A multi-wavelength study of the starburst galaxy NGC 7673

HST/WFPC2 images resolved NGC 7673 into a large number of star clusters. Among these, 31 fall inside the IUE large aperture, which was used to acquire an integrated ultraviolet spectrum of the galaxy. We have fitted Starburst99 evolutionary synthesis model

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002 voN 61 1v5641140/hp-ortsa:viXraAMULTI-WAVELENGTHSTUDYOFTHESTARBURSTGALAXYNGC7673

1PaolaCastangia1,2,AnnaPasquali3,PieroBenvenuti1

3UniversitàdiCagliari(Italy),2INAF-OsservatorioAstronomicodiCagliari(Italy),ETHZürich(Switzerland)

Abstract

HST/WFPC2imagesresolvedNGC7673intoalargenumberofstarclusters.Amongthese,31fallinsidetheIUElargeaperture,whichwasusedtoacquireanintegratedultravioletspectrumofthegalaxy.Wehave ttedStarburst99evo-lutionarysynthesismodelstotheobservedcoloursoftheseclustersandderivedages,reddenings,andmassesoftheclusters.ThenasimplesumoftheclusterssyntheticUVspectrahasbeencomparedtotheobservedIUEspectrum,inordertoinvestigatethestarformationhistoryofNGC7673.

1.Introduction

ThehighangularresolutionoftheHubbleSpaceTelescope,hassigni -cantlyimprovedourunderstandingofnearbystarburstgalaxies.OneofthemostintriguingresultsoftheHSTimaginghasbeenthediscoveryofcompacthigh-luminositystarclusters(oftenreferredtoas"superstarcluster",SSCs),inavarietyofgalacticenviroments.TheseSSCsshowremarkablysimilarproperties:compactsizes(fewtensofparsec);youngages(<1Gyr);andas-sociatedhighluminosities,implyingmassesbetween105and106M⊙.Sofar,severalstudiesindicatethat20%to50%oftherecentstarformationoccursinclusters,regardlessofthelocationwithinthegalaxy(e.g.galacticdisksorcircumnuclearrings)andofitsHubbletype(fromspiralstomergers;Buatetal.,1994;Maozetal.1996;Meureretal.1995).

UVimaginghasshownthatthemorphologyofstarburstregionsisquitecomplex,withcompact,bright,youngclustersembeddedinadiffuseirregularUVbackground.Buatetal.(1994)havefoundthatdiscretesourcesdominatetheUVemissionofM33andthatthefractionoftheUV uxarisingfromdiffuseregionsisnotlargerthan20%.Onthecontrary,Maozetal.(1996)havedeterminedthatthefractionofUV uxemittedbystarclusterscanvarybetween15%and50%ofthetotalUVluminosityofcircumnuclearringsin

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