1985ApJ. . .290 . .602T 12 by Hunter(1982).Thecomplexes inNGC3728arewhatshe massive Hncomplexes,which havebeenwellstudiedoptically higher thanthatinthesolarneighborhood. their globalstar-formingproperties,andhavefoundthatthe galaxy toanotherareeasily made.Thesegalaxiescontain (Hunter 1982),comparisonsof similarlysizedregionsfromone star formationrate(SFR)inirregularscanbeupto1000times optical and21cmsurveyofirregulargalaxiestodetermine gher, andRautenkranz(1982;hereafterHGR)havemadean forming materialpresentinthesesystems.Irregulargalaxies crucial fordeterminingthedistributionandamountofstar- 4449. Adoptedparametersfor thegalaxiesarelistedinTable1. they havenoobviousglobalmechanismsforstarformation, to containgiantHncomplexesandOBassociations,yet show evidenceforrecentstarformation,sincetheyareknown in eachgalaxy,withthestrongest emissionfoundinNGC such ascompressionofgasalongspiralarms.Hunter,Galla- Since thethreeirregularsare allatadistanceof5.4Mpc axies NGC4449,4214,and3738.COwasdetected and Hunter1984). and Scoville1984);theLargeMagellanicCloud(Hugginset al. lars: M82(Rickardetal1977;StarkandCarlson1984;Young limit. COdetectionshavebeenmadeinthefollowingirregu- they expectedtofindCOandmadenodetectionsatthe0.04 K for COat52positionsinsixMagellanic-typeirregularswhere (Elmegreen, Elmegreen,andMorris1980;Young,Gallagher, and Hunter1984).Elmegreen,Morrissearched deficient inCOrelativetospiralsofthesameluminosity in whichobservationshavebeenmadefoundtobe tions haveconcentratedonspiralgalaxies,andthoseirregulars irregular galaxies.Thusfar,mostextragalacticCOobserva- might expecttoobserveahighabundanceofmoleculargasin 1975; Israeletal.1982);andNGC1569(Young,Gallagher, The AstrophysicalJournal,290:602-608,1985March15 © 1985.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. Observations ofthemoleculargasinexternalgalaxiesare Here, wepresentCOobservationsofthethreeirregulargal- Since itisbelievedthatstarsforminmolecularclouds,one 29±0 more opticallyluminous. Subject headings:galaxiesstellarcontent—interstellar:moleculesnebulae:Hnregions late-type spirals(YoungandScoville1982h).Thus,irregulargalaxieswiththesameCOintensityasare (L/d)-. Thisissignificantlydifferentfromtheroughlylinearrelationshipfoundforcentral5kpcin lars withtheluminousScdgalaxyIC342,allofwhichareroughlyatsamedistance,central50" stronger emissionfoundinoneHiicomplexthanthecentralregionofgalaxy.Comparingirregu- irregulars have2-5timeslessblueluminositythanthecentral50"inIC342butfactorsof40-70CO. and NGC3738,alsoinagaintHncomplex4449usingthe14mtelescopeofFiveCollege’ Radio AstronomyObservatory(HPBW=50").TheCOdistributioninNGC4449appearsclumped,with co 2 © American Astronomical Society • Provided by the NASA Astrophysics Data System A powerlawrelationisobservedbetweentheCOandbluefluxesinirregulargalaxieswithL/doc CO emissionhasbeendetectedinthecentral1.3kpcofthreeirregulargalaxiesNGC4449,4214, B CO ABUNDANCESANDSTARFORMATIONINTHETHREEIRREGULARGALAXIES Five CollegeRadioAstronomyObservatoryandDepartmentofPhysicsAstronomy,UniversityMassachusetts,Amherst I. INTRODUCTION stars: formation NGC 4449,4214,AND3738 Linda J.TacconiandJudithS.Young Received 1984July26;acceptedOctober5 ABSTRACT 602 8910 10 h89 9 f89 9 e 3 d c a, ahms _1b scaled toadistanceof5.4Mpc);from HGRforNGC3738. 