Towards a Complete Census of AGNs 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 x 10^9
L_sun) to D<15 Mpc (Goulding & Alexander 2009). A database of accurate SMBH
mass (M_BH) estimates is compiled from literature sources using physically
motivated AGN modeling techniques (reverberation mapping, maser mapping and gas
kinematics) and well-established indirect M_BH estimation methods (the M-sigma
and M_BH-L_(K,bul) relations). For the three sources without previously
published M_BH estimates, we use 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)
x 10^7 M_sun), but with the majority in the poorly studied M_BH ~ 10^6-10^7
M_sun 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-average SMBH growth rate of galaxies in the local Universe and find that
the AGNs hosting SMBHs in the mass range M_BH ~ 10^6-10^7 M_sun 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
Gyrs). Additionally, we find tentative evidence for an increasing
volume-weighted AGN fraction with decreasing SMBH mass in the M_BH ~ 10^6-10^8
M_sun range. Overall, we conclude that significant mass accretion onto small
SMBHs may be missed in even the most sensitive optical surveys due to absent or
weak optical AGN signatures.
