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" (or 110 pc), making it an ideal target for dynamical studies with the Hubble Space Telescope (HST). We have obtained HST spectra of the nuclear disk, using an 0.2"-wide slit at five parallel positions. Measurements of the H-alpha and [NII] emission lines are used to map out the kinematic structure of the disk in unprecedented detail. 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. Our modeling code includes the blurring due to the telescope point-spread function and the nonzero slit width, as well as the instrumental shift in measured wavelength for light entering the slit off-center, so as to simulate the data as closely as possible. Over most of the disk, the models are able to fit the observed rotation curves closely, although there are localized regions within the disk which appear to be kinematically disturbed relative to the overall rotational pattern. The velocity dispersion of [NII] 6584 rises from 50 km/s in the outer disk to 160 km/s at the nucleus, and most of this linewidth cannot be attributed to rotational or instrumental broadening. To account for the possible dynamical effect of the intrinsic velocity dispersion in the gas, we also calculate models which include a correction for asymmetric drift. A central dark mass of 2.1(+0.5/-0.4)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.