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

1.Introduction

Inthepastfewyears,avarietyofhighredshiftstar-formingsourcessuchastheLyman-andBalmer-breakgalaxies(e.g.Steideletal.1999)ortheSCUBAandISOdustystarbursts(Bargeretal.1998;Ausseletal.1999;Elbazetal.1999)werediscoveredatdi erentwavelengthsfromabroadrangeofobservingtechniques.However,eachofthesemethodsofdetectionstronglysu ersfromitsownselectione ects(e.g.,dustextinction, uxlimita-tionorcolourselectionattheobservedwavelength),andtheconnectionbetweenthevariouspopulationsofdistantobjectscurrentlyknownisstillpoorlyunderstood(e.g.,Webbetal.2003).

Inthegoaltotracethestar-formingactivityandtheevolutionofhighredshiftgalaxieswithreducedbiases,analternativeapproachusingthecosmologi-calGamma-RayBursts(GRBs)asprobesofstarfor-mationwasrecentlyproposed(e.g.,Wijersetal.1998;Mirabeletal.2000;Blain&Natarajan2000).SincethediscoveryofX-ray/optical/radiotransientcounterpartstolong-durationGRBs,the“collapsar”model(Woosley1993;MacFadyen&Woosley1999)linkingtheseeventstothecataclysmicdestructionofmassivestarshasindeedreceivedastrongsupportfromagrowingnumberofev-idence.TheseincludethepresenceofdustextinctionintheX-rayandopticaltransients(e.g.,Galama&Wijers2001),thespectralenergydistributionandmorphologyofGRBhostsconsistentwithcompact,irregularormerger-drivenstarbursts(e.g.,Hjorthetal.2002;Sokolovetal.2001;Djorgovskietal.1998),andtheGRBlocaliza-tionswithintheirhostssuggestingapopulationofdisk-residingprogenitors(Bloometal.2002b).Moreover,theironemissionlinesdetectedintheirX-rayafterglows(e.g.,Piroetal.2000)andthelate-timebrighteningsobservedinthelight-curvesofseveralGRBopticaltransients(e.g.,Galamaetal.2000)havebeeninterpretedasthesignatureforthepresenceofanunderlyingsupernovaoccuringwiththeGRBexplosion,andthusprovidedfurthercluesforthe“collapsar”scenario.Becauseoftheshort-livednatureofmassivestars,andbecausegamma-raysdonotsu erfrominterveningcolumnsofgasanddust,GRBscouldthusbeusedtosign-posttheinstantaneousstarformationintheUniverseindependentlyofthee ectsofdustextinction.Inthisperspective,itisespeciallycrucialtoestablishwhetherGRB-selectedgalaxiesarereallyrepresentativeofthestar-formingsourcesinthe eldatsimilarredshifts(Schaefer2000),orwhethertheymayformaparticularcategoryofnewobjectsoranysub-sampleofanalready-knownpopulationofgalaxies.Physicalpropertiesofthecircum-burstenvironmentcouldplayacrucialrolein

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

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