A composite HII region luminosity function in H alpha of unp

A composite HII region luminosity function in H alpha of unprecedented statistical weight

Astronomy&Astrophysicsmanuscriptno.6151msFebruary5,2008

cESO2008

AcompositeH regionluminosityfunctioninHαof

unprecedentedstatisticalweight

T.R.Bradley1,2,J.H.Knapen2,J.E.Beckman3,4,andS.L.Folkes2

12

arXiv:astro-ph/0609607v1 21 Sep 2006

34

CentreforAstrophysics,UniversityofCentralLancashire,PrestonPR12HE,UK

CentreforAstrophysicsResearch,UniversityofHertfordshire,Hat eld,HertsAL109AB,UKe-mail:j.knapen@star.herts.ac.ukInstitutodeAstrof´ sicadeCanarias,E-38200LaLaguna,SpainConsejoSuperiordeInvestigacionesCient´ cas,Spain

Received;accepted15Sep.2006

ABSTRACT

Context.StatisticalpropertiesofH regionpopulationsindiskgalaxiesyieldimportantcluestothephysicsofmassivestarformation.Aims.WepresentasetofH regioncataloguesandluminosityfunctionsforasampleof56spiralgalaxiesinordertoderivethemostgeneralformoftheirluminosityfunction.

Methods.H regionluminosityfunctionsarederivedforindividualgalaxieswhich,afterphotometriccalibration,aresummedtoformatotalluminosityfunctioncomprising17,797H regionsfrom53galaxies.

Results.Thetotalluminosityfunction,aboveitslowerlimitofcompleteness,isclearlybest ttedbyadoublepowerlawwithasigni cantlysteeperslopeforthehighluminosityportionofthefunction.Thischangeofslopehasbeenreportedintheliteratureforindividualgalaxies,andoccursataluminosityoflogL=38.6±0.1(Linergs 1)whichhasbeentermedtheStr¨omgrenluminosity.Asteepfallo intheluminosityfunctionabovelogL=40isalsonoted,andisrelatedtoanupperlimittotheluminositiesofunderlyingmassivestellarclusters.Detaileddataarepresentedfortheindividualsamplegalaxies.

Conclusions.TheluminosityfunctionsofH regionsinspiralgalaxiesshowatwoslopepowerlawbehaviour,withasigni cantlysteeperslopeforthehighluminositybranch.Thiscanbemodelledbyassumingthatthehighluminosityregionsaredensitybounded,thoughthescenarioiscomplicatedbytheinhomogeneityoftheionizedinterstellarmedium.Thebreak,irrespectiveofitsorigin,isofpotentialuseasadistanceindicatorfordiscgalaxies.

Keywords.galaxies:spiral–galaxies:structure–ISM:H regions

1.Introduction

Intheirpapergivingamajoroverviewoftheluminosityfunc-tions(LFs)inHαemissionoftheH regionsindiskgalaxiesKennicutt,Edgar&Hodge(1989;hereafterKEH)notedthatanimportantsubsetoftheirobjectsshowedLFswithaclearbreakinslopeataluminosityofL(Hα)=38.7dex(unitsofergs 1).Atlowerluminositiesthepowerlawslopeis atter,whereasabovethebreakitissteeper.KEHcalledthoseLFsshowingabreak“typeII”,suggestingthatanygalaxywillex-hibittypeIIbehaviourifithassu cienthighluminosityH regions.Agalaxywithsu cientluminousH regionstoshowaclearbreak,M51,wasobservedbyRand(1992),whomea-sureditatL(Hα)=38.6dex.Rand’swork,usingCCDdata,wasespeciallyprecise,andrevealed,aswellasthebreak,anarrowpeakintheLFclosetothebreakluminosity.IntheLFofM33,Hodgeetal.(1999)foundnobreak,whichisnotsurprisingsincethefunctionisde nedonlyuptoL(Hα)=38.4dex,withonlyadozenregionsaboveL(Hα)=38dex.FurtherexamplesofthistypeareNGC6822andthedwarfgalaxyHolmbergII(Hodge,Strobel&Kennicutt1994),smallergalaxieswithsmallH re-gionnumbersatL(Hα)>38dex.Ontheotherhand,M101hassome60H regionswithL(Hα)>38.5dexandshowsaclearbreakatL(Hα)38.6dex(Scowen,Dufour&Hester1992).OurgrouphaspublishedLFsforgalaxiesselectedtohavelargenum-

A composite HII region luminosity function in H alpha of unprecedented statistical weight

2T.R.Bradleyetal.:AcompositeH regionluminosityfunctioninHαofunprecedentedstatisticalweight

OncompletingarecentimagingsurveyinHαofasetofnearbygalaxies(Knapenetal.2004)wesawthatwecouldmakeasigni canttestofthesuggestionthatadualslopeLFwithawellde nedbreakgivesabettergeneraldescriptionthanasinglepowerlawslope,andthattheoriginalresultofKEH,withitscleanbreakataspeci cluminosity,ispresentinadatasetwithasu cientstatisticalbase.Ourobservationshaveallowedustoisolateandcataloguealmost18,000H regionsin56galaxies.Herewegivethekeyresultsofthisstudyandalsoo eron-linecataloguesforthefullsetofobjects.

2.SampleselectionandHαimaging

2.1.Sampleselectionandobservations

WehaveusedthedatasetpublishedbyKnapenetal.(2004),whichcontainsafullsetofcontinuum-subtractedHαimagesof57relativelyface-on(i<50 ),nearby,Northern,spiralgalaxies.Mostoftheimageswereobtainedwiththe1mJacobusKapteynTelescope(JKT),butsomewithothertelescopesorfromthelit-erature.MostoftheHαimagesweretakenthrough≈50Åwide ltersmatchedtotherecessionalvelocityofeachgalaxy.FulldetailsontheobservationsaregiveninKnapenetal.(2004),whereasthesamplegalaxiesarelisted,andtheirdistances,platescales,andresolutionsgiven,inTable1(onlineonly).2.2.TheeffectsofseeingonH regionselection

TheFWHMseeinginourHαimagesvariesfrom0.8to3.7arcsec(1.53arcsecinthemedian),whichcorrespondstoarangeinspatialresolutionof30to295pc(137pcintheme-dian).Wethusneedtoconsiderpossiblee ectsofblendingontheresultingH regioncatalogues.Thetwomajore ectsarethatfainterH regionsmaybespatiallycoincidentwithlarger,brighter,H regions,reducingthenumberoffaintH regionsobserved,andoverlappingofH regions,whichmayleadtoameasuredluminosityhigherthanthetrueluminosity.KEHshowedthatwhereasdegradingimagesuptoresolutionsof200pchadlittlee ectontheluminositiesofinteresthere,andthatdegradingthemtoaresolutionof200-400pcwillaf-fectthefaintendoftheluminosityfunctionwhilepreservingtheshapeoftheupperLF,blendingseriouslya ectstheentireLFandcausesincreasesintheluminositiesof rstrankedH re-gionsinimageswitharesolutionof300-500pc.Rand(1992)modelledthemaine ectsofblendingforthearmLFofM51,andfoundthattheydidnotsigni cantlya ecttheshapeoftheLF.

Morerecently,HαLFshavebeenderivedfromHSTimaging(Pleuss,Heller&Fricke2000;Scovilleetal.2001),whichhassigni cantlyhigherspatialresolutionthanthetypicalsizeofanH regionintheluminosityrangeofinteresthere.Underthesecircumstances,itispossibletoclassifyasingl …… 此处隐藏：23449字，全部文档内容请下载后查看。喜欢就下载吧 ……