A Method of Identifying AGNs Based on Emission-Line Excess and the Nature of Low-Luminosity AGNs in the Sloan Digital Sky Survey. I. A New Method

摘要

Wedevelopedanewtechniqueforidentifyingactivegalacticnuclei(AGNs)andstudiedthenatureoflow-luminosityAGNsintheSloanDigitalSkySurvey.Thisistheformerpartofaseriesofpapers.Wedevelopanew,sensitivemethodofidentifyingAGNsinthispaper.Theemission-lineluminosityinaspectrumisthesumofastar-formationcomponentandanAGNcomponent(ifpresent).Wedemonstratethatanaccurateestimateofthestar-formationcomponentcanbeachievedbyfittingmodelspectra,generatedwitharecentstellarpopulationsynthesiscode,toacontinuumspectrum.Bycomparingtheobservedtotallineluminositywiththatattributedtostarformation,wecantellwhetheragalaxyhostsanAGNornot.Wecompareourmethodwiththecommonlyusedemission-linediagnosticsproposedbyBaldwin,Phillips,andTerlevich(1981,PASP,93,5;hereafterBPT).Bythismethod,weclassify85%ofthestrongemission-lineobjectsinthesamestar-formation/AGNasBPT.Oneuniquefeatureofourmethodisitssensitivity;itisapplicabletonearlytwiceasmanyobjectsasBPT.WefurthermakeacomparisonbetweenourmethodandtheBPTdiagnosticsusingstackedspectraandselectionsinX-rayandradiowavelengths.WeshowthatitisoverallasensitivemethodofidentifyingAGNs,whileourmethodsuffersfromincompletenessandcontaminationasanyAGNidentificationmethoddoesso.Weemphasizethatourmethodcanbeappliedathighredshifts(upto$${z}$$$$sim$$1.7withred-sensitiveopticalspectrograph)withoutmakinganyaprioriassumptionsaboutthehost-galaxyproperties.Anotheruniquefeatureisthatitallowsustosubtracttheemission-lineluminosityduetostarformation,andtoextracttheintrinsicAGNluminosity.