Closing in on a Short-Hard Burst Progenitor Constraints from Early-Time Optical Imaging and(19)

Amorphological ttoWIYNI-bandimagingusinggal t(Pengetal.2002)showsgoodagreementwithadeVaucouleurspro le(S´ersicindex=4),withaχ2/dof=1.22.Thee ectiveradiusisRe=0.96′′=

3.47kpc.Thegalaxyhasanaxisratioof0.81withthesemimajoraxisalignedalongapositionangleeastofnorthat～90 .Therewaslittleimprovementinχ2/dofbyaddingmorecomplicatedmorphologiesorlettingtheS´ersicindexvary.

ThecoincidenceofapointsourceatradiowavelengthswiththeopticalcenterofG1mightsuggestthepresenceofalow-levelactivegalacticnucleusdespitethelackoftelltalefeaturesobservedintheG1opticalspectra.Moreover,inspectionofarchivalimagesofthisgalaxyfromtheNear-EarthAsteroidTrackingProgram(Pravdoetal.1999)on9April2002,20April2002,3May2002,22March2003,and8April2003revealsnoapparentvariabilityoftheopticallightfromG1.However,radioemissionwithoutcorrespondingopticalemissionisnotuncommoningiantellipticalgalaxiesharboringmildlyactivenuclei(Ho1999).TheradioemissioninG1isunlikelytobeassociatedwithstarformation,giventhelowSFRdeducedabove.

Thepropertiesofthisprobablehostgalaxycontrastsigni cantlywiththosemeasuredforthegalaxyhostsoflong-durationGRBs.First,mosthostsoflong-durationGRBsexhibitemission-linefeaturesin-dicativeofhighSFRs(e.g.,Djorgovskietal.2003).Second,theabsoluteK-bandluminosityofthisgalaxy(≈1.6×1011L⊙)exceedsthatofallpreviouslyidenti edGRBhostgalaxies(Charyetal.2002).Third,theimpactparameteroftheGRB(asde nedbythe90%XRTerrorcircle)islargerthanthatofallpreviouslyassociatedGRB-hostgalaxypairs(long-bursto sets<～10kpc;Bloometal.2002).

5.1.S1,S2:FaintBlueGalaxiesinaHigh-RedshiftGroup?

TheGemini/GMOSspectraofS1andS2arefeaturelessandblue.ExaminingtheregionsofthespectrumwhereHαorHβwouldlieifattheredshiftofthecluster,wedetectnomeasurableemission.Assuming,forthemoment,thatthesourcesareattheclusterredshiftof0.22,weputa3σupperlimitontheHαluminosityofLHα<1.5×1039ergs 1andLHα<1.4×1039ergs 1forS1andS2,ing

theequationfromKennicut(1998)relatingtheHαluminositytotheSFR,we ndthattheupperlimitsfortheunextinguishedSFR,assumingthatS1andS2areclustermembers,are～1.1×10 2M⊙yr 1forthegalaxies.IfS1andS2areclustermembers,thentheyarenotformingstars,whichwouldseemtocon ictwiththeirbluecolors.

Amorelikelyscenario,alsomentionedbyCenkoetal.(2005a),isthatS1andS2arebothbackgroundgalaxies.AlthoughtheGeminispectrarangefrom4600 Ato8600 A,thedatahavepoorsignal-to-noiseratio