198 9ApJ. . .344L. .61D 20- -21 20 -1 scope, SEST,atESO/LaSilla,Chile. to getdynamicalmassesifthe molecularcloudsareassumedto and LargeMagellanicClouds (SMCandLMC),isitpossible and globalmethods.Onlyfornearbyobjects,liketheSmall discussed inthiscontext,andhigherY[JF(CO)]valuesby up (Elmegreen, Elmegreen,andMorris1980;Young,Gallagher, A typicalfactor,X[JF(CO)]=N(H)/W(CO\foundforthe by scalingthevelocity-integratedCOline,W(CO)=fT*dv. molecular hydrogencolumndensity,iV(H),isusuallyderived meter lines,itisusedasatracerforthemoleculargas.The and Hunter1984;TacconiYoung1985,1987)havebeen excitation conditionsduetoanincreasedintensityoftheultra- of galaxy(Thronsonetal.1987;Israel1988;Cohen1988). to afactorof10weresuggestedbyseveralauthorsforthis type Milky Wayis2f[JF(CO)]=2.6x10(moleculescm distributed moleculeafterHandhaseasilyobservablemilli- processes andhistory.AstheCOmoleculeismostwidely observed lowCOintensitiesofirregular(Irr)galaxies are discussedindetailbyMaloneyandBlack(1988). The violet (UV)radiationfieldandabundanceseffects.Theseeffects (see alsoHeithausenandMebold1989foradiscussionof (molecules cm[Kkms])foranumberofcirrusclouds Galactic Y[JF(CO)]factors). commonly acceptedforallregionsofourGalaxy.DeVries, of thekeyobservationstounderstandingstarformation Heithausen, andThaddeus(1987)foundvaluesof0.5x10 spiral galaxies(e.g..Youngetal.1989).Butthisfactorisnot values, derivedfromMilkyWayclouds,areusedfornormal [K kms])(e.g.,Bloemenetal.1986)andverysimilar 2 2 G 2 4 The AstrophysicalJournal,344:L61-L64,1989September15 3 2 © 1989.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A 1 Harvard-Smithsonian CenterforAstrophysics. LowellObservatory. RadioastronomischesInstitutderUniversität Bonn. BasedonobservationswiththeSwedish-ESO 15msubmillimetertele- For mostIrrgalaxiesthemassofHisderivedbyindirect The basicexplanationsfortheobservedhigherXvaluesare The contentofmoleculargasingalaxies,mainlyH,isone 2 2 20 7 2A Subject headings:galaxies:individual(NGC55)—interstellarmatterstars:formation into acolumndensityofmolecularhydrogen,N(H),isfoundtobe)/W(CO)=60x10(molecules ing that80%ofthecloudismolecular.Thefactortransformsvelocity-integratedCOline,IT(CO), axis. Assumingvirialequilibrium,thetotalmolecularmassofcloudisestimatedtobe8x10M,imply- j =1(-►0COline.ThedistributionofcorrespondswellwiththeintenseHicloudnearbarNGC is low,M~0.3yr\butmusthavebeenhigherinthepast. 55. TheextentoftheCOcloudisabout975pcalongmajoraxisand390perpendicularto metallicity seemstobeinverselyproportionalthenitrogenabundance.Thepresent-daystarformationrate mass asderivedfromdustmasseswhichcanbeinferredIRASFIRfluxes.Fromacomparisontothe cm [Kkms]^similartothevaluepreviouslyfoundforSmallMagellanicCloudandabout20 © American Astronomical Society •Provided bythe NASAAstrophysics Data System times higherthanvaluesforourGalaxy.Thisisconfirmedbyanindependentestimateofthemoleculargas Magellanic CloudsandIC10,weconcludethatforirregulargalaxiesthedependenceofiV(H)/IF(CO)on 2 0 0 2 CARBON MONOXIDE(CO)INTHEMAGELLANICIRREGULARGALAXYNGC55AND Using theSEST15mtelescopeonLaSillainChile,NGC55wassearchedandsuccessfullymapped I. INTRODUCTION 1 N(H)/IT(CO) RATIOINIRREGULARGALAXIES 2 2,34 R.