The Starburst-Interstellar Medium Interaction of NGC 1569 I.(4)

C2–Figure5b–AclusterlyingSouthwestofSSCA.ThisalsohastheseparationbetweenredstarsandHeiiemission.TheredstarsarefoundatthecenteroftheclusterwhilethebrightestHeiiisatitsEasternedge.TheHeiiemissionisequivalenttofourWNLsandhasanareaof0.0893arcsec2.Also,thecolorsofthesystem(andC1)arepredominantlyin uencedbyredstars.Thisiscluster13inHunteretal.(2000)andourMF555Wmagnitudeagreeswiththeirsgiventheuncertaintiesinbothmeasurements.

C3–Figure5c–Anotherclusterwherethestrongestcontinuumsourceisa“hole”intheHeiiimagesignifyingredstars.ThepeakofHeiiemissionisfoundtotheNortheastofthebrightestcontinuumpixelandhasanareaof0.0496arcsec2.Also,thereisaslightriseinthe[Oiii] uxnearthepeakHeiiandgivesstrongevidenceforanebularorigin.KS97’sspectraalsocoverthisregionwhichthey ndonlynebularemission.However,ifoneassumesthatalltheemissionisfromWRstarsandusingthesameaverageemissionofaWNLstar,thenumberofWNLequivalentsistwo.Thisisnotclassi edasadetectedclusterinHunteretal.(2000).

We present the detection of He II sources in the Im galaxy NGC 1569 from HST WFPC2 imagery. Out of the fifteen detections, seven were Wolf-Rayet stars, five were stellar clusters with associated He II emission, and three were sources of unknown origin. The

C4–Figure5d–SSCA.Gonz´alez-Delgadoetal.(1997)reportthe ndingof25-40WNLequivalentstarsinSSCA.However,ifoneusesournumbersandassumesthatalltheemissionisfromWRstars,thenumberofWNLequivalentsis～50.Thisnumberislargerthantheirestimateandsuggeststhatthereissomenebularcomponent(withamaximumareaof0.823arcsec2)thatwecannotseparateinthisstudy.However,continuumdilutionofthebroadλ4686emissionlinemightbeaproblemintheGonz´alez-Delgadoetal.(1997)spectra.Thus,thenumbermustbesomewherebetweenthetwonumbers,probablynearertheirestimate.ThisisclusterAinHunteretal.(2000)andourMF555Wmagnitudeagreeswiththeirsgiventheuncertaintiesinbothmeasurements.

C5–Figure5e–AclusterontheveryedgeofthestarburstandmaynotbeapartofthestarburstwhichcreatedSSCAanditssurroundings.Interestingly,thisclusterhasnoredstellarpopulationwithinornearit.Itistheonlyclusterlikethisandisfarremovedfromtheotherfourclusters.IfoneassumesthatalltheHeiiemissionisfromWRstars,thenumberofWNLequivalentstarsisthree,similarinnumbertoC1–C3.However,theareaofemissionismuchsmallerat0.0298arcsec2.Thisiscluster39inHunteretal.(2000)andourMF555Wmagnitudeagreeswiththeirsgiventheuncertaintiesinbothmeasurements.

U1–Figure6a–Thebanana-shapedcontouriswherethemaximumlevelofemissionis.ItisontheSouthernedgeofSSCAandisSouthofS2.Theemissionisprobablynebularinoriginwithanareaof0.00992arcsec2,buttheHeiiemissionissimilartotheaverageHeiiemissionofGalacticorLMCWRstars.

U2–Figure6b–TheclustertotheNorth(V=18.1;B-V=0.63)ofU2hasalargepopulationofredstars.Thus,thisHeiiemissionmightberelatedtothegroupofstarsjustlikethesituationintheclusters.Therefore,itispossiblethatoneWRstarisadjacenttothecluster.However,sincenostarisseenitsnebularoriginismorerealistic(withanareaof0.0198arcsec2).

We present the detection of He II sources in the Im galaxy NGC 1569 from HST WFPC2 imagery. Out of the fifteen detections, seven were Wolf-Rayet stars, five were stellar clusters with associated He II emission, and three were sources of unknown origin. The

U3–Figure6c–Thecircularpointsourceissurroundedbythreebrightclusters.Nostellarpointsourcesarefoundbetweenthethree.Theemissionismostprobablynebularinorigin,butcouldbeaWRstarsincetheHeiiemissioniscomparabletotheaverageHeiiemissionofGalacticorLMCWRstars.

4.3.OntheOriginofNebularHeiiinNGC1569

ThepresenceofnebularHeiioutsideofplanetarynebulaeisrarebecausefewthermalsourcesproduceenoughphotonswithenergies>54eV.Garnettetal.(1991)statedthreealternativeionizingmechanismswhichcouldaccountforthisnebularHeii.Hotstellarionizingcontinuaisade nitepossibilityforthisgalaxy.ThestarburstwhichproducedtheWRstarsinSSCAoccurredonly2-3Myrago(Gonz´alez-Delgadoetal.1997)or4-5Myrago(Hunteretal.2000).Schaerer&Vacca(1998)modeledstrongnebularHeiiduetomassivestarsintheearlystarburstphases.Thesemodelshavebeensubsequentlyupdated(Schaerer2000).UsingtheI(Heii4686)/I(Hβ)ratiosfromKS97(theirTable2;valuesof8×10 3and1.2×10 2)andFigure8fromSchaerer(2000),weconcludethattheburstagemustbe3-4Myrold.Thisisinfairagreementwiththeyoungerageofthetwo-stageburstproposedbyGonz´alez-Delgadoetal.(1997)andtherecentlycalculatedagesofSSCAandsurroundingclusters(Hunteretal.2000).

ThesecondmechanismfortheformationofHeiiisshockexcitationsuchthatthestrengthofthenebularHeiidependsonthevelocityoftheshock:(Vshock≈120kms 1;Garnettetal.1991).Ifshockswerepresent,bright[Oiii]emissionwouldbeatthesamelocationwhichmayhavebeenseenforC3.Atthistime,basedonourlimitedimagery,shockscannotbeconsideredaviablesource.PhotoionizationbyX-raysisthe nal,alternativeexplanationfornebularHeii.Pakull&Angebault(1986)foundthatnebularHeiiwasproducedaroundtheblackholecandidatebinaryLMCX-1.DellaCeca

We present the detection of He II sources in the Im galaxy NGC 1569 from HST WFPC2 imagery. Out of the fifteen detections, seven were Wolf-Rayet stars, five were stellar clusters with associated He II emission, and three were sources of unknown origin. The

etal.(1996)statethatNGC1569’shardX-rayspectraisduetothetwobrightX-raypointsourceslocatedintheROSAThighresolutionimagerdata.Thesesourcescouldbeinterpretedaslow-massX-raybinariesoryoungsupernovaremnants.FutureX-rayimageswouldresolvewhetherthesepointsourcesareX-raybinariesandcoincidentwithsomeoftheHeiisourcesstatedhere.ChandraobservationsofthisobjectweretakenMay2000,andwewillhavetowaitandseewhetherthishypothesisisvalid.Preliminary ndingsoftheChandradatadonotshowX-raybinariescoincidentwithanyofourHeiisources(Kobulnicky2000).

4.4.EstimateoftheTotalNumberofWolf-RayetStarsinNGC1569ItwasunlikelythatwedetectedallWRstarswithinNGC1569,sincemanyare“blurred”intoSSCA,andthismethodprincipallydetectsWNstars.WederivedatotalnumberofdetectedWolf-Rayetstarsfromou …… 此处隐藏：5958字，全部文档内容请下载后查看。喜欢就下载吧 ……