The Starburst-Interstellar Medium Interaction of NGC 1569 I.(2)

2.ObservationsandReduction

WFPC2imagesofNGC1569weretaken23September1999fortheCycle8program,GO-8133.NGC1569wasorientedintwoofthewide- eldchips(WF2&WF3)ofthecamerawithanearby10thmagnitudestarplacedoutofthe eldofview(seeFigure1).Thee ectiveplatescaleis0′′.0996pixel 1(1.07pcpixel 1attheadopteddistanceof2.2Mpc).TheGO-8133imagesusedherewereF469N(HeII),F502N([OIII]),andF547M(～Str¨omgreny).NGC1569’sradialvelocityis 104kms 1.Theshiftofeachemissionlineis～2 A,andtherefore,allemissionlineswereobservedverynearthecenterofthe lter

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

transmissioncurve.ObservationalparametersforthesedataarefoundinTable1.

Thesedatawererecalibratedusingthebestreference lesandthenewSTScI

“on-the- y”calibration(OTFC)system.OTFCimagesgavethesameimagestatisticsoverthewide- eldchipscomparedtothemanuallycalibratedones.Additionally,weusedOTFCprocessedarchivalWFPC2imageryofNGC1569fromtheCycle6programsGO-6111andGO-6423,whichwereprimarilybroadbandimagesinUBVR.PleaserefertoGreggioetal.(1998)formoredetailsontheGO-6111imageryandtoHunteretal.(2000)fordiscussionoftheGO-6423imagery.Table1hasobservationalparametersforthesedataaswell.

TheGO-6111datawereditheredbyafewpixelsforasubsetoffourexposures.Theseimagesneededtobealignedwithrespecttooneanother,whichwasdoneusingstandardIRAF2applications.Forallimages,cosmicrayswerebestremovedbyusingtheSTSDASpackagecrrej.Thecosmicrayremovedimageswerethenrotated(inthecaseoftheCycle6data)andalignedwithrespecttotheCycle8F547Mimage.TheF502NandF469Nimageswerecontinuumsubtracted.Thetotalcountsofseveralbright eldstarsweredeterminedintheF547M,F502N,andF469Nimages.Averageratiosoftheemissionlinetocontinuumtotalcountsforthesestarswerefound,scaledtotheF547Mimage,andthenewF547Mimagewassubtractedfromeachinterference- lterimage.Thisprocesswasiterateduntilthe eldstarsweresubtractedout,andtheresidualswereataveragelevelsofthebackgroundnoise.

Drissenetal.(1999)usedaweightedaveragebetweenF547MandF439WimagesforcontinuumsubtractionoftheirF469NimageofNGC2403.Theyindicatedthatthiswould

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

properlysubtractredstarsandnotleavethemas“holes”ifonlytheF547Mwereemployed.WeattemptedthismethodandfoundthattheweightingheavilyfavoredF547M(～99%),andthus,onlyF547Mwasused.Furthermore,theonlyF439W(GO-6111)imagecoversone-halfofthegalaxyonour eld.

Fluxcalibrationofourcontinuum-subtracted,interference- lterimageswasdoneusingtheWFPC2exposuretimecalculatorfoundatSTScI’shomepage.Ageneric uxof1.0×10 16ergscm 2s 1,anexposuretimeof1000seconds,andtheredshiftvelocityofNGC1569( 104kms 1)wereinputtedtodeterminethenumberofobjectelectronsfortheF469NandF502N lters.Thenumberofelectronssec 1wasconvertedtoDNsec 1,andsettingtheDNsec 1valueequaltothegeneric uxof1.0×10 16ergscm 2s 1,wesolvedforthe uxinoneDNsec 1.Theseconversionnumbersareinunitsofergscm 2s 1DN 1andare1.084×10 14forF502Nand1.847×10 14forF469N.

WiththecalibratedHeiiimage,wedeterminedwhethertheHeiiemissionwasduetoaWRstar,nebularsource,orassociatedwithastellarcluster.Allpixelswithcountsover3timesthedeviationinthebackground(3σ)intheF469Nimagewerenotedaspossibledetections.EachpixelwithapparentemissionwascarefullycheckedintheindividualF469Nexposurestoseewhetherthelocationhadbeenstruckbyacosmicray,warmpixelordefectwhichmightaccountforitbeinghigh.Therewasalsothepossibilitythatthepixelhadablemishinthecontinuumimagewhichalsomightaccountforitbeinghigh.Ifapixellocationwasinanyofthesecategories,itwasthrownout.Whatremainedafterthisrejectionprocesswasconsidereda“good”detection.

EachpixelthatmettheabovecriteriawassuperimposedontheF502NandF555Wimages.Wedeterminedfromthesuperpositionwhetheragooddetection’slocationcorrespondedtoabright,point-likesourceinoneorbothoftheimages.ThedetectionsmeetingtherequirementofbeingapointsourceinF502Nbutnocorrespondingpoint-source

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

inF555Wwerenotfoundinoursearch.IfthegooddetectionhadacorrespondingpointsourceinF555WbutnotinF502N,itwaslabeledS(forstellarsources)orC(forclustersources).Ifthegooddetectionwasnotassociatedwithanypointsourceineitheroftheimages,itwaslabeledUinthe guresandtablesbelow.

FortheSandCsources,theHeii uxandWFPC2BandVmagnitudeswerecalculated.WFPC2BmagnitudeswerenotmeasuredforsomesourcesbecausetheGO-6111imagesdidnotcoversomeHeiilocations,orthecorrespondingpointintheF439Wimagehadlowsignal-to-noise.OurmagnitudeandsearchcriteriawerebasedontheIRAFprocedureapphot.TheHeiipixellocationofeachSsourcewaschosenasthestartingpositionforthebrightestpixelsearchintheBandVimages.Thesearchradiuswasrestrictedtowithin2pixelsoftheHeiilocation.Theaperturesforthestellarsourceswere2pixelsbecausemostoftheHeiipixelswereconcentratedinthecrowdedstellarregionofthegalaxy.Backgroundcountsweretakenfromanannulusthatwasonepixeloutsidetheaperture.Thenumberofpixelcentersfoundwithinthe2pixelaperturewerecountedandthetotalnumberofcountswascomputed.TheHeii uxwasfoundbyusingthesameIRAFprogramandparameters,andthetotalcountswereconvertedtoanabsolute ux.ThesameprocedurewasusedinthecaseoftheCsources,excepttheaperturewassettoalargervalue.LookingattheF555Wimages,onevisuallyinspectedthemaximumradiusofeachcluster,andtheaverageradiuswas5pixels.SSCAwassetat10pixels,andthebackgroundannuluswasappropriatelyexpanded.On …… 此处隐藏：6121字，全部文档内容请下载后查看。喜欢就下载吧 ……