-J. DettmarandA.Heithausen Received 1989May24;acceptedJuly10 ABSTRACT L61 low-resolution acusto-optical spectrometer using 728channels encoder, thepointingaccuracy wassometimesaspoor5", but mostofthe timebetterthan2".Spectra weretakenwitha tions wereapproximately12'. Duetoproblemswiththe azimuth theotherathigher.Distance betweenonandoffposi- mode wasusedwithtwooff-positions, onelocatedatlower and weatherconditions.A balanced on-offbeamswitching perature ofabout500-650K depending onpositioninthesky cooled Schottkydiodemixerreceiverwithatotalsystem tem- metallicity ofthisgalaxy. SEST 15mtelescope.Thetelescopewasequippedwith a the Galaxyandthatthisisatleastpartlyrelatedto low conversion factors20timeshigherthanthecanonicalvalue for librium forthemolecularcloudobservedinNGC55leads to (FIR) (Riceetal.1988). Dettmar, andWielebinski1986),far-infraredradiation such astheHi21cmline,6radiocontinuum(Hummel, and tostudyitscorrelationwithotheremissionobservations quate todeterminetheoveralldistributionofmoleculargas NGC 55of2Mpc,the43?5beamSESTtelescopeat115 GHz correspondstoalinearresolutionofabout420pc,ade- is locatedonLaSillainChile.Attheassumeddistancefor detail, weobservedthisgalaxyintheJ=1>-►0COlinewith the 15mSwedish-ESOSubmillimeterTelescope(SEST)which of moleculargaswithinMagellanic-typegalaxiesinmore to theLMC.Inorderstudydistributionandproperties the LocalGroup.ItisclassifiedasSBmandshowssimilarities (Cohen etal.1988)and25timeshigherfortheSMC(Rubio resulting Y[!F(CO)]valuesare6timeshigherfortheLMC within theGalaxyandifappliedtoMagellanicClouds, be invirialequilibrium.Thisassumptionworksverywell 1988) thantheaverageXvalueforourGalaxy. G The COobservationswerecarriedoutin1988Julywiththe In thisLetterweshowthattheassumptionofavirialequi- NGC 55isthenearestgiantMagellanic-typegalaxybeyond II. OBSERVATIONSANDREDUCTION \—IQ

^ L62 DETTMAR AND HEITHAUSEN Vol. 344

-1 and a velocity resolution of 1.792 km s at 115 GHz. The a) The N(H2)/W(CO) Ratio ft integration time was 40 minutes (on + off) resulting in an rms (T) Since the CO complex is the most prominent star-forming 00 noise of about 15 mK. The removed baseline was always of region in NGC 55, we can assume that the molecular gas is in \ 1 first order. The beam efficiency is about rj = 0.68. For more virial equilibrium. We can derive its virial mass from details on the telescope and its equipment, see Booth et al M = 210 At;2R , (1989). vir where we assumed that the density is constant. Av is the aver- III. RESULTS aged velocity distribution (in km s-1), and R is the radius of CO was detected toward the direction where the Ha and 6 the cloud (in pc). As this cloud is near the minor axis and in the cm radiocontinuum emission is strongest (Hummel, Dettmar, region of solid body rotation, the line-of-sight integration does and Wielebinski 1986). The map is fully sampled at a spacing not significantly contribute to the observed velocity dispersion. between two adjacent points of 20". The resulting spectra are For Av we use the full width at half-maximum of the CO line shown in Figure 1. The CO line temperature ranges from 34 to averaged over all CO lines, which is 35 km s-1. The averaged 94 mK; the line width (full width at half-maximum) ranges line amplitude is 57 mK. We find that the cloud’s mass is about _1 7 7 from 20 to 43 km s . The CO complex has an extent of 8 x 10 Mq. The comparison to the H i mass of 1.7 x 10 M© 100" x 40", corresponding to about 975 pc along the major