Towards a Complete Census of AGNs in Nearby Galaxies: A Large Population
  of Optically Unidentified AGNs
  Using Spitzer-IRS spectroscopy, we investigate the ubiquity of Active
Galactic Nuclei (AGN) in a complete (~94%), volume-limited sample of the most
bolometrically-luminous galaxies (L_IR > (0.3-20) x 10^10 L_sun) to D < 15 Mpc.
Our analyses are based on the detection of the high-excitation emission line
[NeV](\lambda 14.32 um; 97.1 eV) to unambiguously identify AGN activity. We
find that 17 of the 64 IR-bright galaxies in our sample host AGN activity
(~27^{+8}_{-6}%), >50% of which are not identified as AGNs using optical
spectroscopy. The large AGN fraction indicates a tighter connection between AGN
activity and IR luminosity for galaxies in the local Universe than previously
found, potentially indicating a close association between AGN activity and star
formation. The optically unidentified AGNs span a wide range of galaxy type
(S0-Ir) and are typically starburst-dominated systems hosting modest-luminosity
AGN activity (L_[NeV] ~ 10^37 - 10^39 erg s^-1). The non-identification of
optical AGN signatures in the majority of these galaxies appears to be due to
extinction towards the AGN, rather than intrinsically low-luminosity AGN
activity. Examination of optical images shows that the optically unidentified
AGNs with evidence for extinction are hosted in either highly inclined galaxies
or galaxies with dust lanes, indicating that obscuration of the AGN is not
necessarily due to an obscuring torus. We therefore conclude that optical
spectroscopic surveys miss approximately half of the AGN population simply due
to extinction through the host galaxy.