3738 andNGC4214vanWoerden etal.forNGC4449. and NGC4449. extensive complexofabout400pcindiametercomprisedan contains twooff-centermassiveHncomplexes—thefirstisan In addition,NGC4449containsaTypeIIsupernovaremnant second iscomprisedofanarcHnregions(cf.Hunter1982). arc ofyoung,bluestarsjoinedtoanHnregions,andthe fined tothehighsurfacebrightnesscentralregion.NGC4449 galaxy, whiletheHnregionsinNGC4214aregenerallycon- confined totwoclumpsawayfromthecentralregionof classifies asa“two-clumped”system,wherethecomplexesare M(H i)/M0.040.100.06 M(H i)/L(M/L)0.460.630.28 Total massM(M)6.8x102.33.4 L(L) 6.6x103.87.2 M(H i)(M)3x102.42 R(H i)(kpc)15x4760 Inclination 39°37°44° £> (kpc)4.312.58.0 Z) (arcmin)Z757!945T3 Distance (Mpc)5.4<,< Classification" ImIAB(s)mIBm 50" ongalaxy(kpc)1.3 Decl. (1950).54°48'0':036°36'30':044°22T8:0 R.A. (1950)ll334812132546 K(kms) 230289200 b0 0 h BQ 0 8 f 25 25 e d 3 c b LSR Luminositiesfrom fromRC2;M=5.48. FromMatteucciandChiosi(1983) forNGC4214and4449(values HGRandvanWoerdenetal. R(Hi)istheregionoverwhichHihas beendetected;fromHGRforNGC Distheisophotaldiameterat25th magnitudefromRC2. deVaucouleurs,andCorwin(1976;hereafterRC2). HGR. PeakHavelocityfromHunter(1982)forNGC3738;RC24214 BO 25 Parameter NGC373842144449 Galaxy Parameters TABLE 1 1985ApJ. . .290 . .602T © American Astronomical Society •Provided bythe NASAAstrophysics Data System

Fig. 1.—Enlargements of the three irregular galaxies NGC 3738, NGC 4214, and NGC 4449 from the blue Palomar Sky Survey plates. The circle indicates the 50" beam size of the 14 m FCRAO telescope, corresponding to 1.3 kpc at a distance of 5.4 Mpc. 1985ApJ. . .290 . .602T _1 -1 1 NGC 4449ispresentedinFigure 2.Inthisgalaxy,moreCO and Scoville1984;Young,Gallagher,Hunter1984). intensities foundintheouterdisksofspirals,suchasIC 342 and M51(YoungScoville1982a;Young tan DistrictCommission,Commonwealth ofMassachusetts. dation undergrantAST82-12252and withthepermissionofMetropoli- axies (seeTable2).Thesevaluesarecomparabletothelowest grated intensitiesof0.5to1.1±0.15Kkmsinthese gal- position inNGC3738.Theemissionwasdetectedwith inte- complex ofNGC4449;marginallydetected(atthe3alevel)in the SWHiicomplexinNGC4449,noreitheroff-center the centersofNGC3738and4214;notdetected in center ofNGC3738.Table2liststhelineparametersforall the observations. complexes inNGC4449andpositions50"oneithersideofthe region ofeachgalaxy,wehaveobservedtwomassiveHn galaxies. Inadditiontotheobservationsmadeincentral (KL). method describedinYoungandScoville(1982a)producinga 1983) andinirregularssuchasM82NGC1569(Young to adiameterof1.3kpcassumingdistance5.4Mpcforthe temperature scalesuchthatT*=70KfortheOrionNebula point. Intensitycalibrationwasperformedaccordingtothe center ofeachgalaxy,foratotal~4hoursintegrationper posed toshowthe50"half-powerbeamwidthoftelescope with a256channelfilterbank1MHzchannels,andwere mixer receiverwithT%170K.Thespectrawereresolved switching every30secondstolocations10'EandWofthe smoothed to10kms.Datawereobtainedbyposition (FCRAO). Thetelescopewasequippedwitha3mmcooled at theCOJ=1-0frequency(115.271203GHz),corresponding NGC 4449fromthePalomarSkySurvey,withcirclessuper- 604 antenna oftheFiveCollegeRadioAstronomyObservatory between 1983MarchandDecemberusingthe14m The extremebrightnessofthisremnantisattributedto lengths (KirschnerandBlair1980;SeaquistBignell1978). which isthemostluminousknownatopticalandradiowave- extraordinarily highdensitymaterialofmetallicity. A SSB 1 The spectrumofCOemission fromthestrongestpositionin CO hasbeendetectedinthecenterandSEHn TheFCRAOisoperatedwithsupport fromtheNationalScienceFoun- Figure 1showsenlargementsofNGC3738,4214,and CO observationsofthethreegalaxieswereobtained © American Astronomical Society • Provided by the NASA Astrophysics Data System II. OBSERVATIONS a) NGC4449 in. RESULTS NGC 37381(0,0)0.020.50±0.1540 NGC 42141(0,0)0.020.62±0.1560 NGC 44491(0,0)0.020.78±0.1550 Upper limitsare2cr;forIupperintegralsover50kms~L co -1 Position (arcminfromcenter)(K)(Kkms)(kms 2 (-0.33,-0.68)...