Diffuse Ionized Gas in irregular galaxiesI- GR 8 and ESO 245-G05

We have studied the spectral characteristics of the Diffuse Ionized Gas in two irregular galaxies with low metallicities and intermediate Star Formation Rates: ESO 245-G05 and Gr 8. The [OIII]/Hbeta in these galaxies is higher than in the DIG of spiral gal

Di useIonizedGasinirregulargalaxies:I-Gr8andESO245-G05

A.M.Hidalgo-G´amez

InstitutodeAstronom´ a,UNAM,Ciudad

Universitaria,Aptdo.70264,C.P.04510,Mexico

City,Mexico

and

EscuelaSuperiordeF´ sicayMatem´aticas,IPN,U.P.AdolfoL´opezMateos,MexicoCity,Mexico

arXiv:astro-ph/0603241v1 9 Mar 2006

Abstract

WehavestudiedthespectralcharacteristicsoftheDi useIonizedGasintwoirregulargalaxieswithlowmetallicitiesandintermediateStarFormationRates:ESO245-G05andGr8.The[OIII]/HβinthesegalaxiesishigherthanintheDIGofspiralgalaxiesbutnotashighasinotherirregulargalax-iespreviouslystudied,suchasIC10andNGC6822.The[NII]/Hαand[SII]/Hαratioshaveverysmallvalues,indicatingtheabsenceofshocksastheion-izationsourceforthisgas.ThisionizationcanbeexplainedinbothgalaxieswithphotonleakagefromtheHiiregionsastheonlysource.ThepercentageofphotonsescapedfromtheHiiregionsissmallinESO245-G05,ofonly35%,butvariesfrom35%upto60%inGr8.Wealsoinvestigatedifthedi erencesfoundbetweenspiralandirregulargalaxiesinthe[OIII]/Hβandthe[NII]/Hαratioareduetodi erencesinthemetalcontentbetweenthesetypesofgalaxies.Al-thoughthenumberofgalaxiesstudiedisnotverylarge,itcanbeconcludedthatthe[OIII]/Hβisnotrelatedwiththeoxygencontent,whilethesituationismoreambiguousforthe[NII]/Hαratio.

highvaluesof[NII]/Hα,and[SII]/Hαbothincreasingwiththedistancetotheplane,andhighvaluesoftheionization,asde nedby[OII]/[OIII](T¨ullmann&Dettmar2000).Ontheotherside,theDIGpropertiesofirregulargalaxiesarenotknownverywell,asveryfewgalaxieshavebeenstudiedsofar.Thesefewdatahavepreventedustomakeanyconclusionontheirspectralcharacteristics(Martin1997;Hidalgo-G´amez2005;Hidalgo-G´amez&Peimbert2006).Thesedatashowthatthelineratiosdi erfromthoseinspiralgalaxies.The[OIII]/Hβislarge.Forsomeofthegalaxies,e.g.IC10,islargerthan1.5.The[NII]/Hαand[SII]/Hαratiosaresmallerthan0.5.Moreover,ifthelineratiosareverydi erentforspiralandir-regulargalaxies,itislikelythattheionizationsourceoftheDIGwillalsodi erduetodi erentmetalcon-tents,gasmassesorStarFormationRates(SFR).

InthepresentinvestigationwestudytwodwarfirregulargalaxieswithlowchemicalabundancesandintermediateSFRs:Gr8andESO245-G05.The rstoneisanearbygalaxyatadistanceofD=2.24Mpc(Hidalgo-G´amez&Olofsson1998)dominatedintheopticalbytwolargeHiiregions.Themetallicityisverylow(12+log(O/H)=7.4±0.1,Hidalgo-G´amez&Olofsson2002)andalsotheSFR,whichis2.12×10 3M⊙yr 1(Hunter&Elmegreen2004).ESO245-G05isoneoftheSculptorgroupdwarfgalaxies.Theoxygencontentisalsolow(7.6-7.8withsmallvari-ationsalongthebar,seeHidalgo-G´amezetal.2001)andtheSFRis1.1×10 2M⊙yr 1(Miller1996).Inbothgalaxies,alargeextensionofionizedgasintheirHαimagesispresent.Therefore,theyrepre-sentgoodcandidatesforstudyingthepropertiesoftheDIG.Duetotheirlowmetallicitiestheycould

keywordsgalaxies:irregular–galaxies:–inter-beusedtostudythedependenceoftheDIGproper-stellarmedium:Hiiregions:general–galaxies:in-tieswithsomeofthecharacteristicsoftheirregulardividual:Gr8–galaxies:individual:ESO245-G05galaxies.

Thepaperisstructuredasfollows:theobser-vationsandthereductionofthedataaredescribed

1.Introduction

inthenextSection.InSection3,thelineratios

ExtensivestudiesoftheDi useIonizedGasalongtheslitforeachgalaxyarepresented.Possible(hereafterDIG)havebeencarriedoutforspiralsourcesofionizationoftheDi useGasaredescribedgalaxies(e.g.Otte&Dettmar1999).ThesestudiesinSection4.Section5discussestherelationbetweenshowthatthespectrallineratiosformostofthespiralthemetallicityandtheDIGproperties.Conclusionsgalaxiesareverysimilar,withaverylow[OIII]/Hβ,arepresentedinSection6.

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