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 1.4×105Me. Keplerian rotation curves fitted to these data yield enclosed masses between 4.1×104Me and 1.4×105Me, with a mode of 7.2×104Me. Fitting the optical spectrum, we measure a nuclear stellar velocity dispersion s =-110.7+13.4 12.1∗kms-1. From near-infrared photometry, we fit a bulge mass of (7.3±2.7)×108Me and a stellar mass of 1.4×1010Me. 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.