<0.30 3 (0.33,0.68)...<0.30 2 (0.32,-0.47)0.041.13±0.1540 3 (-0.32,-0.57)...<0.40 TACCONI ANDYOUNG a (Aa, AÔ)T*/AV CO Observations aco TABLE 2 _1 -1 1 -1 1 to 10kms^resolution. has madehigherresolution(58"x96")Hiobservationsand the velocitycentroidsareboth~300kms.Allsopp(1979) resolution and73kmsforHiatlower(HGR); line widthatthe20%intensitylevelis60kms~forCO50" with thehighestsurfacebrightnessportionofNGC4214.The regions. that theadditionalmotionsmaybeduetogasintheseouter which ismuchlargerthantheCOresolution.Thus,itlikely observed inthisgalaxyoutto~10timestheopticaldiameter, the twovelocityrangesareprobablyduetofactthatHiis Woerden, Bosma,andMebold1975).Thedifferencesbetween 260 kms(Rogstad,Rougoor,andWhiteoak1967;van from 200-250kmswhiletheHiemissionoveramuch larger regionischaracterizedbyvelocitiesbetween-140and (position 3inTable2). regions connectedtoanarcofolder,bluestarsisfound on thewesternsideofgalaxy,whereanotherarcHn Table 2).However,similarlystrongemissionisnotobserved cident withanarcofHnregionsinthegalaxy(position2 tours fromtheeastern,low-velocitypartofgalaxyiscoin- diagram forNGC4449inFigure3.Thehighdensityofcon- and Palmer1981;YoungScoville1982a).Theanomalous and decreasingintensitieswithradiusinthedisks(cf.Rickard that oftheluminousScgalaxieswhichexhibitcentralpeaks ing thatthemoleculargasdistributionismuchdifferentfrom CO intensitydistributionisillustratedinthespatial-velocity was detectedatposition2thanthecentralposition,indicat- Fig. 2—SpectrumoftheSEHncomplex positioninNGC4449smoothed The COdetectedinthisgalaxy(atthe3alevel)iscoincident The COvelocitiesinthecentral50"ofNGC4449range 0 100200300 400500 b) NGC4214 Vkm s LSR (~') Vol. 290 1985ApJ. . .290 . .602T -1 _1 No. 2,1985 -1 1 _1- measured thevelocitiesforNGC4214torangefrom257327 0.003 Kwiththelowestcontourat0.015K. km s.ThesevaluesagreequitewellwiththeCOdata. the threeobservedpositionsinNGC4449.Contourlevelsareintervalsof attributed totheinner1.1kpc.Thisindicates,then,that Allsopp hasalsofoundrotationalvelocitiesof±7kms motions inthezdirection. primarily torotation,butrathernoncircularmotionssuch observed COlinewidthinthesameregionmaybeduenot as radialflows,randomvelocitiesofindividualclouds,or whereas thatoftheHiprofile(HGR)overaregion10'acrossis sity levellinewidthforthecentralCOprofileis~40kms, km s”fortheCOandHiprofiles,respectively.Since resolution ismuchgreaterthanboththeCOand 3 alevel),butnotineitheroff-centerposition.The20%inten- motions fromtheentiregalaxy.FromnarrowCOline 2'.6 opticalsizeofthegalaxy(deVaucouleurs,de width, andthesinglepeakedHiprofile,itisevidentthatthis and Corwin1976;hereafterRC2),theHiprofileincorporates ~75 kms;thevelocitycentroidsare225and240 Fig. 3.—ObservedCOintensitiesasafunctionofvelocityandpositionfoi CO emissionwasdetectedinthecenterofNGC3738(at 6 6 6 76 017a04 a02 b6 5 8 8 8 8 2c © American Astronomical Society • Provided by the NASA Astrophysics Data System measurements <14%ofthearea 50"CObeamonthegalaxy. M (M)4.6x106.6<1.8 4 x103.2<1.9 M\{M) 2x10 1.7 x101.6 MJL{M/) 0.005 ...0.0080.014 M +^Hi/Mdyn°- ••• M/M 0.005... 