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

Abstract

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 $1.4\times 10^5{M}_{\odot }$, with a mode of $7.2\times 10^4{M}_{\odot }$. Fitting the optical spectrum, we measure a nuclear stellar velocity dispersion $\ddot{I}{ƒ}_{\ast }=110.7_{-13.4}^{+12.1}$~{\rm km~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 $M_{\mathrm{BH}}-\ddot{I}{ƒ}_{\ast }$ relation and roughly one order of magnitude below the $M_{\mathrm{BH}}-M_{\mathrm{Bulge}}$ and $M_{\mathrm{BH}}-M_{\ast }$ 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.

Accepted for publication in ApJ