A Radio Perspective on Star-Formation in Distant Galaxies

inDistantGalaxies

EricA.Richards

arXiv:astro-ph/9811120v1 8 Nov 1998NationalRadioAstronomyObservatory&UniversityofVirginia

520EdgemontRoadCharlottesville,VA22903email:erichard@nrao.edu

Abstract.

Determinationoftheepochdependentstar-formationrateoffieldgalaxiesisoneoftheprincipalgoalsofmodernobservationalcosmology.Deepradiosurveys,sensitivetostarburstsouttoz∼1-2,mayholdthekeytounderstandingtheevolutionofthestarburstphenonemonunhinderedbytheeffectsofdust.Usingdeep,highresolutionradioobservationsoftheHubbleDeepField,weshowthattheµJyradioemissionfromfieldgalaxiesatz∼0.4−1isprimarilystarburstinorigin.Inaddition,wehavediscoveredapopulationofopticallyfaint,possiblyobscuredsystemsthatarecandidatehigh-zprotogalaxies.Atleastoneoftheseradiosourcesisidentifiedwithasub-mmdetection.

RADIOEMISSIONASASTARFORMATIONTRACER

Thediffuseradioemissionobservedinlocalstarburstsisbelievedtobeamix-tureofsynchrotronradiation(excitedbysupernovaeremnantsandhencedirectlyproportionaltothenumberofsupernovaeproducingstars)andthermalradiation(fromHIIregionsandhenceanindicatorofthenumberofOandBstarsinagalaxy).Asthethermalandsynchrotronradiationofastarburstdissipatesonaphysicaltimescaleof107−108years,theradioluminosityisatruemeasureoftheinstantaneousstar-formationrate(SFR)inagalaxy,uncontaminatedbyolderstellarpopulations.Sincesupernovaeprogenitorsaredominatedby∼8M⊙stars,synchrotronradiationhastheadditionaladvantageofbeinglesssensitivetouncer-taintiesintheinitialmassfunctionasopposedtoUVandopticalrecombinationlineemission.However,themostobviousadvantageofusingtheradioluminosityasaSFRtracerisitsunsusceptibilitytodustobscuration,asgalaxiesandtheinter-galacticmediumaretransparantatcentimeterwavelengths.

Comparisonofthelocalradioluminosityfunction(LF)ofstar-forminggalaxies[1]withthosederivedindependentlyatFIR[2],Hα[3],andUVwavelengths[4]showssurprsingagreement.Figure1showsthefourLFsinunitsofSFRs.ThisplotsuggeststhatthebulkoflocalstarformationisoccurringinmodeststarburstswithSFR∼10M⊙yr−1.However,pastthepeakintheLF,theHαandUVestimatesbegintodropbelowtheradio/FIRrates,andbeyond50M⊙yr−1hasentirelyvanished.Thisisdirectevidencethatopticallyselectedsurveysareincapableofdetectingthemostextremeanddustobscuredstarbursts.

RADIOSTAR-FORMATION

=FIGURE1.Shownisthecontributiontothelocalstar-formationluminositydensity(u)perluminosityintervalofstar-forminggalaxy.Theradio,IRAS,Hα,andfar-ultraviolet(FUV)luminositieshavebeenconvertedtoSFRsassumingaSalpeterIMFover0.1-100M⊙.NoticethatthefourmeasuresofSFRsallpeakat∼10M⊙yr−1(seeCramm,ApJL,506,85forafullerdiscussion).TheradioandIRASpointsareinparticularlygoodagreement,reflectingthetightFIR/radiocorrelationinstar-forminggalaxies.Theshadedregionrepresentswhatmaybeadustcurtainbeyondwhichopticalsurveysareblindtostarformation.IftheSFRluminosityfunctionevolvesasL∝(1+z)3.5,thenbyz=1,itwillappearasthesolidline.Thisanalysissuggeststhatthebulkofglobalstar-formationathigh−zishiddenfromopticalsurveys.

RADIOEMISSIONFROMDISTANTGALAXIES

Our1.4/8.5GHzstudyoftheHubbleDeepFieldusingtheVLAandMERLINhasdemonstratedthat70%ofµJysourcesareidentifiedwithmorphologicallype-culiar,mergingand/orinteractingdiskgalaxies,manywithindependentevidenceofstar-formation(bluecolors,infra-redexcess,HIIopticalspectra)[5][6].RadiomorphologiesfromthehighresolutionVLA/MERLINobservationsoftheHDF[7]indicatethat95%ofµJyradiosourcesareresolvedat0.2′′resolutionandsuggestsamediansizeof2′′,comparabletotheopticalextentofthesez∼0.4−1systems.ThesedataexcludeAGNasthedominantcontributortotheradioluminosityinthemajorityofthesesystems.Thusthecosmologicalfaintradiopopulationisdom-inatedbythedistantanalogsoflocalIRASgalaxieswithsuggestedstar-formationratesof10-1000M⊙yr−1.