0.032 0.007 M/M 0.012... 0.007 0.013 Mh/M 2.33.3<0.9 0.2 2.0<1.2 L(L) 2.3x10... 7.3 x105.0 L(L) 9x10... 4.8 xio2.3 M(M) 3.8x10... 5.4 x102.4 M/L(M/L) 0.290 ...0.2870.460 h0 hq hbq0 H2 HIdyn H2dyn hi Ha0 bq dyn0 H2HaQ 3 b c Hmassesforthesepositionsare2 a upperlimits. Lisobtainedbysummingseveral 5"-7"aperturemeasurementsfromHunter(1982).Inallcasesthetotal areacoveredbytheHa Misscaleddown totheareacoveredbyHaobservations. 2 Ha h a a Position 123 1 23 c) NGC3738 NGC4449 THREE IRREGULARGALAXIES NGC 444942143738 Galaxy Masses TABLE 3 1 2 6 62 6 irregular galaxyhasamuchlowerpeakrotationalvelocity coupled withavastlydifferentHidistributionthanthatof sities, wehavefollowedtheempiricalanalysisofYoungand late-type spiral,whichtypicallyshowsadouble-hornedHi Galaxy inordertoobtainanestimateofthemassH.At tions ofdarkcloudsandgiantmolecularinourown profile andaFof100-300kms“. is atbestuncertaininviewofpossibleabundanceandexcita- present, theapplicabilityofthisanalysistoanirregulargalaxy Scoville (1982a)whousedCOandvisualextinctionobserva- tion differencesbetweenmolecularcloudsinirregularand luminosities amongthegalaxieswehavestudied,derived spiral galaxies.However,duetothesimilarmetallicitiesand H massesofoneirregularrelativetoanothershouldbereli- able. 4449 fromequation(3a)ofYoungandScoville(1982a): H intheinnerregionsofNGC3738,4214,and inclination ofthegalaxy.TheHmassesarethenfoundfrom where IistheobservedCOintegratedintensityandi where Aistheareaofbeamongalaxyinpc.For 2 max assumed distanceof5.4Mpcforthesethreegalaxies,thearea complex positionhasahigherHmassthanthecentralposi- roughly thesame,withvaluesrangingfrom~3x10Mto resulting Hmassesforthecentral50"oftheseirregularsare of theFCRAO50"beamis1.35x10pconeachobject.The tion. However,intheSWposition,wefindlessthanhalfof H thatisobservedinthecenterofgalaxy. 2 for thecentral50"in3irregulargalaxies(seeTable3).Since 2 2 co 2 0 2 ~5 x10M©(seeTable3).InNGC4449,theSEHn 2 To deriveHcolumndensitiesfromtheobservedCOinten- We haveobtainedvaluesfortheface-onsurfacedensityof 2 We havecomparedthederivedHmasseswithi 2 20 N «4x10/(cosi)[H/cm],(1) H2co2 M(H) ä6/(cosi)[M],(2) 2co0 IV. DISCUSSION a) Masso/H 2 b) MassofHi 605 1985ApJ. . .290 . .602T 8 8 _1 6 7 -1 9 under theassumptionthatobservedvelocityspreadispri- (Young andScoville1982h). literature. Theblueluminosities,derivedfrom50"magni- in NGC4214.Thus,thetotalgasmass(M+Mi)makesup imately 0.5%ofthedynamicalmassinNGC4449and while inthecentral50"ofNGC4214Hmakesuponly0.5% NGC 3738,Hconsistsofroughly1%thedynamicalmass, with thefindingsofHunter(1982)thatirregulargalaxies 606 galaxies theMJLratioislessthan0.014/,while the 9 x10L,whereNGC4449isthemostluminous,and galaxy, wehaveusedmulti-aperturephotometrydatafromthe fractions willbecorrespondinglyhigher. irregulars and4%inNGC4214.Wenotethatifpartofthe Table 3indicatethatforthecentral50"ofNGC4449and display littleornoevidenceforrotation.Theresultsgivenin marily duetorotation.TheCOlinewidthsforthecentral50"in spiral galaxiesgenerallyhavevaluesmorethan10timeshigher tudes andassumingM(0)=5.48,rangefrom2x10to the truedynamicalmasseswillbelower,andHmass velocity widthoftheCOlinesisduetononcircularmotions, about 2%ofthecentraldynamicalmassinformertwo of thedynamicalmass.Accordingly,Hiconstitutesapprox- these galaxiesaretypically~50kms,whichisconsistent vations todeterminedynamicalmassesintheirregulargalaxies for