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Properties of the Molecular Gas in Starburst Galaxies and AGN 9 0 0 2 n S. Mühle∗ a JointInstituteforVLBIinEurope J Postbus2,7990AADwingeloo,TheNetherlands 2 E-mail: [email protected] 2 E.R. Seaquist ] A DepartmentofAstronomyandAstrophysics,UniversityofToronto G 50St. GeorgeStreet,Toronto,ONM5S3H4,Canada h. E-mail: [email protected] p - C. Henkel o Max-Planck-InstitutfürRadioastronomie r t AufdemHügel69,53121Bonn,Germany s a Email: [email protected] [ 1 Thereisgrowingevidencethatthepropertiesofthemoleculargasinthenucleiofstarburstgalax- v iesandinAGNmaybeverydifferentfromthoseseeninGalacticstarformingregionsandthat 8 6 ahighkinetictemperatureinthemoleculargasmayleadtoanon-standardinitialmassfunction 4 in the nextgenerationofstars. Unfortunately,amongthe fundamentalparametersderivedfrom 3 . molecularlineobservations,thekinetictemperatureof themoleculargasin externalgalaxiesis 1 0 oftennotwelldetermineddue to a lack ofsuitable tracer molecules. We discussthe diagnostic 9 power of selected transition lines of formaldehyde(H CO), which can be used as a molecular 0 2 : thermometeraswellasanexcellenttracerofthemoleculargasdensity.Asaproofofconcept,we v i presenttheresultsofourmulti-transitionlinestudyoftheH COemissionfromtheprototypical X 2 starburstgalaxyM82. Usingourlargevelocitygradientmodel,wetightlyconstrainthephysical r a propertiesofthedensegasintheprominentmolecularlobes,completelyindependentofthestan- dard"cloudthermometer"ammonia(NH )orothermoleculartracers.Ourresultsagreewellwith 3 thepropertiesofthehigh-excitationmoleculargascomponentfoundinthemostcomprehensive CO studies. Our observations also indicate that there may be an asymmetry between the two molecularlobes. The9thEuropeanVLBINetworkSymposiumonTheroleofVLBIintheGoldenAgeforRadioAstronomy andEVNUsersMeeting September23-26,2008 Bologna,Italy ∗Speaker. (cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/ PropertiesoftheMolecularGasinStarburstGalaxiesandAGN S.Mühle N 60" E Figure1:Left:HubbleHeritageimageofthestarburstgalaxyM82[14].Right:High-resolutionCO(2→1) mapofthecircumnuclearmolecularringinM82[20].Theobservedpointingpositionsofourstudyandthe correspondingbeamwidthsat218GHz(11′′)aremarkedbythesolidcircles. 1. Introduction Star formation, one of the fundamental processes that shape the evolution of a galaxy and its surroundings, isthoughttobefuelledbymoleculargas,inparticular byitsdensecomponent. Sev- eralobservational studieshavealreadyshownthecorrelation betweenthedensemoleculargasand starformation[e.g.4]. Butdoesstarformationproceedinthesamemannerinalltypesofgalaxies or does it depend significantly on the physical properties of the molecular gas? In recent years, moreandmoreevidence hasbeenfoundforthepresence ofasignificant, orevendominant, warm molecular gas component in starburst galaxies and AGN [10,17] and for nonstandard initial mass functions in active environments [16,6]. Unfortunately, the physical properties of the molecular gasinexternal galaxies,inparticular thekinetictemperature, arerarelywellconstrained. Theeasily thermalized and optically thick CO(1→0) and CO(2→1) transitions would be a goodtemperaturetracer,ifthefillingfactorofextragalacticcloudswasbetterknown. Theinversion lines of the symmetric top molecule ammonia (NH ) are frequently used as “cloud thermometer” 3 in ourGalaxy. However, variations in thefractional abundance ofupto three orders ofmagnitude within the disk of the Milky Way may indicate that ammonia traces only a specific component of the molecular gas. Other commonly observed molecules like HCN are good density tracers, but require an a priori knowledge of the kinetic temperature. Therefore, the dust temperature is often assumedtobeagoodmeasureofthekinetictemperature ofthegas. Inthiscontribution, wepresenttheresultsofourrecentstudyonthephysicalpropertiesofthe moleculargasintheprototypicalnearbystarburstgalaxyM82,demonstratingthediagnosticpower ofselected para-formaldehyde linesforexternalgalaxies [11]. Formaldehyde(H CO)isaslightly 2 asymmetrictopmoleculewithawealthofmillimeterandsubmillimetertransitionsinitssubspecies para-H COandortho-H COandshowsonlylittlevariationinitsfractional abundance inavariety 2 2 of galactic environments [e.g. 5]. In general, line intensity ratios involving different K ladders of a energy levels aregood tracers ofthekinetic temperature, because therelative populations ofthese ladders are governed by collisions. By contrast, line ratios within a single K ladder sensitively a probe the gas density, once the kinetic temperature is known [7]. Thus, H CO is not only one of 2 thefewdirectmolecularthermometers, butalsoanexcellent tracerofthegasdensity. 