for this complex found by Hummel, Dettmar, and Wielebinski axis and 390 pc perpendicular to the major axis. It is in posi- (1986) shows that this cloud is mainly molecular. Comparing it tional coincidence with the prominent H i cloud T SE of the to an averaged IF(CO) value of about 3 K km s- \ we derive a bar structure, and the CO velocities also fit very well with the X[W(CO)] ratio of 60 xlO20 (molecules cm-2 [K km H i velocities found by Hummel, Dettmar, and Wielebinski s-1]-1). This is approximately 20 times the average value for (1986). As the radio continuum and Ha emission also peaks in our Galaxy. This conversion factor is of course at best an order this region, it is probably associated with the dominant star- of magnitude estimate. Uncertainties arise because (1) NGC 55 forming region in NGC 55. On the spatial resolution of our is seen edge-on and the geometry therefore is unknown; (2) the map, the CO emission resembles clearly a number of clouds. complex is clearly not resolved, and thus there might be Nevertheless, the emission is quasi-continuous, similar to the regions with higher IT(CO) values; or (3) the cloud may have a large molecular complex found near the 30 Dor complex in the dense, unobserved core and, subsequently, lower -values. LMC (Cohen et al 1988), so that we can regard it as one cloud This should be checked with higher resolution, e.g., by obser- complex. Since the size of the NGC 55 complex is smaller than vations in the J = 21—► 1 CO line. the one near the 30 Dor in the LMC, but the mass is compar- An independent estimate for the mass of the molecular gas able, the NGC 55 complex is denser. can be obtained from the IRAS FIR fluxes. We will follow here A second star-forming region, visible in 6 cm radiocontin- the procedure described by Hunter et al (1989) in their study of uum emission (Hummel, Dettmar, and Wielebinski 1986), was properties of Irr galaxies from IRAS observations. The IRAS also searched but not detected in CO. This region is centered fluxes for NGC 55 are taken from the catalog by Rice et al at R.A.(1950) = 00h12m43s6, decl.(1950) = -39°29'29". The (1988). The FIR emission is described as emerging from three upper detection limit for the CO line temperature at this posi- point sources which can be identified with the star-forming tion is 30 mK, which corresponds to the 3 a rms noise. regions as indicated by the Ha emission (Hummel, Dettmar, and Wielebinski 1986). This and the FIR colors [S(100)/ 5(60) = 2.1] exclude large contributions by a cirrus com- ponent. The use of the S(60)/S(100) flux ratio results in a dust tem- perature of Td = 29K. The relation Md = 0.256 x S(60)/Jy 2 240 k/Td x D /Mpc x (e — 1) then leads to a dust mass of Md = 3.37 x 105 Mq. We note that the use of different relations for the dust temperature and for the dust mass as inferred from FIR fluxes (e.g., Thronson and Telesco 1986) results in masses that are higher by up to 60%. On the other hand, a different composition of the dust grains, e.g., graphites instead of sili- cates, could reduce the dust mass. If we finally make use of the relation M(H2) ~ 500 Md (Young et al 1989) that holds for a wide range of spiral gal- axies, including the Milky Way (Thronson and Mozurkewitch 8 1989), we find M(H2) ^ 1.7 x 10 M©, which is within a factor of 2 in agreement