NGC4214is~0.3.Thatis,hasahigherfrac- for thecentral50"informertwogalaxiesare~2,ratio able forthegalaxiesstudiedhere,wehaveusedCOobser- tion ofitsISMinatomicformthantheothertwogalaxies. roughly 2x10MinNGC3738and4449,nearly from thelowerresolutiondataofHGR.The50"Himassesare NGC 4449,hasnearlyhalfthe blueluminosityofthecentral 3738 istheleastluminous(seeTable3).Thus,ineachofthese 3738 whileitcomprisesmorethan3%ofthedynamicalmass times lessthaninNGC4214.Thus,whiletheM/Mratios agrees towithinafactorof2withtheHimassdetermined from Allsop(1979)wascalculatedtobe1.7x10M,which mass ofNGC4214obtainedfrom58"x90"resolutiondata contain activestar-formingregions,andthedifferences may Ables’s (1971)luminosityprofile indicatesablueluminosityof have foundaCOintegrated intensityof38Kkms,and as anSedIgalaxy,easilylendsitselftocomparisonwith the lars tothoseoflate-typespiralgalaxies,sincebothgalaxytypes unknown detailsoftheHidistribution.Thecentral50" the Himasseswerescaledtowhatwouldbeobservedwitha the HidataforNGC3738and4449wereobtainedwith the central50"ofthisgiantspiral, YoungandScoville(1982a) three irregularsbecauseitsdistanceof4.5Mpc(Rogstad and shed somelightongalaxyevolution.IC342,whichisclassified only anapproximatevaluefortheHimassin50"dueto 50" inIC342,buthasafactor of46lessCO.Theleastlumi- Shostak 1972)issimilartothatofthegalaxiesstudiedhere. In 50" beam,assumingaflatHidistribution.Thisestimateyields 1.4 x10Lq.Thecentral50" inthemostluminousirregular, H2H 2 2 10' and9'resolutionsrespectively(HGR;vanWoerdenetal.), HBQG q B 2 0 H2HI 0 To obtainblueluminositiesforthecentral50"ofeach Because nohigh-resolutionkinematicinformationisavail- It isofinteresttocompareCOandHipropertiesirregu- © American Astronomical Society • Provided by the NASA Astrophysics Data System e) ComparisonwithLate-TypeSpirals d) OpticalLuminosity c) DynamicalMass TACCONI ANDYOUNG 9 9 29±0 2 821A±022 luminosity inthecentral5kpcoflate-typespirals.Based on the studyofSearle,Sargent,andBagnuolo(1973),Young, its central50",whileonlybeing6timeslessluminous. indicator oftheabundance moleculargas,oneinterpretation lation Istars(<2x10years old).IftheCOemissionisan the numberofstarsformed overthelast~10yearsina relationship betweentheamount ofmoleculargaspresentand nosity inalate-typespiraloriginates fromtheyoungPopu- blue luminosityisproportionaltothefirstpowerof CO irregulars, ratherthanthefirstpowerasiscasefor the with analmostperfectcorrelation.Thefactthatthe blue with acorrelationcoefficientof0.69.Fortheirregulars introduced whendividingbythesquareofdistancetoeach nous ofthethreeirregulars,NGC3738,has70timeslessCOin of theCO-bluelightcorrelation isthatthereaone-to-one the findingsofYoungandScoville(1982b)whofoundthat the of galaxiesgiventhepresentstatisticsandgalaxysamples. spirals, indicatesasignificantdifferencebetweentheseclasses galaxy. Here,wefindthatforthespiralsofalltypes Scoville, andBrady(1984)showthat~50%ofthebluelumi- surface brightnessdependsonthecubeofCOfluxfor the relative tothedataplottedinYoungandScoville(1982b),is galaxy, thelogarithmicrelationshipbetweenCOandblueflux galaxies (YoungandScoville1982b),13Virgospirals(Young, in Figure4forthethreeirregularswithvaluesnineSc shown inFigure4isroughlylinear.Thescatterforthespirals, Scoville, andBrady1984),anotherirregulargalaxy,NGC (Lco/d)'- spiral galaxiesfromYoung,Scoville,andBrady(1984),thefilledcircles spiral galaxiesfromYoungandScoville(1982b),thecrossesrepresentVirgo disk ofM101fromBlitzetat(1981).ThebestfittothespiraldataisL/d= 2.3 x10(L/d)-whilethatfortheirregularsisLJd=5.8 show theirregulars.TheXrepresentsNGC5461,anHnregioninouter 50" ofspiralandirregulargalaxies.Theopencirclesrepresentthelate-type 1569 (Young,Gallagher,andHunter1984).Forbothtypesof B co The relationship(3)derivedaboveforthespiralssummarizes We havecomparedthe50"COandbluesurfacebrightnesses Fig. 4.