Wehavedetectedover100radiosourcesincompletesampleswithin4.5′oftheHDFwheredeepopticalimagingisavailable.NinetypercentoftheseradiosourcesareidentifiedwithgalaxiesofmeanmagnitudeIAB∼22.However,approximately10%remainunidentifiedtoIAB=28intheHDFandIAB=26intheHDFflanking

RICHARDS

FIGURE2.Thegreyscaleshowsa20′′×20′′HDFI-bandimagecontainingtheSCUBAde-tectionHDF850.1.Thecontourscorrespondto1.4GHzemissionatthe-2,2,4and6σlevel(σ=7.5µJy).ThethreesigmapositionerrorcircleforHDF850.1isshownaftershiftingtotheVLAcoordinateframe.TheoriginalpositionofHDF850.1takenfromHughesetal.(1998)isdenotedbythediamond.TheISOdetectionismarkedwithacrosswiththreesigmapositionerrors(Ausseletal.1998,A&Ainpress).The0.1′′radio/opticalregistrationclearlyrulesoutassociationwiththebrightspiral.VLA3649+1221maybethemostobscuredpartofalargergalaxy3-633.1atz=1.72(locateddirectlyunderneaththeSCUBAerrorcircle;Fernandez-Sotoetal.1998,AJinpress).

fields.NICMOSimagingofoneoftheseunidentifedradiosources(VLA3642+1331)hasrevealedaH=22.7diskgalaxyatunknownredshift(seeI.Waddingtonelse-whereintheseproceedings).Anotherradiosource,VLA3649+1221,isidentifiedwithaIAB=28objectintheHDF.NICMOSimagingbyDickinsonetal.(1998,privatecommunication)showsthatthisobjecthasasteeplyrisingoptical/nearin-fraredspectrumpossiblysuggestingahighredshift.Wehavesubsequentlyshownthatthebrightestsub-mm/SCUBAsourceintheHDF(HDF850.1)(Hughesetal.1998,Nature,393,241)isidentifiedwithVLA3649+1221.Itisourhypothesisthatthesesystemsareextreme,dustobscuredstarburstgalaxies.Thesurfacedensityofthesecandidatehigh-zradioobjectsiscomparabletothesub-mmpopulationat∼0.1arcmin−2.

Ofthefive850µmsourcesintheHDF,twoaresolidlydetectedatradiowave-lengths(Fig.2and3),whiletwohavelesssecureradioidentifications[8].Twooftheseradio/sub-mmsourcesareassociatedwithz∼0.5starbursts(HDF850.3andHDF850.4).Theothertwodetections(HDF850.1andHDF850.2)musthaveredshiftslessthan1.5orbecontaminatedbyAGNbasedonradioluminosityar-guments.Thisradioanalysissuggeststheclaim,basedonlowresolutionsub-mmobservationsalone,thattheopticalsurveysunderestimatethez>2globalstar-

RADIOSTAR-FORMATION

FIGURE3.Radio1.4GHzcontoursdrawnatthe-2,2,4and6σlevelareoverlaidontheHDFI-bandimagecenterdonthepositionofHDF850.4(20′′onaside).A15µmdetectionfromthecompletecatalogofAusseletal.(1998)hasbeenassociatedwiththisradiosourceandsuggestslikelystarburstactivityinthediskgalaxy.ThesymbolsarethesameasforFigure2.WeestimateaSFR=150M⊙yr−1.

formationratearepremature.Ontheotherhand,thez<1star-formationhistorymayhavebeenunderestimatedifasignificantfractionofthesub-mmpopulationliesatrelativelylowredshift,inagreementwiththelargenumberofz=0.4-1radiostarburstswearefindingintheHDF.Analysisoftheevolvingradiostar-formingluminosityfunctionpromisestoshedlightontheprevalenceofdustenshroudedstarbrustactivityathigh−z(seeHaarsma&Partridge,elsewhereintheseproceed-ings).

ItisapleasuretothankmycollaboratorsK.Kellermann,E.Fomalont,B.Partridge,R.Windhorst,T.Muxlow,I.Waddington,andD.Haarsma.SupportforpartofthisworkwasprovidedbyNASAthroughgrantAR-6337.*-96AfromtheSTSI,whichisoperatedbyAURA,Inc.,underNASAcontractNAS5-2655,andbyNSFgrantAST93-20049.

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