Cold dust and its relation to molecular gas in the dwarf irregular galaxy NGC 4449

We present observations of the dwarf irregular galaxy NGC 4449 at 850 $\mu$m and 450 $\mu$m obtained with SCUBA at the JCMT. The distribution of the cold dust agrees well with that of the CO and H$\alpha$ emission. To explain the integrated mm- through far

Astronomy&Astrophysicsmanuscriptno.H4087(DOI:willbeinsertedbyhandlater)

February2,2008

Colddustanditsrelationtomoleculargasinthedwarfirregular

galaxyNGC4449

C.B¨ottner,U.Klein,andA.Heithausen

RadioastronomischesInstitutderUniversit¨atBonn,AufdemH¨ugel71,53121Bonn,Germany

arXiv:astro-ph/0306627v1 30 Jun 2003

Received05Nov2002/Accepted24Jun03

Abstract.WepresentobservationsofthedwarfirregulargalaxyNGC4449at850µmand450µmobtainedwithSCUBAat

theJCMT.ThedistributionofthecolddustagreeswellwiththatoftheCOandHαemission.Toexplaintheintegratedmm-throughfar-infraredcontinuumspectrumthreedustcomponentsarerequired,withtemperaturesof16K,39Kand168K,

respectively.Thedustmassisdominatedbythecolddustcomponent;wederiveatotaldustmassof～3.8×106M⊙,andwiththelocalgas-to-dustratioof～130atotalgasmassofMHI+H2～4.9×108M⊙.ComparisonwiththeHImassleadstoatotalmoleculargasmassof～3.4×108M⊙.WederiveaconversionfactoroftheCOlineintensitytomolecularhydrogencolumndensityXCO=NH2/ICOwhichisatleast11timeslargerthantheGalacticvalue.ThesevaluesareinaccordwiththelowermetallicityofNGC4449.

Keywords.galaxies:individual:NGC4449–galaxies:ISM–galaxies:irregular–ISM:dust,extinction

1.Introduction

Carbonmonoxide(CO)iscommonlyusedasatracerformoleculargasbecausethemolecularhydrogen(H2),thedom-inantspecies,lacksstrongemissionlinesfromwhichthecol-umndensitycouldbedetermined.Becausetherotationaltran-sitionsofCOarerelativelyeasytoexcite,itispossibletouse theCOlineintensityintegratedoverallvelocities(ICO=Tmbdv)ortheCOluminosity,LCO,inoneoftheselinestoes-timatethecolumndensity,N(H2),andmass,M(H2),ofmolec-ularhydrogen,providedoneknowsthecorrectconversion.FortheMilkyWaytheconversionfactor,XCO=NH2/ICO(1 0),hasbeenfoundtobebetweenXCO=1.6×1020cm 2(Kkms 1) 1(Hunteretal.1997)andXCO=2.3×1020cm 2(Kkms 1) 1(Strongetal.1988).Throughoutthepaperwewillusethelat-terbecauseitiswidelyusedforcomparisonintheliterature.Earlyon,thequestionarosewhetheronecanapplythislocalvaluetoexternalgalaxies,astherelationmaystronglydependonthephysicalconditionssuchastheradiation eldandthemetallicity(Dettmar&Heithausen1989;Israel1997,Baroneetal.2000).Dwarfirregulargalaxiesprovideexcellentlabo-ratoriestoinvestigateXCOinenvironmentswithlowermetalabundancesandvaryingexcitationconditions.

Analternativetracerforthemoleculargasisthecolddustthatonecanmeasureatmm/submmwavelengths(Gu´elinetal.1993;Dumkeetal.1997).Thedustmasscanbedeter-minedfromtheemissionspectrumwithsomeassumptionsonthedustemissivityandcomposition.Adoptinga

gas-to-dust

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