Are the hosts of Gamma-Ray Bursts sub-luminous and blue galaxies

We present K-band imaging observations of ten Gamma-Ray Burst (GRB) host galaxies. We compare their observed and absolute K magnitudes as well as their R-K colours with those of other distant sources detected in various optical, near-infrared, mid-infrared

Astronomy&Astrophysicsmanuscriptno.(DOI:willbeinsertedbyhandlater)

1February2,2008

ArethehostsofGamma-RayBurstssub-luminousandblue

galaxies? ,

E.LeFloc’h1,P.-A.Duc1,2,I.F.Mirabel1,3,D.B.Sanders4,5,G.Bosch6,R.J.Diaz7,C.J.Donzelli8,I.Rodrigues1,T.J.-L.Courvoisier9,10,J.Greiner5,S.Mereghetti11,J.Melnick12,J.Maza13,and

D.Minniti14

arXiv:astro-ph/0301149v1 9 Jan 2003

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121314

CEA/DSM/DAPNIAServiced’Astrophysique,F-91191Gif-sur-Yvette,FranceCNRSURA2052,FranceInstitutodeAstronom´ ayF´ sicadelEspacio,cc67,suc28.1428BuenosAires,Argentina

InstituteforAstronomy,UniversityofHawaii,2680WoodlawnDrive,Honolulu,HI96822,UnitedStatesMax-Planck-Institutf¨urExtraterrestrischePhysik,D-85740,Garching,GermanyFacultaddeCs.AstronomicasyGeof´ sica,PaseodelBosques/n,LaPlata,ArgentinaObservatorioAstron´omicodeC´ordoba&SeCyT,UNC,Laprida854,Cordoba[5000],ArgentinaIATE,ObservatorioAstron´omico&CONICET,Laprida854,Cordoba[5000],ArgentinaINTEGRALScienceDataCenter,Ch.d’Ecogia16,CH-1290Versoix,SwitzerlandGenevaObservatory,Ch.desMaillettes11,1290Sauverny,Switzerland

IstitutodiAstro sicaSpazialeeFisicaCosmica,SezionediMilano“G.Occhialini”,viaBassini15,I-20133Milan,Italy

EuropeanSouthernObservatory,AlonsodeCordova3107,Santiago,ChileDepartamentodeAstronom´ a,UniversidaddeChile,Casilla36-D,Santiago,ChileDepartmentofAstronomy,Ponti caUniversidadCat´olica,Av.Vicu naMackenna4860,Santiago,Chile

ReceivedDecember6,2002/AcceptedDecember23,2002

Abstract.WepresentK–bandimagingobservationsoftenGamma-RayBurst(GRB)hostgalaxiesforwhichanopticaland/orradioafterglowassociatedwiththeGRBeventwasclearlyidenti ed.DatawereobtainedwiththeVeryLargeTelescopeandNewTechnologyTelescopeatESO(Chile),andwiththeGemini-NorthtelescopeatMaunaKea(Hawaii).AddingtooursamplenineotherGRBhostswithK–bandphotometryanddeterminedredshiftspublishedintheliterature,wecomparetheirobservedandabsoluteKmagnitudesaswellastheirR Kcolourswiththoseofotherdistantsourcesdetectedinvariousoptical,near-infrared,mid-infraredandsubmillimeterdeepsurveys.We ndthattheGRBhostgalaxies,mostofthemlyingat0.5<~z<~1.5,exhibitverybluecolours,comparabletothoseofthefaintbluestar-formingsourcesathighredshift.Theyaresub-luminousintheK–band,suggestingalowstellarmasscontent.Wedonot ndanyGRBhostsharbouringR–andK–bandpropertiessimilartothosecharacterizingtheluminousinfrared/submillimetersourcesandtheextremelyredstarbursts.ShouldGRBsberegardedasanunbiasedprobeofstar-formingactivity,thislackofluminousand/orreddenedobjectsamongtheGRBhostsamplemightrevealthatthedetectionofGRBopticalafterglowsislikelybiasedtowardunobscuredgalaxies.Itwouldmoreoversupporttheideathatalargefractionoftheoptically-darkGRBsoccurwithindust-enshroudedregionsofstarformation.Ontheotherhand,ourresultmightalsosimplyre ectintrinsicpropertiesofGRBhostgalaxiesexperiencinga rstepisodeofverymassivestarformationandcharacterizedbyaratherweakunderlyingstellarpopulation.Finally,wecomputetheabsoluteBmagnitudesforthewholesampleofGRBhostgalaxieswithknownredshiftsanddetectedatopticalwavelengths.We ndthatthelatterappearstatisticallyevenlessluminousthanthefaintbluesourceswhichmostlycontributedtotheB–bandlightemittedathighredshift.ThisindicatesthattheformationofGRBscouldbefavouredinparticularsystemswithverylowluminositiesand,therefore,lowmetallicities.Suchanintrinsicbiastowardmetal-poorenvironmentswouldbeactuallyconsistentwithwhatcanbeexpectedfromthecurrently-favouredscenarioofthe“collapsar”.TheforthcominglaunchoftheSWIFTmissionattheendof2003willprovideadramaticincreaseofthenumberofGRB-selectedsources.Adetailedstudyofthechemicalcompositionofthegaswithinthissampleofgalaxieswillthusallowustofurtheranalysethepotentiale ectofmetallicityintheformationofGRBevents.Keywords.galaxies:starburst–galaxies:evolution–cosmology:observations–gammarays:bursts–

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