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 × 10⁹ L_⊙) to D < 15 Mpc. 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) × 10⁷ M_⊙], but with the majority in the poorly studied M_BH ≈ 10⁶-10⁷ M_⊙ region. Using sensitive hard X-ray (2-10 keV) 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 ≈ 10⁶-10⁷ M_⊙ 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 ≈6 Gyr). Additionally, we find tentative evidence for an increasing volume-weighted AGN fraction with decreasing SMBH mass in the M_BH ≈ 10⁶-10⁸ M_⊙ 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.