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 similar to 0.2 pc in diameter, coincident with the compact nuclear X-ray source that lies at the base of the similar to 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 x 10(5) M-circle dot. Keplerian rotation curves fitted to these data yield enclosed masses between 4.1x10(4)M(circle dot) and 1.4x10(5)M(circle dot), with a mode of 7.2x10(4) M-circle dot. Fitting the optical spectrum, we measure a nuclear stellar velocity dispersion sigma(*) = 110.7 -(12.1)(13.4)s(-1). From near-infrared photometry, we fit a bulge mass of (7.3 +/- 2.7) x 10(8) M-circle dot and a stellar mass of 1.4 x 10(10)M(circle dot). The mass upper limit of the intermediate-mass BH in IC 750 falls roughly two orders of magnitude below the M-BH-=M-* relation M-BH-M-Bulge and M-BH-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.