—ComparisonofCOandbluesurfacebrightnessesinthecentral 2 9±0 28i l±0 (L/d) =5.8x10(L(4) Bco (L/d) =2.9x10(Lo/a-(3) BC Vol. 290 1985ApJ. . .290 . .602T No. 2,1985 galaxy. Thesituationfortheirregulargalaxiesmaybesimilar. and foundthatonly10%ofthebluelightcomesfroman young, bluestars.Sincetheothertwoirregulargalaxiesofthis underlying oldpopulationofstars,andtht90%arisesfromthe Schild (1984)hasstudiedthestellarcomponentsofNGC4214 emission isanindicatoroftheamountHpresentinirregu- study havesimilarcolorsandluminosities,theirbluelumin- indicates thatthesegalaxiesformmoreluminousregionsfora lar galaxies,thentherelationshipdescribedinequation(4) osities mayalsobeprimarilyfromyoungstars.IftheCO the nextsection. given amountofCO.Thissubjectwillbediscussedfurtherin masses. YoungandScoville(1982h)havefoundthatforSc irregular galaxiesistheratioofmoleculartoatomichydrogen their centersthanthelowluminositygalaxies.Theyfind galaxies, thehighluminosityobjectshavemoremassinH irregular galaxiesofthisstudy,NGC1569,andM33have spirals to-30forthehighluminositygalaxies.Thethree M/Mhi ratiotovaryfrom-0.4forthelowluminosity 2 M/Mi ratiosof<2.5whilehighluminosityIC342has densities becomecomparable(YoungandScoville1982a;Sco- M/M =59.IntheouterpartsofluminousspiralsIC ville andYoung1983).Thecentralregionsoftheirregulars, luminosity spirals,andtheouterregionsofluminous then, haverelativegasabundanceslikethecentersof low 342, NGC6946,andM51,however,theHisurface molecular form.However,theouterregionsofluminous that is,withcomparablefractionsoftheirISMinatomic and 2 spirals havehigherMJLratiosthantheirregulars. H2 lower thaninspiralsduetodecreasedcosmicrayheating, one deficiency inirregulargalaxies,asdiscussedbyElmegreen, would observelessCOemissionperunitmolecularmass. H2H However, onemightexpectan increaseincosmicrayheating Elmegreen, andMorris(1980).First,iftheCOexcitationwere H2HI due tothelargenumberof youngstars(andpresumably supernovae) inthissampleof irregular galaxies. 2 may beuncertaininirregular galaxiesifthereisalargemetal- licity differencebetweenourGalaxy andirregulars.HGRhave studied theabundancesofionized gasinalargesampleof nB /) CODeficienciesandStarFormationinIrregularGalaxies Another interestingquantitytocompareforthespiraland There areseveralpossibleexplanationsfortheobservedCO Second, theCOintensityto H surfacedensityconversion 2 © American Astronomical Society • Provided by the NASA Astrophysics Data System 68 678 a679 68 367 869 57 NGC 37383.2x101.62.32.00.014 NGC4214 4.0x101.74.80.20.008 NGC 15696.7x101.11.80.60.004 NGC 44494.6x102.092.30.005 IC 342(disk)8.9x105.65.11.60.170 IC 342(center)2.3x103.91.4590.164 M33 1.9x101.511.30.019 3 b Young,Gallagher,andHunter(1984)referencestherein. MandfromYoungScoville(1982h). H2hi Galaxy (Af)(-^o)(M/Lq) 0q Gas MassesandMass-to-LightRatiosforCentral50"in THREE IRREGULARGALAXIES Irregular andSpiralGalaxies Mh AfLM/L 2HIbH2 8 TABLE 4 Irregulars Spirals NGC 4214,and4449,findoxygenabundancesinthe (Smith 1975).TheyhavedeterminedO/HratiosforNGC3738, low inmetallicity,asisobservedtheMagellanicClouds