A Search for Ionized Gas in the Draco and Ursa Minor Dwarf Spheroidal Galaxies

The Wisconsin H Alpha Mapper has been used to set the first deep upper limits on the intensity of diffuse H alpha emission from warm ionized gas in the Local Group dwarf spheroidal galaxies (dSphs) Draco and Ursa Minor. Assuming a velocity dispersion of 15

ASearchforIonizedGasintheDracoandUrsaMinorDwarf

SpheroidalGalaxies

J.S.Gallagher,G.J.Madsen,R.J.Reynolds

arXiv:astro-ph/0301228v1 13 Jan 2003

DepartmentofAstronomy,UniversityofWisconsin–Madison,475NorthCharterStreet,Madison,WI

53706-1582;jsg@astro.wisc.edu,madsen@astro.wisc.edu,reynolds@astro.wisc.edu

E.K.Grebel

Max-PlanckInstituteforAstronomy,K¨onigstuhl17,D-69117Heidelberg,Germany;grebel@mpia.de

T.A.Smecker-Hane

DepartmentofPhysics&Astronomy,4129FrederickReinesHall,UniversityofCalifornia,Irvine,CA

92697-4575;smecker@carina.ps.uci.edu

ABSTRACT

TheWisconsinHαMapperhasbeenusedtosetthe rstdeepupperlimitsontheintensityofdi useHαemissionfromwarmionizedgasintheDracoandUrsaMinordwarfspheroidalgalaxies.Assumingavelocitydispersionof15kms 1fortheionizedgas,wesetlimitsofIHα≤0.024RandIHα≤0.021RfortheDracoandUrsaMinordSphs,respectively,averagedoverour1 circularbeam.Adoptingasimplemodelfortheionizedinterstellarmedium,theselimitstranslatetoupperboundsonthemassofionizedgasof 10%ofthestellarmass,or～10timestheupperlimitsforthemassofneutralhydrogen.NotethattheDracoandUrsaMinordSphscouldcontainsubstantialamountsofinterstellargas,equivalenttoallofthegasinjectedbydyingstarssincetheendoftheirmainstarformingepisodes 8Gyrinthepast,withoutviolatingtheselimitsonthemassofionizedgas.1.

Introduction

(1999,2000)yieldedupperlimitsonanydi useneutralgasatcolumndensitiesof<1018cm 2,implyingthatanydistributedgaswouldhavelowmeanvolumedensityandthereforecouldbeion-ized.Bowenetal.(1997)searchedforahotion-izedISMintheoutskirtsoftheLeoIdSphbyseekinginterstellarultravioletlineswithnodetec-tion,therebysettingionizedgascolumndensitylimitsalongtwosightlinestobackgroundQSOssimilartothoseoftheHisurveys.Gizis,MouldandDjorgovsky(1993)studiedtheFornaxdSphwithROSATandfoundnoextendedcomponentthatcouldbeassociatedwithveryhotgasandsetalimitof≤105M⊙.

ThelimitsoninterstellargascolumndensitiesfromtheultravioletandHiobservationscorre-Amongtheunusualcharacteristicsofdwarfspheroidal(dSph)galaxiesistheapparentlackofanyinterstellargas,eveninsystemssuchasFornaxorCarina,wherestarformationoccurredinthelast1to3Gyr(Skillman&Bender1995;alsoseereviewsbyGallagher&Wyse1994;Mateo1998;Grebel1999).Stetson,Hesser,&Smecker-Hane(1998) ndasequenceofstarsincolor-magnitudediagramoftheFornaxdSphthatap-pearstobeonlyafewtimes108yrsold.Thema-jorexceptiontothistrendmaybetheSculptordSph,whereanHicloudcouldbeassociatedwiththegalaxy(Carignanetal.1998;Carignan1999).Recentsensitive21cmHiobservationsbyYoung

第1页下一页