An Accreting, Anomalously Low Mass Black Hole at the Center of Low Mass
  Galaxy IC 750
  We present a multi-wavelength study of the active galactic nucleus in the
nearby ($D=14.1$ Mpc) low mass galaxy IC 750, which has circumnuclear 22 GHz
water maser emission. The masers trace a nearly edge-on, warped disk $\sim$0.2
pc in diameter, coincident with the compact nuclear X-ray source which lies at
the base of the $\sim$kpc-scale extended X-ray emission. The position-velocity
structure of the maser emission indicates the central black hole (BH) has a
mass less than $1.4 \times 10^5~M_\odot$. Keplerian rotation curves fitted to
these data yield enclosed masses between $4.1 \times 10^4~M_\odot$ and $1.4
\times 10^5~M_\odot$, with a mode of $7.2 \times 10^4~M_\odot$. Fitting the
optical spectrum, we measure a nuclear stellar velocity dispersion $\sigma_* =
110.7^{+12.1}_{-13.4}$~{\rm km~s}$^{-1}.$ From near-infrared photometry, we fit
a bulge mass of $(7.3 \pm 2.7) \times 10^8~M_\odot$ and a stellar mass of $1.4
\times 10^{10}~M_\odot$. The mass upper limit of the intermediate mass black
hole in IC 750 falls roughly two orders of magnitude below the $M_{\rm
BH}-\sigma_*$ relation and roughly one order of magnitude below the $M_{\rm
BH}-M_{\rm Bulge}$ and $M_{\rm BH}-M_*$ relations -- larger than the relations'
intrinsic scatters of (0.58 $\pm$ 0.09) dex, 0.69 dex, and (0.65 $\pm$ 0.09)
dex, respectively. These offsets could be due to larger scatter at the low mass
end of these relations. Alternatively, black hole growth is intrinsically
inefficient in galaxies with low bulge and/or stellar masses, which causes the
black holes to be under-massive relative to their hosts, as predicted by some
galaxy evolution simulations.
