Properties of AGN selected by their mid-IR colours: evidence for a
  physically distinct mode of black hole growth
  We study the narrow emission line properties and stellar populations of a
sample of 1385 AGN selected to have strong excess emission at mid-infrared
wavelengths based on comparing Wide-field Infrared Survey Explorer W1-W2 band
colours with optical stellar absorption line indicators. Our goal is to
understand whether the physical conditions in the interstellar medium of these
objects differ from those of local AGN selected by their optical emission line
ratios. To enable this comparison, we construct a control sample of 50,000
optically-selected AGN with the same redshifts that do not have strong mid-IR
excess emission. The mid-IR excess and control samples differ strongly in
[OIII] line luminosity, ionized gas excitation mechanism, ionization state and
electron density. We show that the radio-detected, mid-IR excess AGN constitute
the most luminous and highly ionized AGN in the local Universe and they
contribute primarily to the growth of black holes in the most massive galaxies.
At least half of this black hole growth is occurring in galaxies with recent
starbursts. The morphologies of these systems indicate that the starbursts have
probably been triggered by galaxy-galaxy mergers and interactions. The most
luminous AGN in our mid-IR excess sample have properties that are similar to
the Type II quasars studied at higher redshifts. In contrast, the control
sample constitute a class of lower ionization, less luminous AGN in more
quiescent galaxies that contribute primarily to the growth of low mass black
holes.
