Nuclear Disks of Gas and Dust in Early-Type Galaxies and the Hunt for Massive Black Holes: Hubble Space Telescope Observations of NGC 6251

We discuss Hubble Space Telescope (HST) optical images and spectra of NGC 6251, a giant E2 galaxy and powerful radio source at a distance of 106 Mpc (for H0=70 km s-1 Mpc-1). The galaxy is known to host a very well-defined dust disk; the exceptional resolution of our V and I images allows a detailed study of the disk structure. Furthermore, narrowband images centered on the Hα + [N II] emission lines reveal the presence of ionized gas in the inner 0.″3 of the disk. We used the HST Faint Object Spectrograph with the 0.″09 aperture to study the velocity structure of the disk. Dynamical models were constructed for two extreme (in terms of central concentration) analytical representations of the stellar surface brightness profile, from which the mass density and corresponding rotational velocity are derived assuming a constant mass-to-light ratio (M/L)V~8.5 M☉/L☉. For both representations of the stellar component, the models show that the gas is in Keplerian motion around a central mass ~4-8×108 M☉ and that the contribution of radial flows to the velocity field is negligible.