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 $1.4 \times 10^5~M_\odot$,withamodeof$7.2 \times 10^4~M_\odot$. Fitting the optical spectrum, we measure a nuclear stellar velocity dispersion $\sigma_* = 110.7^{+12.1}{-13.4}$\mathrm{k}\mathrm{m}\mathrm{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 $M_{\rm BH}-\sigma_$relationandroughlyoneorderofmagnitudebelowthe$M_{\rm BH}-M_{\rm Bulge}$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.