Narrowband HST images of M87: Evidence for a disk of ionized gas around a massive black hole

We present HST WFPC2 narrowband Hα + [N II] images of M87 which show a small disk of ionized gas with apparent spiral structure surrounding the nucleus of M87. The jet projects ~19.5^deg^ from the minor axis of the disk, which suggests that the jet is approximately normal to the disk. In a companion Letter, Harms et al. measure radial velocities at r = +/- 0.25" along a line perpendicular to the jet, showing that one side of the disk is approaching at 500 +/- 50 km s^-1^ and the other side of the disk is receding at 500 +/- 50 km s^-1^. Absorption associated with the disk and the sense of rotation imply that the apparent spiral arms trail the rotation. The observed radial velocities corrected for a 42^deg^ inclination of the disk imply rotation at +/-750 km s^-1^. Analysis of velocity measurements at four positions near the nucleus gives a total mass of ~2.4 +/-0.7 x 10^9^ M_sun_ within 18 pc of the nucleus, and a mass-to-light ratio (M/L)_I_ = 170. We conclude that there is a disk of ionized gas feeding a massive black hole in the center of M87.