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

2K.Chy˙zyetal.:MagneticFieldsandIonizedGasinIrregularGalaxies

Spiralgalaxiesshowmuchmoreregularmagnetic elds

intheinterarmspacethaninstronglystar-formingspi-ralarms(e.g.Beck&Hoernes1996).Nothingisknownwhethersuchadi erencealsooccursbetweensmallverystronglyandveryweaklystar-formingirregulars.Forallabovereasonsweperformedasensitivesearchforex-tended,di usetotalpowerandpolarizedemissionintwosmallirregulars:IC10andNGC6822re ectingtwoex-tremitiesofstar-formingactivity.Thestar-formingbodyofIC10hasthediameterofonly1.6kpcassumingthedis-tanceof0.8Mpc(Wilsonetal.1996).WithinthisradiusitrotatesslowlyatthespeedVrot 30km/s(Wilcots&Miller1998)andshowstworegionsofaveryhighstar-formingactivity.ThemeanHαsurfacebrightnessofthisgalaxyis4timeshigherthaninNGC4449,makingIC10arapidlystar-formingobjectsimilartoyoungbluecom-pactgalaxies(BCG,Richeretal.2001).TheopticalbodyofNGC6822isphysicallyabouttwicelargerthanIC10.Itlacksprominentregionsofrecentstarformation.BothgalaxiespossessimilarHimasscontent:1.7·108M⊙inthecaseofIC10(Huchtmeier1979)and1.3·108M⊙forNGC6822(deBlok&Walter2000).

Asasearchforweakextendedemissionisparticularlydi cultforNGC6822becauseofitslargeangularsizeandlowradiosurfacebrightnesswhichmakessingle-dishobservationspreferred.Forthesakeofcomparisonbothobjectswerestudiedinawayprovidingthebestcompro-misebetweensensitivityandresolution,ensuringasim-ilarnumberofbeamspergalaxysize.ThiswasdonebydeepmappingofNGC6822at4.85GHzandofIC10at10.45GHzusingthe100-mE elsbergradiotelescope.Tostudytheassociationsofmagnetic eldswiththeionizedgasandstarformationweobtainedsensitiveim-agesofbothobjectsintheHαline.TheHiiregioncon-tentofIC10wasanalyzedbyHodge&Lee(1990)andNGC6822byHodgeetal.(1988)butinthelattercaseonlypartofthegalaxywasobserved.Hunteretal.(1993)foundthatIC10isembeddedinanimpressiveweboflarge lamentsandshells,whiletheycouldnotdetectsimilarstructuresinNGC6822.OurnewHαdatacovereachgalaxyononeCCDframewitharcsecondresolutionandhighsensitivity.InSect.2wepresenttheobservingtechniquesusedtoobtainthemapsoftotalpower,po-larizedintensityandHαemission.Section3.presentstheobservationalresults.Therelativecontentofthermalandnonthermalemissionandthequestionofglobalmagnetic eldsisdiscussedinSect.4.

2.Observationsanddatareduction2.1.Radioobservations

ThetotalpowerandpolarizationmapsofIC10at10.45GHzwereobtainedusingthefour-hornsysteminthesecondaryfocusoftheE elsberg100-mMPIfR

telescope

Fig.1.HαimageofIC10,freefromthestellarcontin-

uum.

Fig.2.TotalpowercontourmapofIC10at10.45GHz,withB-vectorsofpolarizedintensitysuperimposedonto

theHαimage.Theresolutionis1.′

13.Contourlevelsare2,3,4,5then7,11,15etc.mJy/b.a.,thevectorof1′lengthcorrespondstopolarizedintensityof1.5mJy/b.a.(Schmidtetal.1993)1,thedetailsoftheobservingproce-dureweredescribedindetailinourpreviousworks(e.g.Soidaetal.1996).Intotalweobtained28coveragesof

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