The black hole mass in NGC 4258 from gas kinematics

NGC 4258 (or M106) 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 ($3.82\pm.01\times 10^7 M_\odot$), and the distance to the galaxy (7.2 Mpc). Hubble Space Telescope (HST) archival data allow the measurement of the BH mass in two additional, independent ways: stellar and gas kinematics, thus providing a crucial test of these more widely-used methods. Here we report on progress in a re-analysis of the archival data allowing gas kinematics. These data consist of HST 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 distance from the BH. The gas only shows organized rotation out to $\sim$0.4$''$. The H$\alpha$ emission shows a broad-line (BL) component from the central AGN, and regions outside of the BL region show greater line widths than expected for a kinematically ``cold'' gas disk. We report initial results of modeling the kinematics as resulting from a thin, inclined disk of line-emitting gas orbiting under the influence of gravity only.