An Accreting, Anomalously Low-mass Black Hole at the Center of Low-mass Galaxy IC 750

We present a multiwavelength 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 ∼0.2 pc in diameter, coincident with the compact nuclear X-ray source that lies at the base of the ∼kiloparsec-scale extended X-ray emission. The position-velocity structure of the maser emission indicates that the central black hole (BH) has a mass less than 1.4 × 105 M . Keplerian rotation curves fitted to these data yield enclosed masses between 4.1 × 104 M and 1.4 × 105 M , with a mode of 7.2 × 104 M . Fitting the optical spectrum, we measure a nuclear stellar velocity dispersion km s−1. From near-infrared photometry, we fit a bulge mass of (7.3 2.7) × 108 M and a stellar mass of 1.4 × 1010 M . The mass upper limit of the intermediate-mass BH in IC 750 falls roughly two orders of magnitude below the MBH- * relation and roughly one order of magnitude below the MBH-MBulge and MBH-M* relations-larger than the relations' intrinsic scatters of 0.58 0.09 dex, 0.69 dex, and 0.65 0.09 dex, respectively. These offsets could be due to larger scatter at the low-mass end of these relations. Alternatively, BH growth is intrinsically inefficient in galaxies with low bulge and/or stellar masses, which causes the BHs to be undermassive relative to their hosts, as predicted by some galaxy evolution simulations.