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
  In order to perform a more complete census of active galactic nuclei (AGN) in
the local Universe, we investigate the use of the HeII~$\lambda 4685$ emission
line diagnostic diagram by Shirazi & Brinchmann (2012) in addition to the
standard methods based on other optical emission lines. The He II based
diagnostics is more sensitive to AGN ionization in the presence of strong star
formation than conventional line diagnostics. We survey a magnitude-limited
sample 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 diagnostic
diagrams, as well as the HeII diagram to identify AGN. In this sample, 1,075
galaxies are selected as AGN using the BPT diagram, while an additional 234
galaxies are identified as AGN using the HeII diagnostic diagram, representing
a 22% increase of AGN in the parent galaxy sample. We explore the host galaxy
properties of these new HeII selected AGN candidates and find that they are
most common in star-forming galaxies on the blue cloud and on the main sequence
where ionization from star-formation is most likely to mask AGN emission in the
BPT lines. We note in particular a high HeII AGN fraction in galaxies above the
high-mass end of the main sequence where quenching is expected to occur. We use
archival Chandra observations to confirm the AGN nature of candidates selected
through HeII based diagnostic. Finally, we discuss how this technique can help
inform galaxy/black hole co-evolution scenarios.