2 PropertiesoftheMolecularGasinStarburstGalaxiesandAGN S.Mühle NE lobe SW lobe 30 145.60 GHz a 145.60 GHz a 20 20 b Tmb b Tmb (mK) (mK) 10 10 0 0 0 500 0 500 velocity (km/s) velocity (km/s) 20 20 218.48 GHz 218.48 GHz c d e f c d f T(mmKb)10 Tmb 10 (mK) 0 0 0 500 0 500 velocity (km/s) velocity (km/s) Figure 2: Observed spectra of the NE (left) and the SW lobe (right) of M82. Each spectrum is labeled with the frequency the receiver was tuned to. Thus, the velocity scale of the 218.48GHz spectra refers to the H CO(3 →2 ) line. The H CO(3 →2 ) and the H CO(3 →2 ) transitions are offset by 2 22 21 2 03 02 2 21 20 348.5kms−1 and−389.7kms−1, respectively,whiletheCH OH(4 →3 E)lineisoffsetby49.4kms−1. 3 2 1 In the 146GHz spectra, the HC N(16→15) line is offset by 86.46kms−1 from the H CO(2 →1 ) 3 2 02 01 emission. ThecurvesshowtheGaussianfittoeachindividuallineaswellasthespectrumresultingfroma superpositionofallidentifiedlines: a=H CO(2 →1 ),b=HC N(16→15),c=H CO(3 →2 ),d= 2 02 01 3 2 21 20 H CO(3 →2 ),e=CH OH(4 →3 E),f=H CO(3 →2 ). 2 22 21 3 2 1 2 03 02 2. The Properties oftheMolecular GasinM82 Asthemostprominentnearbygalaxyinthenorthernhemispherewithanuclearstarburst,M82 has been the target of many investigations, including numerous molecular line studies [e.g. 3,18]. Figure1showsanopticalimageofthehighlyinclinedgalaxyanditsprominentgalacticwindout- lined intheHa emission (red). Themolecular gasisconcentrated inacircumnuclear ring around the center of the starburst, which is seen nearly edge-on. The target of our observations were the two lobes of the ring, which are easily spatially separable with single-dish telescopes. Tuning the receiversoftheIRAM30-mtelescopeto145.60GHzand218.48GHz,wedetectedtheH COtran- 2 sitions 2 →1 , 3 →2 , 3 →2 , and 3 →2 at 145.60GHz, 218.22GHz, 218.48GHz, 02 01 03 02 22 21 21 20 and218.76GHz,respectively(Fig.2). Notethatthelatterthreelineswereobservedsimultaneously ina1GHzwidespectrum, whicheliminated uncertainties duetopointing errors, calibration issues ordifferent beamwidths inthetemperature-tracing lineratioH CO(3 →2 )/H CO(3 →2 ). 2 03 02 2 21 20 TheHC N(16→15)lineisalsopresentinboth145.60GHzspectra, buteasilyseparated fromthe 3 nearbyH COline. OurobservationsalsorevealedCH OH(4 →3 E)emissioninthespectrumof 2 3 2 1 thenortheasternlobe. ThepresenceofmethanolinthemolecularlobesofM82wasindependently 3 PropertiesoftheMolecularGasinStarburstGalaxiesandAGN S.Mühle log N(pH2CO)/D v = 13.36 Tkin = 191 K 300 14 200 13 Tkin log N(pH2CO)/D v 12 100 11 3.0 3.5 4.0 4.5 5.0 3.0 3.5 4.0 4.5 5.0 log n(H2) log n(H2) Figure3: ResultsofourLVGanalysisfortheNEandSWlobesshownascutsthroughthe3-Dparameter space (T in K, n in cm−3, N /D v in cm−2km−1s) alonga plane of constantpara-H CO column kin H2 pH2CO 2 density per velocityinterval(left) and of constantkinetic temperature(right). The lines representthe fol- lowinglineratios: H CO(3 →2 )/H CO(3 →2 )(red),H CO(2 →1 )/H CO(3 →2 )(blue), 2 03 02 2 21 20 2 02 01 2 03 02 H CO(2 →1 )/H CO(3 →2 ) (green). The correspondingdashed lines outline the uncertaintiesof 2 02 01 2 21 20 each line ratio. The adoptedsource size is q =7.′′5 and the assumed para-H CO abundanceper velocity s 2 gradientisL =1×10−9km−1spc. confirmedby[9]. The basic assumption that the observed emission lines all originate from the same volume of molecular gas with the same average physical properties implies that all lines should have the same velocity profile, parametrized here by a Gaussian curve with the same central velocity and the same line width for all lines. With these constraints, we fitted all detected lines of a spectrum simultaneously using the technique of “fitting dependent lines” in class, a part of the GILDAS software package. Accounting for the small difference in beam widths between the observations at 145.60GHz and those at 218.48GHz, we derived the integrated line intensities of the detected H COlines and compared theline intensity ratios ofour observations with theresults ofour non- 2 