Magnetic Fields and Ionized Gas in the Local Group Irregular Galaxies IC10 and NGC6822

We performed a high-sensitivity search for galaxy-scale magnetic fields by radio polarimetry at 10.45GHz and 4.85GHz with the Effelsberg 100m radio telescope, accompanied by Halpha imaging, for the two Local Group irregular galaxies IC10 and NGC6822. Their

Astronomy&Astrophysicsmanuscriptno.chyzy(DOI:willbeinsertedbyhandlater)

February2,2008

MagneticFieldsandIonizedGasintheLocalGroupIrregular

GalaxiesIC10andNGC6822

K.T.Chy˙zy1,J.Knapik1,D.J.Bomans2,U.Klein3,R.Beck4,M.Soida1,andM.Urbanik1

1

arXiv:astro-ph/0308500v1 28 Aug 2003

234

AstronomicalObservatory,JagiellonianUniversity,ul.Orla171,30-244Krak´ow,PolandAstronomischesInstitutderRuhr-Universit¨atBochum,Universit¨atsstr.150,44780Bochum,GermanyRadioastronomischesInstitutderUniversit¨atBonn,GermanyMax-Planck-Institutf¨urRadioastronomie,Postfach2024,D-53010Bonn,Germany

Received28February2003/Accepted24April2003

Abstract.Weperformedahigh-sensitivitysearchforgalaxy-scalemagnetic eldsbyradiopolarimetryat10.45GHzand4.85GHzwiththeE elsberg100mradiotelescope,accompaniedbyHαimaging,forthetwoLocalGroupirregulargalaxiesIC10andNGC6822.Theirstar-formingbodiesaresmallandrotateslowly.IC10isknowntohaveaveryhighstar-formingactivity,resemblingbluecompactdwarfs,whileNGC6822hasalowoverallstar-formationlevel.Despiteverydi erentcurrentstarformationrates,ourHαimagingrevealedalargewebofdi useHα lamentsandshellsinbothIC10andNGC6822.Someofthemextendfarawayfromthegalaxy’smainbody.Thetotalpoweremissionofbothobjectsshowsbrightpeakseitheratthepositionsofopti-callystrongstar-formingclumps(IC10)orindividualHiiregionsorsupernovaremnants(NGC6822).However,inbothcaseswedetectasmoothlydistributed,extendedcomponent.InIC10wefoundclearevidenceforthepresenceofadi use,mostlyrandommagnetic eldof 14µGstrength,probablygeneratedbya uctuationdynamo.OneoftheHα-emitting lamentsappearstobeassociatedwithenhancedmagnetic elds.Wealsore-discussthereddeningofIC10anditsimplicationsforitsdistance.InthecaseofNGC6822wefoundonlyveryweakevidencefornonthermalemission,exceptperhapsforsomeregionsassociatedwithlocalgascompression.Wedetectinbothgalaxiessmallspotsofpolarizedemission,indicativeofregular elds( 3µG),atleastpartlyassociatedwithlocalcompressionalphenomena.

Keywords.Polarization–Galaxies:irregular–Galaxies:magnetic elds,Galaxies:individual:IC10,NGC6822–Radiocontinuum:galaxies

1.Introduction

Irregulargalaxiesarelow-massobjectsexhibitingavarietyofrotationalpropertieswithasubclassofthemrotatingveryslowly(rotationalspeedsVrot≤30km/s)andoftenchaotically(e.g.Loetal.1993).Theyconstituteimportantlaboratoriesforlarge-scaleinteractionsofstarswiththeinterstellarmedium:thelowgravitationalpotentialandrelativelysmallsizeincreasetheprobabilitythatsuper-bubbles,formingclosetostar-formingregions,maybreakoutofthegalaxy(e.g.MacLow&Ferrara1998).Manyirregulargalaxiesexhibitgiantarcsor lamentsofionizedgas(e.g.Sabbadin&Bianchini1979,Hunteretal.1993,Bomansetal.1997).Theroleofmagnetic eldsintheoriginandcon nementoftheseionizedstructures(e.g.Hunter&Gallagher1990)isstillamatterofdebate.Inspiralgalaxiesmagnetic elds,whicharesu cientlystrongtotriggerstarformationviamagneticinstabilities

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