The Black Hole Mass in NGC 4258 from Gas Kinematics

NGC 4258 is an important galaxy for the demographics of supermassive black holes (SMBH). Radio (VLBA) observations of its nuclear disk of water masers has allowed a very precise estimate of the mass of the central SMBH (3.9$\pm0.1\times 10^7 M_{\odot}$), and the distance to the galaxy (7.2 Mpc). Hubble Space Telescope (HST) archival data allow the measurement of the BH mass in two independent ways: stellar and gas kinematics, providing a crucial test of these methods. Here we report on an analysis of the archival data which allow gas kinematics: STIS long-slit spectroscopy from two programs, a total of 6 slit positions. We have fitted the H$\alpha$ + [NII] and [SII] lines in order to determine radial velocities and velocity dispersions as a function of position along the slits. We use primarily the velocities to constrain our thin, inclined, gas disk models. We see evidence (in modeling and narrowband imaging) for a nuclear gas disk extending out to about 0.4'' (14 pc) which is greatly inclined relative to the galaxy's spiral disk. It may be unrelated to the maser disk (r$\sim$0.3 pc), given that it's major axis and inclination each differ from the maser disk by about 25 degrees. Our best models show a BH mass of 3-8$\times 10^7 M_{\odot}$, consistent with maser results.