  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.
