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

Abstract

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 ${\sigma }_{* }={110.7}_{-13.4}^{+12.1}$ 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 M_BH-σ_* relation and roughly one order of magnitude below the 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.