LTE models. These models are based on the LVG approximation and a spherical cloud geometry andcoverawideparameterspaceinkinetic temperature, gasdensityandpara-H COcolumnden- 2 sity per velocity interval (see [11] for details). The comparison suggests that the average kinetic temperatureofthemoleculargastracedbytheobservedH COlinesisabout200Kinbothlobes,a 2 moderate gasdensity of7·103cm−3 andatotalmolecular gasmassofabout1.5·108M⊙ perlobe (Fig. 3). Whiletheexactnumbersdependslightly ontheH COabundance andtheexactextentof 2 the emission region, the kinetic temperature of the molecular gas is in any case much higher than thedusttemperatureof48K[2]. At first, a kinetic gas temperature of about 200K may seem odd considering that the dust temperature is significantly lower. However, recent comprehensive studies of multiple CO lines suggest the presence of at least two distinct molecular gas components in M82, where the highly excitedgascomponentmaybethedominantone[19,8]. Inthesestudies,thephysicalpropertiesof the high-excitation molecular gasphase are remarkably similar toour results, which werederived completely independently. Evidence for awarm molecular gas component has also been found in theanalysis ofhighlyexcitedammonialinesinnearbystarburst galaxies [10]andintheinvestiga- tionofIRrotational H linesinasampleofstarburst andSeyfertgalaxies[17]. Shocksandstrong 2 UV and/or X-ray irradiation and cosmic-ray heating are among the heating mechanisms that are 4 PropertiesoftheMolecularGasinStarburstGalaxiesandAGN S.Mühle Figure 4: Left: Superresolved NH (3, 3) emission displayed as black contours on a logarithmic HST 3 WFPC2 F814W image of the core of NGC 253 [15]. Right: Preliminary map of the H CO(1 →1 ) 2 10 11 lineseeninabsorptioninthemolecularringofM82.ThewhitecontoursoutlinetheCO(2→1)emissionas mappedby[20]. likelytobefoundinactiveenvironments likestarburstsandAGNandthatcanheatalargefraction ofthemoleculargastotemperatures ofabout150K[1,17]. 3. Conclusions andOutlook 1. We have demostrated the diagnostic power of selected formaldehyde lines in deriving the physical properties ofthemolecular gasinexternal galaxies. 2. Wedetectedseveralpara-H COlinesat146GHzandat218GHzinbothmolecularlobesof 2 M82,including H COlinesoftheK =2ladder. 2 a 3. Ournon-LTElineratioanalysissuggeststhepresenceofwarm(∼200K),moderatelydense (7·103cm−3)moleculargasnearthenuclearstarburst,whichisinverygoodagreementwith thepropertiesofthehigh-excitationmoleculargascomponentfoundinrecentcomprehensive studies ofmultipleCOlines. 4. OurcasestudysupportstheviewthatinactiveenvironmentslikestarburstgalaxiesandAGN, alargefractionofthemoleculargasmayhaveakinetictemperatureofafewhundredKelvin. 5. We find strong evidence for the presence of methanol in at least the northeastern lobe in M82,confirmingtheresultsof[9]. Duetotheinstrumentallimitationsofthecurrenttelescopesandcorrelators,therearestillvery few investigations of the properties of the molecular gas in external galaxies at resolutions <10′′ using sensitive temperature probes. NGC253, anearby starburst galaxy similar toM82, has been observedwiththeATCAinammoniainversionlines(Fig.4),whichsuggestkinetictemperaturesof 140Kinthenortheastern molecular cloudcomplexesandof200Kinthesouthwestern complexes [15]. Preliminaryresultsofhigh-resolutionobservationsofM82centeredontheH CO(1 →1 ) 2 10 11 line(Fig.4)showthetransition inabsorption againstthecontinuum [12]. TheH COmorphology 2 5 PropertiesoftheMolecularGasinStarburstGalaxiesandAGN S.Mühle and kinematics seem to follow the distribution and kinematics of the CO emission [20] with the exceptionofthesouthwesternlobe,whichshowslessH COabsorptionthanthenortheastern lobe. 2 With the commissioning of ALMA and the new correlator of the EVLA, we will be able to detect molecules like formaldehyde and ammonia with high spatial and frequency resolution in a large number of galaxies, employing their full diagnostic power to derive the physical properties of the molecular gas in galactic environments very different from the Milky Way disk and thus to revealtheinterdependence ofmolecular gas,starformationandfeedback processes. Acknowledgments We wish to thank the staff at Pico Veleta/Granada for their support during the observations. E.R.S. acknowledges a Discovery Grant from NSERC. 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