+/HCNratioshowsdifferences between the two metallicities for A .10mag. The subsolar v modelshowshigherratiosthanthe solar modelfora given A , 5. Molecularabundances v FUVfield,anddensity.StrongFUVfieldsandlowdensitiesin- WeusetheobservedlineintensitiesofHCO+,HCN,HNC,and creasetheratio.Athigheropticalextinctionstheratiosarehardly C Htoestimatethemolecularcolumndensitiesandabundances, influenced by changes in metallicity, A or FUV. This reflects 2 v assuminglocalthermodynamicequilibrium(LTE)andoptically thevariationinHCNandHCO+ abundances.AtlowA ,subso- v thinemission.DetailsonthecalculationsaregivenAppendixD, lar HCN abundancesare lower than solar ones by up to a fac- withresultsshowninTableD.1.Theabundancesareonlylower torof 0.6dex,while HCO+ isenhancedin the subsolarmodels limitsincasetheemissionofHCNandHCO+isopticallythick. byup to 0.7dex.Also the CO abundancesshow a cleardepen- In comparison with our PDR-model analysis below we find, denceonmetallicity.Forallinputparameters,itsabundancein however,thatforthebest-fittingmodelstoourobservationsthe thesubsolarmodelsis 0.6dexlowerthaninthesolarmodels. opticaldepths(τ)inthecentersofthelinesofHCNandHCO+ Thisdirectlyreflectsthe∼underabundanceofcarbonof0.6dexin areτ<0.1,suggestingthatemissionislikelytobeopticallythin thesubsolarmodels.Asaresult,theHCN/COlineratioisfairly (cf.Sect.7).InTable4,wecomparetheabundanceswiththose independentofmetallicity. found in other galaxies (LMC, NGC253, M82, IC342), in se- For optical extinctions in the range of A 8 mag, where v ≤ lectedGalacticsources,thephoton-dominatedregionOrionBar, thebulkofmoleculargasingalaxiesresides(Tielens2005),the the dark cloud TMC-1, and a translucent cloud. The estimated HCN/COratioincreaseswithincreasingdensities.Ingeneralin columndensitiesforthestackedvaluesaregiveninTable3. LIRGs/ULIRGs where most of gas has higher density than in The abundances of HCO+, HCN, and HNC found in M33 normalgalaxies,theHCN/COisalsohigher(GS04a,b). are very similar to those found in the LMC cloud N159. The abundancesderivedfromthestackedspectrumofM33agreeto within 0.5dex with those of N159. Galactic sources have val- 7. ComparisonwithPDRmodels ues that are higher by more than an order of magnitude. The Figure7 shows the range in observed intensity ratios Orion Bar, for example, shows 1.8dex to 0.8dex higher abun- (cf.Table2), where the plot shows that the optical extinc- dances. The good agreement with the LMC may be driven by tions play a decisive role in determining the line ratios. High its similar metallicity of 0.3–0.5 relative to the solar metallic- extinctions of A >16mag are inconsistent with the observed ity (Hunteretal. 2007), which is only slightly lower on aver- v HCN/CO ratios. Similarly, high densities of 105cm 3 cannot agethaninM33(Magrinietal.2007,2010).However,theC H − 2 abundanceobservedinM33andintheOrionBaragreewithin 2 Habing units correspond to an average interstellar radiation field 0.3dex. (ISRF)between6eV hν 13.6eVof1.610 3ergcm 2s 1 (Habing − − − The LTE HCO+/HCN abundance ratios in M33 range be- 1968).Anotherunittha≤tisfr≤equentlyusedistheDraineunitofthelocal tween0.2and0.5(TableD.1).Godardetal.(2010)measuredan averageISRF,whichis1.7 G inHabingunits. × 0 8 C.Buchbenderetal.:DensegasinM33(HerM33es), 7 6 nH=1×103cm−3 nH=5×103cm−3 nH=1×104cm−3 nH=5×104cm−3 5 N C4 H +/O3 C H2 1 0 7 0.4 0.6 0.8 1.0 1.2 1.4 1.6 6 nH=1×105cm−3 nH=5×105cm−3 nH=1×106cm−3 logAv(mag.) 5 subsolar N C4 solar H +/O3 G0=10 C H2 G0=50 1 G0=100 0 observedM33 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.4 0.6 0.8 1.0 1.2 1.4 1.6 logAv(mag.) logAv(mag.) logAv(mag.) 1.5 nH=1×103 nH=5×103 nH=1×104 nH=5×104 1.0 O) /NC 0.5 C 0H 0.0 0 1 g(−0.5 o l −1.0 1.5 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.0 nH=1×105 nH=5×105 nH=1×106 logAv(mag.) O) subsolar /NC 0.5 solar C 0H 0.0 G0=10 0 g(1−0.5 