with the virial mass estimate for the molecu- lar mass. This reassures us that the assumption of virial equi- librium for the cloud is appropriate and the molecular mass found indicates that the conversion factor for the molecular mass in Irr galaxies as inferred from CO line emission is indeed higher by up to a factor of 20 compared to the canonical value Right Ascension Offset [arcmin] for the Galaxy. Fig. 1The distributions of the CO lines in NGC 55. Positions are offsets b) Dependence on Metallicity in arcminutes relative to R.A.(1950) = Q(^12m23s5, decl.(1950) = -39°28'30". The box in the lower right-hand corner gives the amplitude, TJ, in K, and the Metal abundances are considered to be the most important heliocentric velocity. controlling parameters for the relation between the observable

© American Astronomical Society • Provided by the NASA Astrophysics Data System 198 9ApJ. . .344L. .61D 46 6 6 4 46 CO andthetotalcontentofmoleculargas(e.g.,Maloney Black 1988).NGC55haslow-metallicityHnregions(Webster No. 2,1989 and Smith1983;Vigroux,Statsiñska,Comte1987, references therein);thismakesitsinterstellarmediumcompa- rable tothatoftheLargeandSmallMagellanicClouds,which are alsometal-poorIrr’s(Dufour,Shields,andTalbot1982). CO, andtheconversionfactorsXfoundthereencompassour Both MagellanicCloudshavebeenstudiedindetailrecently value forNGC55. (2.5 x10“;110"forSMC),theN/Hratio(4.8)is compare thedifferentXvalueswithoxygenandnitrogen (3.1 x10").AstheconversionfactorXforNGC55found in betweenthevaluesforLMC(8x10")andSMC regions inNGC55(2.2x10~)iscomparabletotheLMC abundances directly.WhiletheO/HratioformeasuredHn (1988), thismightindicateaninverseproportionaldependence here isalmostashighthatoftheSMCfoundbyRubio measured COdistribution,namelyIC10,isincluded.Wehave on thenitrogenabundances. calculated virialmassesforthethreeextendedCOcloudcom- plexes inthisgalaxyfoundbyOhta,Sasaki,andSaito(1988) the samewayasaboveforNGC55,assumingadistanceof1 the differentregions.ThemeanabundancesforIC10areO/ Mpc. Thisresultsinconversionfactorsbetween7and10for H =1.25x10"andN/H6.5(Vigroux,Statsiñska, and Comte1987).InFigure2wehaveplottedX/Xqagainst from Dufour,Shields,andTalbot(1982).Whilearoughdepen- Way wehavealsoincludedthevaluesforOrionnebula O/H andN/Habundances.ForcomparisonwiththeMilky interpreted intermsofcarbonabundancessinceC/Hdoesnot dence ofX/XqonO/Habundancesisfoundforthewhole Talbot 1982).Theratiooftheseelementsisbelievedtobea scale withN/HintheMagellanicClouds(Dufour,Shields,and range ofmetallicities,agoodcorrelationexistsbetweenX/X and N/Hforthelow-metallicityIrrgalaxies. product ofbothprimaryandsecondarynucleosynthesispro- G to theaveragevalueofourGalaxy, on O/HandN/Habundances.Forcom- (1982) areincluded. Thecontinuouslineindicatesthedependence ofX/Xqon parison withtheGalaxy,abundances fortheOrionnebulafromDufouretal. N/H forIrrgalaxies. In theabsenceofknowncarbonabundances,wecanonly This dependenceisstrengthenedifanotherIrrgalaxywith Unfortunately thenitrogenabundancescannoteasilybe pio. 2.