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The S0 galaxy NGC 3245 contains a circumnuclear disk of ionized gas and dustwith a radius of 1.1" (110 pc), making it an ideal target for dynamical studieswith the Hubble Space Telescope. We have obtained spectra of the nuclear diskwith the Space Telescope Imaging Spectrograph, using an 0.2"-wide slit at fiveparallel positions. Measurements of the H-alpha and [N II] emission lines areused to map out the kinematic structure of the disk in unprecedented detail.The data reveal a rotational velocity field with a steep velocity gradientacross the innermost 0.4". We construct dynamical models for a thin gas disk incircular rotation, using HST optical images to map out the gravitationalpotential due to stars. The H-alpha+[N II] surface brightness measured from anHST narrow-band image is folded into the models, and we demonstrate that manyof the apparent small-scale irregularities in the observed velocity curves arethe result of the patchy distribution of emission-line surface brightness. Acentral dark mass of (2.1+/-0.5)x10^8 solar masses is required for the modelsto reproduce the steep central velocity gradient. This value for the centralmass is consistent with recently discovered correlations between black-holemass and bulge velocity dispersion.Comment: Revised version, accepted for publication in ApJ. 53 pages, including  20 figures and 2 table

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