G0=50 lo G0=100 −1.0 observedM33 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.4 0.6 0.8 1.0 1.2 1.4 1.6 logAv(mag.) logAv(mag.) logAv(mag.) Fig.7:PDRmodellineratiosforsubsolar(solidlines)andsolarmetallicity(dashedlines):HCO+/HCN(top)andHCN/CO(bottom). DifferentcolorsindicatedifferentFUVfieldstrengthsG =10(red),50(green),and100(blue).Everypanelofasubfigureshows 0 theresultsforonedensity;fromlefttorightandtoptobottomn =0.1,0.5,1,5,10,50,and102 104cm 3.Grayareasmarkthe H − × rangeofobservedratiosinM33.Thedashedhorizontallinesshowthevaluesfromthestackedspectra. reproduce the measured HCO+/HCN ratios. Interestingly, withaverageline-of-sightcolumndensitiesof8mag.Thebeam- metallicityonlyplaysa minorrole.Ingeneral,bothmetallicity fillingfactorΦ derivedfromratioofthebeam-averagedTIR FUV modelsallowtheobservedrangeoflineratiostobereproduced. intensity to the fitted local FUV field is 30%; i.e., the fitted ∼ Toquantifytheagreementbetweenthelineratiosofthedif- FUVfieldstrengthsaresignificantlyhigherthanexpectedfrom ferentmodelsand the individualobservedcloudsincludingthe theobservations.Thesameholdsforthebeamfillingfactorsde- stacked spectrum,we use a χ2 fit routine.To geta handleonto ducedfromtheratiosofextinctionsderivedfromCOandtheA v the errors of the best-fitting models, a Monte Carlo analysis is of the best-fitting models, which are about Φ =50%. This is Av employed.The details on the fitting and Monte Carlo methods notsurprising,however,andindicatesthatemissionisclumped are given in Appendix B. Table 5 shows the input parameters withinthe114pcbeam.Fromthemodelsofthecalculatedgrid A ,n ,andFUVofthebest-fittingsubsolarandsolarmetallic- closest to the best fit, i.e., Av = 6 and 10mag, n = 1104, v H H itymodels,i.e.thosehavingthelowestχ2,forthestackedvalues and FUV = 50G , we find optical depths (τ) in the centers of 0 andeachobservedcloud. the lines of HCN and HCO+ that lie between 0.02–0.1 and 0.07–0.1,respectively.Thusbothlinesareopticallythin.12CO ismoderatelyopticallythickwithopticaldepthsofτ4–25.The 7.1.Stackedratios linewidthassumedintheMeudonPDRcodeis 3kms 1 (cf. − ∼ Thebest-fittingmodelsforreproducingthestackedHCO+/HCN AppendixA). and HCN/CO ratios of 1.4 and 1%, respectively, that describe theaveragedGMCpropertiesare A =8mag,n =3104cm 3, v H − and FUV=68G . Emission stems from moderately dense gas 0 9 C.Buchbenderetal.:DensegasinM33(HerM33es), Table5:Best-fittingPDRmodels HCO+/HCN HCN/CO A n FUV Φ a Φ b bestχ2c v H Av FUV [%] [mag.] [cm 3] [G ] − 0 subsolarmetallicityModels Stacked 1.4 0.2 1.0 0.1 8 3 (3 4)104 68 24 0.5 0.2 0.3 0.1 0.1 0.1 NO6 2.5±0.2 1.4±0.3 4±2 (6±4)103 41±16 1.7±1.1 0.9±0.4 1.4±0.5 NO3 1.9±0.1 0.8±0.2 8±2 (3±2)103 27±20 0.7±0.2 0.8±0.6 0.6±0.3 GMC91 1.4±0.3 0.4±0.1 6±2 (1±5)104 54±44 0.9±0.3 0.2±0.2 0.1±0.1 NO1 1.4±0.2 1.7±0.4 7±4 (3±4)104 42±40 0.3±0.2 0.3±0.3 0.2±0.1 GMC1 1.1±0.1 2.9±0.4 10± 3 (1±2)104 30±37 0.6±0.2 1.7±2.1 0.7±0.5 GMC26 1.2±0.1 1.0±0.2 9 ±2 (1±3)104 67±24 0.3±0.1 0.2±0.1 0.1±0.1 ± ± ± ± ± ± ± ± solarmetallicitymodels Stacked 1.4 0.2 1.0 0.1 10 1 (5 1)103 47 12 0.4 0.0 0.5 0.1 0.6 0.4 NO6 2.5±0.2 1.4±0.3 2 ±0 (5±1)103 100± 4 3.1±0.8 0.4±0.0 3.4±1.2 NO3 1.9±0.1 0.8±0.2 6±3 (2±2)103 81 ±32 0.9±0.5 0.3±0.1 3.4±0.9 GMC91 1.4±0.3 0.4±0.1 4±1 (1±1)103 98±11 1.4±0.3 0.1±0.0 0.5±0.4 NO1 1.4±0.2 1.7±0.4 7±4 (5±3)103 50±43 0.4±0.2 0.3±0.2 0.2±0.2 GMC1 1.1±0.1 2.9±0.4 11± 2 (3±6)103 26±34 0.6±0.1 1.9±2.6 0.5±0.2 GMC26 1.2±0.1 1.0±0.2 9 ±2 (5±1)103 52±17 0.2±0.1 0.2±0.1 0.4±0.3 ± ± ± ± ± ± ± ± Notes.(a)beam-fillingfactorderivedfromA (12CO)/A (Model);(b)beam-fillingfactorderivedfromFUV(TIR)/FUV(Model);(c)averageχ2ofthe v v best-fittingmodels. 