Towards a complete census of active galactic nuclei in nearby galaxies: the incidence of growing black holes

We investigate the local supermassive black hole (SMBH) density function and relative mass accretion rates of all active galactic nuclei (AGNs) identified in a volume-limited sample of infrared (IR) bright galaxies (L_IR > 3 × 10⁹L_solar) to D < 15Mpc. A data base of accurate SMBH mass (M_BH) estimates is compiled from literature sources using physically motivated AGN modelling techniques (reverberation mapping, maser mapping and gas kinematics) and well-established indirect M_BH estimation methods (the M-σ_* and M_BH-L_K,bul relations). For the three sources without previously published M_BH estimates, we use Two Micron All Sky Survey (2MASS) K-band imaging and GALFIT to constrain the bulge luminosities, and hence SMBH masses. In general, we find the AGNs in the sample host SMBHs which are spread over a wide mass range [M_BH ~ (0.1-30) × 10⁷M_solar], but with the majority in the poorly studied M_BH ~ 10⁶-10⁷M_solar region. Using sensitive hard X-ray (2-10keV) and mid-IR constraints we calculate the bolometric luminosities of the AGNs (L_Bol,AGN) and use them to estimate relative mass accretion rates. We use these data to calculate the volume-averaged SMBH growth rate of galaxies in the local Universe and find that the AGNs hosting SMBHs in the mass range M_BH ~ 10⁶-10⁷M_solar are dominated by optically unidentified AGNs. These relatively small SMBHs are acquiring a significant proportion of their mass in the present day, and are amongst the most rapidly growing in the local Universe (SMBH mass-doubling times of ~6Gyr). Additionally, we find tentative evidence for an increasing volume-weighted AGN fraction with decreasing SMBH mass in the M_BH ~ 10⁶-10⁸M_solar range. Overall, we conclude that significant mass accretion on to small SMBHs may be missed in even the most sensitive optical surveys due to absent or weak optical AGN signatures.