Gas Kinematics Determination of the Black Hole Mass of NGC 4258

ONU McIntosh Center; Activities Room

NGC 4258 is an important galaxy for comparing methods of super- massive black hole mass measurement. Radio (VLBA) observations of water masers in its nuclear disk has allowed a very precise estimate of the mass of the central SMBH (3.9 ± .1 × 10⁷M_⊙), and the distance to the galaxy (7.6 Mpc). Hubble Space Telescope (HST) archival data allow the measurement of the BH mass in two additional, independent ways: stellar kinematics 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 spectra from two programs, for a total of 6 slit positions. We have fitted the H + [NII] and [SII] lines in order to determine radial velocities, velocity dispersions, and emission line strengths as a function of distance from the BH. The thin disk model matches velocity profiles well in all slits to approximately 0.4′′. We use chi-squared functions to measure the fit quality of our models compared to real data, with the best models finding a mass of about 5.6 × 10⁷M_⊙ and a disk inclination of 46^◦. This result is between the aforementioned water maser value and prior gas kinematics work by Pastorini et al. (2007) of 7.9 × 10⁷M_⊙.