Black Holes at the Centers of Nearby Dwarf Galaxies
  Using a distance-limited portion of the Sloan Digital Sky Survey (SDSS) Data
Release 7, we have identified 28 active galactic nuclei (AGNs) in nearby (d <
80 Mpc) low-mass, low-luminosity dwarf galaxies. The accreting objects at the
galaxy centers are expected to be intermediate-mass black holes (IMBHs) with
M_BH < 1e6 M_sun. The AGNs were selected using several optical emission-line
diagnostics after careful modeling of the continuum present in the spectra. We
have limited our survey to objects with spectral characteristics similar to
those of Seyfert nuclei, excluding emission-line galaxies with ambiguous
spectra that could be powered by stellar processes. The host galaxies in our
sample are thus the least massive objects in the very local universe certain to
contain central black holes. Given our focus on the nearest objects included in
the SDSS, our survey is more sensitive to low-luminosity emission than previous
optical searches for AGNs in low-mass galaxies. The [O III] lambda5007
luminosities of the Seyfert nuclei in our sample have a median value of L_5007
= 2e5 L_sun and extend down to 1e4 L_sun. Using published data for broad-line
IMBH candidates, we have derived an [O III] bolometric correction of log
(L_bol/L_5007) = 3.0 +/- 0.3, which is significantly lower than values obtained
for high-luminosity AGNs. Applying this correction to our sample, we obtain
minimum black-hole mass estimates that fall mainly in the 10^3 M_sun -- 10^4
M_sun range, which is roughly where the predicted mass functions for different
black-hole seed formation scenarios overlap the most. In the stellar mass range
that includes the bulk of the AGN host galaxies in our sample, we derive a
lower limit on the AGN fraction of a few percent, indicating that active nuclei
in dwarf galaxies are not as rare as previously thought.
