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 ∼ 1 ′′ of the ionized gas disk in [O II]λ3727 to a distance as close as 0. ′′ 07 (≃ 5pc) 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 -2maximum radius is 0. ′′ 05 (≃3.5pc) 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)×10 9 M ⊙ for its mass.Subject headings: Galaxies -individual (M87); Galaxies -Seyfert; Galaxies -active; Black -Holes * Positions are nominally the same as the preceding ones but there was a small misplacement during observations.

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