THE SUPERMASSIVE BLACK HOLE IN M84 REVISITED

The mass of the central black hole in the giant elliptical galaxy M84 has previously been measured by two groups using the same observations of emission-line gas with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope, giving strongly discrepant results: Bower et al. found M-BH = (1.5(-0.6)(+1.1)) x 10(9) M-circle dot, while Maciejewski & Binney estimated M-BH = 4 x 10(8) M-circle dot. In order to resolve this discrepancy, we have performed new measurements of the gas kinematics in M84 from the same archival data and carried out comprehensive gas-dynamical modeling for the emission-line disk within similar to 70 pc from the nucleus. In comparison with the two previous studies of M84, our analysis includes a more complete treatment of the propagation of emission-line profiles through the telescope and STIS optics, as well as inclusion of the effects of an intrinsic velocity dispersion in the emission-line disk. We find that an intrinsic velocity dispersion is needed in order to match the observed line widths, and we calculate gas-dynamical models both with and without a correction for asymmetric drift. Including the effect of asymmetric drift improves the model fit to the observed velocity field. Our best-fitting model with asymmetric drift gives M-BH = (8.5(-0.8)(+0.9)) x 10(8) M-circle dot (68% confidence). This is a factor of similar to 2 smaller than the mass often adopted in studies of the M-BH-sigma(star) and M-BH-L relationships. Our result provides a firmer basis for the inclusion of M84 in the correlations between black hole mass and host galaxy properties.