7.2.Individualregions GMC1andGMC26 Thesetwocloudshostthelowestobserved ratiosofHCO+/HCNof1.1–1.2.Here,thesolarmodelsprovide Here,wefocusonindividualregionsgroupedbytheirparticular slightly better or equalfits than the subsolar models. However, HCO+/HCN ratios and thus their best-fit Av values:no6 show- sincetheISMofM33issubsolar,herewediscussonlythebest ingahighratioof2.5,GMC91,no3,andno1haveintermediate fitstothesubsolarmodels.TheseGMCshavesimilarbest-fitting ratiosof1.4-1.9andGMC26,GMC1havingratiosof1.1-1.2. inputparametersof A 9–10magandn 104cm 3,while the v H − ∼ best-fitting FUV field strengths are 30G and 70G , respec- 0 0 ∼ tively.Howevercomparingtheirphysicalpropertiesin Table2, GMCno6 This cloud shows the highest HCO+/HCN ratio of again, these two clouds are actually not at all alike. GMC1 is 2.5,while atthe same time the HCN/CO ratiois relativelylow located in the very center of M33 and is by far the most mas- with1.4andweakerthanexpectedfromthelinearfittotheM33 sivecloudinoursample.Itisactivelyformingstarsatarateof data(cf.Sect.4.4.1andFig.5).GMCno6isbestfittedbysubso- 65M Gyr 1pc 2,thehighestinoursample,andhasancorre- − − larmodelsthatyieldalowbest-fittingvalueforAv of 4mag, spond⊙inglyhighFUVfieldof50.7G .GMC26hasmuchlower while the best-fitting density and FUV strength are 6∼103cm−3 HCN/COratios,anditsSFRisonly60.6M Gyr 1pc 2thesec- − − and40G0,respectively.Thebeam-fillingfactorderivedfromAv ondlowestinthesamplewithexceptionof⊙no2. is 1.7, indicating that emission completely fills the beam with For all best-fitting solutions of the six individual positions severalcloudsalongtheline-of-sight.Thiscloudhasthesecond wefindthatthemodeledopticaldepthsofHCNandHCO+ are higheststarformationrateofoursampleof35.9M Gyr 1pc 2, − − τ 0.1,whichrendersemissionoftheselinestobelikelyopti- andthesameholdsfortheFUVfieldstrengthof37⊙.3G . ≤ 0 callythin.Thisalsojustifiestheassumptionofopticalthinemis- sion for the LTE analysis in Sect5. Indeed, the PDR modeled abundances of both molecules are comparable to the ones de- GMC91, no3, and no1 The line ratios of these three clouds rivedfromLTE(cf.Table4andE.2). are best described by subsolar models. The best-fitting A are v In conclusion,it is noteworthyto repeat that the line ratios similar with 6–8mag. So are the FUV 30–50G and the den- 0 studiedherearefairlyindependentofthemetallicity,SFactivity, sities 3103cm 3 – 3104cm 3, and no1 and no3 have similar − − and FUV field strength of the parent GMC, while the optical SFRratesof 13M Gyr 1pc 2, whileno3isa factoroffour more massive∼than n⊙o1 wit−h M− =8105M . GMC91 lies at extinctionhasamajorinfluenceonthemodeledlineratios. only 320pc distance in close viHc2inity of GM⊙Cno3 and is only slightly moremassive than the same. Itis the mostCO intense 8. Summary cloud in our sample while its HCN and HCO+ emission is rel- atively weak. This renders GMC91 somewhat peculiar and re- We present IRAM 30m observations of the ground-state tran- sults in a low HCO+/CO ratio of 0.6% and, as already found sitions of HCN, HCO+, 12CO and 13CO of seven GMCs dis- byRPG11,a particularlylow HCN/CO intensityratioof 0.4% tributed along the major axis in the disk of the nearby spiral (Table2).TheHCN/COratioofGMC91ismuchlowerthanthe galaxy M33. We achieve a spatial resolution of 114pc at a ∼ ratiosobservedin thedisk ofthe MilkyWay byHelfer&Blitz frequencyof89GHz. (1997), who find 2.6% 0.8% and also in the inner disks (5– The molecular gas masses of the target GMCs vary by a 10kpc) of normalgalax±iesby GS04b who find 4% 2%. The factorof 130between0.1105M (GMCno2)and13105M relatively weak HCN and HCO+ emission may indi±cate a low (GMC1)a∼ndthestarformationrate⊙sderivedfromHαand24µm⊙ fraction of dense mass in GMC91, which thusmay be a rather imagesvarybyafactorofmorethan50.TheFUVfieldstrengths quiescentGMCwith onlya low SFR. Indeed,ithasthe lowest showavariationofmorethanafactor20.Below,wesummarize starformationrateof4M Gyr 1pc 2ofallobservedclouds. themainresults. − − ⊙ 10

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