Towards a complete census of AGN in nearby Galaxies: A large population of optically unidentified AGN

Using Spitzer-infrared spectrograph (IRS) spectroscopy, we investigate the ubiquity of active galactic nuclei (AGN) in a complete (≈94 per cent), volume-limited sample of the most bolometrically luminous galaxies [L _{IR,8-1000 μm} ≳ (0.3-20) × 10¹⁰ L _⊙] to D < 15 Mpc. Our analyses are based on the detection of the high-excitation emission line [Ne v]λ14.32 μm (97.1 eV) to unambiguously identify AGN activity. We find that 17 of the 64 infrared (IR) bright galaxies in our sample host AGN activity (≈27⁺⁸ _-6 per cent), ≃50 per cent of which are not identified as AGN 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 AGN span a wide range of galaxy type (S0-Ir) and are typically starburst-dominated systems hosting modest-luminosity AGN activity (L_{[Ne V]} ≈ 10 ³⁷-10³⁹ 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; however, we note that the AGN optical signatures are diluted in some galaxies due to strong star formation activity. Examination of optical images shows that the optically unidentified AGN 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.