The supermassive black hole of M87 and the kinematics of its associated gaseous disk

We have obtained long-slit observations of the circumnuclear region of M87 at three different locations, with a spatial sampling of 0''.028 using the Faint Object Camera f/48 spectrograph on board HST. These data allow us to determine the rotation curve of the inner similar to 1'' of the ionized gas disk in CO rrl lambda 3727 to a distance as close as 0''.07 (similar or equal to 5 pc) to the dynamic center, thereby significantly improving on both the spatial resolution and coverage of previous FOS observations. We have modeled the kinematics of the gas under the assumption of the existence of both a central black hole and an extended central mass distribution, taking into account the effects of the instrumental PSF, the intrinsic luminosity distribution of the line, and the finite size of the slit. We find that the central mass must be concentrated within a sphere whose maximum radius is 0''.05 (similar or equal to 3.5 pc) and show that both the observed rotation curve and line profiles are consistent with a thin disk in Keplerian motion. We conclude that the most likely explanation for the observed motions is the presence of a supermassive black hole and derive a value of M-BH = (3.2 +/- 0.9) x 10(9) M-. for its mass.