irregular galaxiesandhavefoundthatirregularsaregenerally three irregularsofapproximatelyone-halfthesolarvalue. licity andCOemissioningalaxieswehavecompared luminosity asafunctionofO/Hratioinregions1.3kpc diameter forthethreeirregularsofthisstudyandNGC1569, (1981) andMcCall(1984)foropticalHnregions.TheCO data forthespiralswereobtainedfromPagelandEdmunds as wellforasampleofspiralgalaxies(seeFig.5).Metallicity luminosities plottedhavebeenscaledtowhatwouldbe observed inregions1.3kpcacross(YoungandScoville1982b; circles representthecentersofspiral galaxies (YoungandScoville1982b;Stark ratios forthecentersandouterdisks ofspiralandirregulargalaxies.Theopen et al.(1981).Metallicitydataaretaken fromHGRfortheirregularsandPagel spirals fromYoungandScoville(1982u), ScovilleandYoung(1983),Blitz and Edmunds(1981)McCall(1984) forthespirals. 1979; Solomonetal.1983);thecrosses representtheouterdiskpositionsof In ordertoinvestigateapossiblecorrelationbetweenmetal- Fig. 5.—ComparisonofCOluminosity inregions1.3kpcacrossandO/H 12+ LOGEO/HU 607 1985ApJ. . .290 . .602T 8 8 7 8 67 Amherst, MA01003 Linda J.TacconiandJudith S.Young:DepartmentofPhysicsandAstronomy,GRC TowerB,UniversityofMassachusetts Rickard, L.J.,Palmer,P.,Morris,M., Turner,B.E.,andZuckerman,1977, Rickard, L.J.,andPalmer,P.1981,Asir. A.,102,LI3. Pagel, B.E.J.,andEdmunds,M.G.1981,Ann.Rev.Astr.A.,19,77. Israel, F.P.,deGraauw,Th.,Lidholm,S.,vandeStadt,H.,anddeVries,C. 1982, Miller, G.,andScalo,J.1979,Ap.Swpp/.,41,513. McCall, M.,1984,preprint. Matteucci, F.,andChiosi,C.1983,Astr.Ap.,123,121. Kirschner, W.P.,andBlair,P.1980,Ap.J.,236,135. Hunter, D.A.,Gallagher,J.S.,andRautenkranz,1982,Ap.Suppl,49, 53 Hunter, D.A.1982,Ap.J.,260,81. deVaucouleurs, G.,A.,andCorwin,H.G.1976,SecondRefer- Huggins, P.J.,Gillespie,A.R.,Phillips,T.G.,Gardner,F.F.,andKnowles, F. Elmegreen, B.G.,D.M.,andMorris,M.1980,Ap.J.,240,455. tion ratecanonlylastfor6x10yearswithoutreplenishment Allsopp, N.J.1979,M.N.R.A.S.,188,765. Abies, H.D.1971,Pub.U.S.NavalObs.,20. of gasinthesameregion,theyfindthatcurrentstarforma- rate of1M/yrforthecentral50"M51.Given6x10 4449. Blitz, L.,Israel,F.P.,Neugebauer,G.,Gatley,I.,Lee,T.J.,andBeattie, D. H. NGC 4449,respectively.FortotalHi+gasmassesof years forNGC3738and4214,-10 star formationratescanbesustainedfornomorethan~10 function, HGRgivestarformationratesforthethreeirregulars of 0.02,0.17,and0.23M/yrforNGC3738,4214, equivalent widthsandaMillerScalo(1979)initialmass lars. than thespirals,atleastcurrentepoch.BasedonH/f 5 x10to1Mforthecentral50"ofthesegalaxies, be muchmoreefficientinturningtheirmoleculargasintostars is thatthesegalaxiescouldhaveextremelylowmolecular of highmassstarformation.Thatis,theirregulargalaxiesmay abundances andhighluminositiesbyhavingaefficiency additional factorscontributingtotheCOdeficiencyinirregu- and metallicityisuncertain.Weconcludethattheremustbe galaxies, theexactrelationshipbetweenmolecularabundance (Blitz etal.1981).Thus,whilemetallicityvariationsmay account forsomevariationsinCOluminositytheirregular galaxies ofthiswork,althoughithasafactor2moreCO NGC 5461,hasthesameO/Hratioasthreeirregular region inthediskofM101withlowestCOluminosity, cates thateventhoughagalaxyisnotmetalpoor,itcanstill 0 outer disksofthespiralsarelowerthanincenters.TheHn have alowmolecularabundance.TheCOluminositiesinthe metallicities afactorof5higherthantheirregulars.Thisindi- 2 CO luminosities,eventhoughthespiralsinthiscategoryhave note