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L. Ferrarese (UCLA)
As recently as five years ago, the chaotic motions of stars in the dense cores of early-type galaxies was the only key to unlocking the properties of the gravitational field and the distribution of mass in the inner few parsecs. A powerful alternative is now offered by the study of the regular velocity fields of small disks of dust and gas (typically a few hundred parsecs in diameter) discovered in the nuclei of an increasing number of early-type galaxies. The first such disk was discovered in the radio galaxy NGC 4261 thanks to the high resolving power of the HST/WFPC, and it is discussed extensively in Ferrarese, Ford & Jaffe (1996). Detailed modeling of HST/FOS data shows that the flattened and regular disks are dominated by circular motion in the central potential of the galaxy (e.g. Ferrarese & Ford 1999), and that gas flows (which are a concern in less organized structures, Fillmore, Boronson & Dressler 1986) play only a secondary role. Observationally, the velocity field in the disk is easily derived from the bright emission lines; theoretically, the two dimensional models do not require complicated projections of the orbital velocities (which present a major obstacle in the study of the stellar dynamics). More importantly, they avoid ambiguities associated with the presence of velocity anisotropy, which have so far seriously undermined determinations of central masses from stellar dynamical studies (e.g. Gerhard 1993, van der Marel 1994, Dehnen 1995). Measurements of black hole masses based on gas dynamics are reviewed.