Evidence for a Supermassive Black Hole in the S0 Galaxy NGC 3245

The S0 galaxy NGC 3245 contains a circumnuclear disk of ionized gas and dust with a radius of 1.1" (110 pc), making it an ideal target for dynamical studies with the Hubble Space Telescope. We have obtained spectra of the nuclear disk with the Space Telescope Imaging Spectrograph, using an 0.2"-wide slit at five parallel positions. Measurements of the H-alpha and [N II] emission lines are used to map out the kinematic structure of the disk in unprecedented detail. The data reveal a rotational velocity field with a steep velocity gradient across the innermost 0.4". We construct dynamical models for a thin gas disk in circular rotation, using HST optical images to map out the gravitational potential due to stars. The H-alpha+[N II] surface brightness measured from an HST narrow-band image is folded into the models, and we demonstrate that many of the apparent small-scale irregularities in the observed velocity curves are the result of the patchy distribution of emission-line surface brightness. A central dark mass of (2.1+/-0.5)x10^8 solar masses is required for the models to reproduce the steep central velocity gradient. This value for the central mass is consistent with recently discovered correlations between black-hole mass and bulge velocity dispersion.