thattherearebothspiralandirregulargalaxieswithlow 608 0 have bothlowCOandmetalabundances.Itisinterestingto NGC 6946,whichhas100timesmoreCO.Thus,forthese CO content.Theirregulargalaxies,ontheotherhand,tendto galaxies, metallicitycannotbethecauseofvariationsin 0 CO luminosity(M33)hasessentiallythesamemetallicityas galaxy COluminositiescover2ordersofmagnitudeforasmall range inmetallicity;fact,thespiralgalaxywithlowest Stark 1979;Solomoneial.1983;Blitzet1981).Thespiral Ap. J.,213,673. (HGR). Ap. J.,262,100. H. 1975,M.N.R.A.S.,173,69. ence CatalogofBrightGalaxies(Austin:UniversityTexasPress)(RC2). 1981, Ap.J.,249,76. Scoville andYoung(1983)havedeterminedastarformation A thirdexplanationforthelowCOabundancesinirregulars © American Astronomical Society • Provided by the NASA Astrophysics Data System TACCONI ANDYOUNG REFERENCES 6 6 7 -.1984,Ap.J.,287,153. .1982h,Ap.J.(Letters),260,L11. Young, J.$.,Scoville,N.Z.,andBrady, E.1985,Ap.J.(Letters),submitted. Young, J.S.,andScoville,N.Z.1982«,Ap.J.,258,467. Young, J.S.,Gallagher,andHunter,D.A.1984,Ap.J.,276,476. van Woerden,H.,Bosma,A.,andMebold,U.1975,inLaDynamiques des Stark, A.A.,andCarlson,E.R.1984,Ap.J.,279,122. Stark, A.1979,Ph.D.thesis,PrincetonUniversity. Solomon, P.M.,Barrett,J.,Sanders,D.B.,anddeZafra,R.1983Ap J Smith, H.1975,Ap.J.,199,591. Searle, L.,Sargent,W.,andBagnuolo,W.1973,Ap.J.,179,427. Seaquist, E.R.,andBignell,R.C.1978,Ap.J.{Letters),226,L5. Scoville, N.Z,andYoung,J.S.1983,Ap.J.,265,148. Schild, R.1984,A.Letters,preprint. College AstronomyDepartment. Rogstad, D.H.,andShostak,G.S.1972,Ap.J.,176,315. David Sanders.ThisiscontributionNo.583oftheFive helpful insightfulcommentsfromDrs.StevenStromand final stagesofcompletion.Wealsogratefullyacknowledge Rogstad, D.H.,Rougoor,G.W.,andWhiteoak,J.B.1967,Ap.J.,150,9. undergoing moreefficientstarformationthanthespirals. first poweroftheCOcontent.Thusirregularsmaybe and Scoville1982h),wherethebluelightisproportionalto flux. Thisisunliketherelationshipfoundforspirals(Young we findthatbluefluxisproportionaltothecubeofCO irregular galaxies. must beadditionalfactorscontributingtotheCOdeficiencyin both lowCOandmetalabundances.Wesuggestthatthere disks ofspiralgalaxiesshowawiderangeinCOluminosityfor a smallrangeinmetallicity,theirregulargalaxiestendtohave roughly atthesamedistance,irregularshavefactorsof comprises ~2%-4%ofthecentraldynamicalmassesinthese 50-70 lessCO,whileonlybeing2-5timesluminous. three irregulars. masses are~2x10inNGC3738and4449,but-1 position hasahigherHmassthanthecentralposition.i more efficientinformingstarsthanM51. axies rangefrom3-5x10M.InNGC4449,theSEHn with muchstrongerCOemissionobservedintheSEportionof stars thaninM51.Thus,atthecurrentepoch,NGC4449is irregular galaxies,wefindthatfromafactorof-100lessgasin the galaxythanSWregion. to theinterstellarmedium.Comparingthisresultthatof 50" inNGC4449,onlyafactorof4lessmassisconvertedinto x 10MinNGC4214.Thetotalgasmass(M+) 3738, NGC4214,and4449wehavefound: 2 0 0H2HI National delaRechercheScientifique),p.439. (Letters),267,L29. Galaxies Spirales,ed.L.Weliachew(ColloquesInternationauxduCentre We wishtothankLowellGarmanforhisassistanceinthe 4. Whileregions1.3kpcacrossinthecentersandouter 5. FromacomparisonofCOandbluesurfacebrightnesses, 3. WhencomparedtotheScdgalaxyIC342,whichis 2. TheHmassesforthecentral1.3kpcofthesethreegal- From COobservationsofthethreeirregulargalaxiesNGC 1. ThemoleculargasdistributioninNGC4449isclumped, 2 V. CONCLUSIONS