—ThedependenceofX/X,the conversionfactorX[JP(CO)]scaled G © American Astronomical Society •Provided bythe NASAAstrophysics Data System CO INNGC55 13 9 7 1 1 9 1 8 1 cesses andthereforedependsontheinitialmassfunctionsof indicative, anditisofcoursestillpossiblethattheradiation the producingstars. field playsanimportantrole.Toexcludeexcitationeffectsasa reason forthehighX-value,COobservationsNGC55 NGC 55inthesenseofstarformationhistory.Themolecu- are needed. lar andatomicgasinthewholegalaxytogetheraccountsfora total of-2x10M,ifoneincludesacorrectionforthe cloud isthemainreservoirforongoingstarformation. presence ofHe.Themoleculargas8x10Minthecentral most “normal”spiralandIrrgalaxies.Evenifthereis,com- total massofyoungstars.ForNGC55thismethodresultsin an efficiencyof€=M/M(H)-1%,averytypicalvaluefor pared totheatomicgas,onlyverylittlemoleculargasavail- Telesco (1986)haveusedtheinfraredluminositytoderive with themassofstarsthatwasjustformed.Thronsonand turned intostarscanbeestimatedifthegasmassiscompared 1 comparable amountsofatomicandmoleculargas. able, itisconvertedwiththesameefficiencyasingalaxies star-forming region,wecanusetherelationforpresent-day star formationrate(SFR)fromThronsonandTelesco(1986). yr "fromtheradiocontinuumflux.Withpresent-daySFR 0 and Wielebinski(1986)havededucedaSFRofM=0.2 From thiswefindM=0.4yr"andHummel,Dettmar, 0 for -7x10yr,butthecentralcloudwillbeusedupinless of 0.3Myr"asameanvalue,thetotalgassupplywilllast stars2 than 5x10yr. within thelastbillionyearsthatcanbededucedfromL (Gallagher, Hunter,andTutukov1984).TheresultofM=1.6 colors ofthegalaxyshowthatNGC55isina“low”phase M yr"forthismeanSFRinthepastandveryblue star formation. 0 IR0 lar galaxyNGC55andfoundamolecularcloudcomplex of q tional coincidencewiththestar-formingregionandprominent B H icloud1'totheSEofbar. good agreementwithmassesinferredfromFIRfluxesunder conversion factorfortherelationbetweenIF(CO)andiV(H ) the assumptionofatypicaldust-to-gasratio.Thisleads to a q Galaxy. that is20timeshigherthantypicalvaluesforspiralsor the 100" x40",correspondingto975390pc,whichisinposi- abundances andthatitincreases linearlywithdecreasingnitro- mass ofyoungstarstothe oftheavailablemoleculargas gen abundanceforIrr’s. in NGC55is~1%,similarto spiralgalaxies.Thepresent-day mean starformation rateinthepast. star formationrateof-0.3 M yr"islowcomparedtoa 10 wefindthatthisconversion factorisdependentonmetal 2 0 The dependenceonmetallicitydiscussedherecanonlybe 7 Finally wewanttocheckbrieflytheevolutionarystatusof The efficiencywithwhichtheavailablemoleculargasis As theFIRradiationofNGC55isdominatedbymain The present-daySFRshouldbecomparedwithamean 2. Thederivedvirialmassforthiscloudof8x10M©is in 1. WehavesearchedforCOintheMagellanic-typeirregu- 4. Thestarformationefficiency definedastheratioof 3. FromacomparisonwiththeMagellanicCloudsandIC c) StarFormationRateandEfficiency IV. SUMMARY L63 198 9ApJ. . .344L. .61D 4 DETTMARANDHEITHAUSEN manuscript. BothauthorsaresupportedbytheDeutscheFor- P. Thaddeusforhelpfuldiscussionsandcommentsonthekindhospitality, L6 Cohen, R.S.,Dame,T.,Garray,G.,Montani,J.,Rubio,M.,andThaddeus,P. Bloemen, J.B.G,M.,etal.1986,Astr.Ap.,154,25. Booth, R.S.,etal.1989,Astr.Ap.,260,350. Dufour, R.J.,Shields,G.A,andTalbot,J.1982,Ap.252,461. De Vries,H.W.,Heithausen,A,andThaddeus,P.1987,Ap.J.,319,723. Elmegreen, B.G.,D.M.,andMorris,M.1980,Ap.J.,240,455. Ralf-Jürgen Dettmar:LowellObservatory,MarsHillRoad,1400West,Flagstaff,AZ86001 Gallagher, J.S.,Hunter,D.A.,andTutukov,V.1984,Ap.J.,284,544. Heithausen, A.,andMebold,U.1989,Astr.Ap.,214,347. 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