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

摘要

Wehavedevelopedanewmethodofidentifyingactivegalacticnuclei(AGNs)andstudiedthenatureoflow-luminosityAGNsintheSloanDigitalSkySurvey.ThisisthelatterpartofaseriesofpapersinwhichweconsidercorrelationsbetweentheAGNactivitiesandthehost-galaxyproperties.BasedonasampleofAGNsidentifiedbyanewmethoddevelopedintheformerpart(2012,PASJ,64,36),wefoundthatAGNstypicallyshowextinctionof$$ au_V$$$$=$$1.2,andexhibitawiderangeofionizationlevels.Thefindingofionizationlevelsmotivatedustouse[OII]$$+$$[OIII]asanindicatorofAGNpower.WefoundthatAGNsarepreferentiallylocatedinmassive,red,early-typegalaxies.TakingintoaccountaselectionbiasoftheOxygen-excessmethod,weshowedthatstrongAGNsarelocatedinactivestar-forminggalaxies,andthatrapidlygrowingsuper-massiveblackholesarelocatedinrapidlygrowinggalaxies,whichclearlyshowsthecoevolutionofsuper-massiveblackholesandtheirhostgalaxies.Thisisasurprisingphenomenon,giventhatthegrowthsofblackholesandhostgalaxiesoccurontheirrespectivephysicalscaleswhichareverydifferent.Interestingly,theAGNpowerdoesnotstronglycorrelatewiththehost-galaxymass.Itseemsthatthemassworksasa``switch''foractivatingAGNs.TheabsenceofAGNsinlow-massgalaxiesmightbeduetotheabsenceofsuper-massiveblackholesthere,butadedicatedobservationofthenuclearregionofnearbylow-massgalaxieswouldbenecessarytoobtainadeeperinsightintoit.