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In order to perform a more complete census of active galactic nuclei (AGN) inthe local Universe, we investigate the use of the HeII~$\lambda 4685$ emissionline diagnostic diagram by Shirazi & Brinchmann (2012) in addition to thestandard methods based on other optical emission lines. The He II baseddiagnostics is more sensitive to AGN ionization in the presence of strong starformation than conventional line diagnostics. We survey a magnitude-limitedsample of 63,915 galaxies from the Sloan Digital Sky Survey Data Release 7 at$0.02<z<0.05$ and use both the conventional BPT emission line diagnosticdiagrams, as well as the HeII diagram to identify AGN. In this sample, 1,075galaxies are selected as AGN using the BPT diagram, while an additional 234galaxies are identified as AGN using the HeII diagnostic diagram, representinga 22% increase of AGN in the parent galaxy sample. We explore the host galaxyproperties of these new HeII selected AGN candidates and find that they aremost common in star-forming galaxies on the blue cloud and on the main sequencewhere ionization from star-formation is most likely to mask AGN emission in theBPT lines. We note in particular a high HeII AGN fraction in galaxies above thehigh-mass end of the main sequence where quenching is expected to occur. We usearchival Chandra observations to confirm the AGN nature of candidates selectedthrough HeII based diagnostic. Finally, we discuss how this technique can helpinform galaxy/black hole co-evolution scenarios.

monthly notices of the royal astronomical society

Active Galactic Nuclei from HeII: a more complete census of AGN in SDSS  galaxies yields a new population of low-luminosity AGN in highly star-forming  galaxies