The CO content of the Local Group dwarf irregular galaxies IC5152, UGCA438, and the Phoenix

We present a search for CO(1->0) emission in three Local Group dwarf irregular galaxies: IC5152, the Phoenix dwarf, and UGCA438, using the ATNF Mopra radio telescope. Our scans largely cover the optical extent of the galaxies and the stripped HI cloud West

Mon.Not.R.Astron.Soc.000,(2006)Printed5February2008

A(MNLTEXstyle lev2.2)

TheCOcontentoftheLocalGroupdwarfirregular

galaxiesIC5152,UGCA438,andthePhoenixdwarf

1 2131

P.Buyle ,D.Michielsen,S.DeRijcke ,J¨urgenOtt§,andH.Dejonghe1

SterrenkundigObservatorium,GhentUniversity,Krijgslaan281,S9,B-9000Ghent,Belgium

SchoolofPhysicsandAstronomy,UniversityofNottingham,UniversityPark,NottinghamNG72RD,UKCSIROAustraliaTelescopeNationalFacilityCnrVimiera&PembrokeRoadsMars eldNSW2122,Australia

arXiv:astro-ph/0609401v1 14 Sep 2006

23

ABSTRACT

WepresentasearchforCO(1→0)emissioninthreeLocalGroupdwarfirregulargalaxies:IC5152,thePhoenixdwarf,andUGCA438,usingtheATNFMopraradiotelescope.OurscanslargelycovertheopticalextentofthegalaxiesandthestrippedHIcloudWestofthePhoenixdwarf.Apartfromatentativebutnon-signi cantemissionpeakatonepositioninthePhoenixdwarf,nosigni cantemissionwasdetectedintheCOspectraofthesegalaxies.Foravelocitywidthof6kms 1,wederive4σupperlimitsof0.03Kkms 1,0.04Kkms 1and0.06Kkms 1forIC5152,thePhoenixdwarfandUGCA438,respectively.Thisisanimprovementofoverafactorof10comparedwithpreviousobservationsofIC5152;theothertwogalaxieshadnotyetbeenobservedatmillimeterwavelengths.AssumingaGalacticCO-to-H2conversionfactor,wederiveupperlimitsonthemoleculargasmassof6.2×104M⊙,3.7×103M⊙and1.4×105M⊙forIC5152,thePhoenixdwarfandUGCA438,respectively.WeinvestigatetwopossiblecausesforthelackofCOemissioninthesegalaxies.Ontheonehand,theremaybeagenuinelackofmoleculargasinthesesystems,inspiteofthepresenceoflargeamountsofneutralgas.However,inthecaseofIC5152whichisactivelyformingstars,moleculargasisatleastexpectedtobepresentinthestarformingregions.Ontheotherhand,theremaybealargeincreaseintheCO-to-H2conversionfactorinverylow-metallicitydwarfs( 2 [Fe/H] 1),makingCOapoortracerofthemoleculargascontentindwarfgalaxies.

Keywords:galaxies:ISM–galaxies:dwarf–galaxies:LocalGroup–individualgalaxies:IC5152,Phoenixdwarf,UGCA438

1INTRODUCTION

Dwarfgalaxiesformthemostabundantgalaxypopulationinthenearbyuniverse.Theyrepresentawideclassoflow-mass,low-metallicitystellarsystems,withsomebeinggas-richandpresentlyactivelyformingstars(dwarfirregularsordIrrs),whileothersaregas-poorandapparentlyformedtheirstarslongago(dwarfspheroidalsordSph).Inbetweentheaboveextremesexistsasubclassofgas-richdwarfs,calledtransition-typegalaxies,thatarenotcurrentlyfor-mingstars.Theseobjectsoccupyanevolutionarystatein-

termediatebetweendIrrsanddSphs2001).

Moststudiesoftheinterstellarmedium(ISM)ofdwarfgalaxiesuptonowhavefocusedontheiratomicgas(HI)content(Youngetal.2003;Conseliceetal.2003;Buyleet2005;Bouchardetal.Neverthelessif,inaccordancewithmassivegalaxies(e.g.theMilkyWay),thestarformationpredominantlytakesplaceinmolecularclouds,thatareobservedtoformwithincoldHIregions(withvelocitydispersionsoftheorderof3kms 1),onewouldexpectacorrelationbetweenthestarformationrateandtheCOemissionratherthanbetweenthestarfor-mationrateandtheamountofcoldHIgas.However,sofarnosuchrelationhasbeenobservedfordIrrs,neitherforCOorHI.GenerallyindividualHIcloudsarefoundclosetostarformationsites,withusuallyano setbyafew100pc.Also,someverydenseHIclouds,whichsurelyexceedtheden-sitythresholdabovewhichstarformationisexpectedtobepossible,shownosignsofongoingorrecentstarformation(Mateo1998;Begumetal.2006).Theseresultsshowthat

E-mail:Pieter.Buyle@UGent.be,dolf.michielsen@nottingham.ac.uk,sven.derijcke@UGent.be,Juergen.Ott@csiro.au

Post-doctoralFellowoftheFundforScienti cResearch-Flan-ders,Belgium(F.W.O.)

Post-doctoralFellowoftheFundforScienti cResearch-Flan-ders,Belgium(F.W.O.)§BoltonFellow

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