The nature of GRB-selected submillimeter galaxies hot and young

We present detailed fits of the spectral energy distributions (SEDs) of four submillimeter (submm) galaxies selected by the presence of a gamma-ray burst (GRB) event (GRBs 980703, 000210, 000418 and 010222). These faint ~3 mJy submm emitters at redshift ~1

Draftversion,February1,2008

APreprinttypesetusingLTEXstyleemulateapjv.10/09/06

THENATUREOFGRB-SELECTEDSUBMILLIMETERGALAXIES:HOTANDYOUNG

´nandD.WatsonM.J.Micha lowski,J.Hjorth,J.M.CastroCero

DarkCosmologyCentre,NielsBohrInstitute,UniversityofCopenhagen,JulianeMariesvej30,2100CopenhagenØ,Denmark

Draftversion,February1,2008

arXiv:0708.3850v1 [astro-ph] 29 Aug 2007

ABSTRACT

Wepresentdetailed tsofthespectralenergydistributions(SEDs)offoursubmillimeter(submm)galaxiesselectedbythepresenceofagamma-rayburst(GRB)event(GRBs980703,000210,000418and010222).Thesefaint~3mJysubmmemittersatredshift~1arecharacterizedbyanunusualcombinationoflong-andshort-wavelengthproperties,namelyenhancedsubmmand/orradioemissioncombinedwithopticalfaintnessandbluecolors.Weexcludeanactivegalacticnucleusasthesourceoflong-wavelengthemission.FromtheSED tsweconcludethatthefourgalaxiesareyoung(ages<2Gyr),highlystarforming(starformationrates~150M⊙yr 1),low-mass(stellarmasses~1010M⊙)anddusty(dustmasses~3×108M⊙).Theirhighdusttemperatures(Td 45K)indicatethatGRBhostgalaxiesarehotter,younger,andlessmassivecounterpartstosubmm-selectedgalaxiesdetectedsofar.FuturefacilitieslikeHerschel,JCMT/SCUBA-2andALMAwilltestthishypothesisenablingmeasurementofdusttemperaturesoffainterGRB-selectedgalaxies.

Subjectheadings:galaxies:evolution—galaxies:high-redshift—galaxies:starburst—galaxies:ISM

—dust,extinction—gamma-ray:burst

1.INTRODUCTION

Ithasbeenclaimedthatsubmillimeter(submm)galax-ies(SMGs,seeBlainetal.2002,forareview)aresignif-icantcontributorstothestarformationhistoryoftheUniverseatredshiftsz~2 3(Chapmanetal.2005)andhavebuiltupasubstantialfractionofthepresent-daystellarpopulationetal.1999).SMGsarelu-minous(withinfraredluminositiesLIR~1012 14L⊙)andcold(withmeandusttemperatureTd=36±7K,Chapmanetal.Galaxieswithsimilarluminosi-tiesbutwithhigherdusttemperatures(Td>45K)aredi culttodetectinthesubmmwithcurrenttechnologyduetothefactthatthepeakoftheinfrareddustemis-sionisshiftedoutof850µshorterwavelengths2004;2004,2005)insuchgalaxies.

Attheotherendofthegalaxyluminosityfunction,thehostgalaxiesoflong-durationgamma-raybursts(GRBs,originatinginthecollapsesofverymassivestarsattheendoftheirevolution,e.g.Staneketal.Hjorthetal.donothavemuchincommonwithSMGsexceptforthefactthatthistypeofgalaxiesisalsothoughttocontributesigni cantlyto,oratleasttrace,theglobalstarformation(e.g.2005,IncontrasttoSMGs,GRBhostsarefoundtobeintheoptical(LeFloc’hetal.2003;andmetal-poor(Fynboetal.2003).Themajorityofthemhavenotbeendetectedatmid-infrared(MIR),submmorradiowavelengthsetal.Tanviretal.Bergeretal.indicatingthat,asaclass,theyarenotheavilyobscuredorvio-lentlystarforminggalaxies.Alowinternaldustcon-tentisconsistentwithlowextinctionfoundintheanaly-sisofopticalafterglows(Strattaetal.2004;2006;Kannetal.2006)andopticalspectralenergydis-tributions(SEDs)ofthehostgalaxies(Christensenetal.

Electronicaddress:michal@dark-cosmology.dk

However,fourGRBhosts(980703,000210,000418and010222)havebeen rmlydetectedinsubmmradio(Tanviretal.2001b,providingasomewhatcomplexpicture:as-sumingthatthisemissionispoweredbystarbursts,thederivedstarformationrates(SFRs)areoftheorderofafewhundredsolarmassesperyearandtheamountofdustinthesegalaxiesissigni cant.Ontheotherhand,theyexhibitbluecolors,lowextinctionandlowextinction-correctedoptical/UVSFRs1998;2001;Charyetal.2002;etal.Bergeret2003;etal.2004;Kannetal.likethemajorityofknownGRBhosts.ThispuzzlingsituationwashighlightedbytheirFigure3)—theseGRBhostsaremuchfainterintheopticalthantheprototypicalUltraLuminousInfraredGalaxy(ULIRG)Arp220andhavemuchbluerspectra,butaremoreluminousinsubmmandradio.Infact,notemplateSEDmodel(e.g.2001;couldgiveconsistencywiththedata(seealsolMicha lowski2006).More-over,noneofhavebeendetectedat24µm(LeFloc’hetal.CastroCer´onetal.2006,LeFloc’h,privatecommunication).

ThelocationoftheGRBeventswithintheirhostsaddsfurthercomplexitytothispicture.AsfoundbyFruchteretal.GRBstracethelocationofthebrightestrest-UVpartsoftheirhosts(seealsoIfthemajorityofthestarformationinthehostsofGRBs980703,000210,000418and010222washiddenbydust,thentheyshouldpreferentiallybefoundinobscured(UV-dim)partsoftheirhosts(aslongasGRBstracestarformation),contrarytoobservations.Inthispaperweinvestigatethisseemingdiscrepancybetweenshort-andlong-wavelengthdatathroughstel-

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