ar X iv : a st ro - p h / 98 01 18 0 v 1 1 9 Ja n 19 98 The Nuclear Disk in M 87 : A Review

The disk in the center of M87 is a prototype for gas orbiting a massive central object. Three sets of HST+COSTAR FOS and FOC observations provide strong evidence that the nuclear disk in M87 is in Keplerian rotation around a black hole with a mass of (2 − 3) × 10 9 M⊙. A deep (6 orbits), high resolution Hα+[N ii] PC2 HST image shows a trailing, three arm spiral superposed on the underlying nuclear disk. Several of the extended filaments appear to connect directly to the disk. The filaments extending to the NW appear to be twisted, as in NGC 4258. Earlier arguments that the NW filaments are flowing from the nucleus are supported by the presence of blue shifted non-Keplerian components within 20 pc of the nucleus. The gas in the blue and red shifted non-Keplerian components has negative energy and will fall back into the nucleus. The morphological and kinematical observations can be explained by assuming that the filaments originate in a bidirectional wind emanating from the disk. Such a wind will carry away angular momentum, enabling gas in the disk to move toward the black hole. Small (r ∼ 1 ′′ ; r ∼ 100 − 200 pc), well-defined dusty (D-type) and ionized (I-type) " nuclear " disks are common in elliptical galaxies. We suggest that the size of the black hole's radius of influence RBH relative to the radius of the nuclear disk R disk determines whether the disk will be a D-type or I-type. I-type disks (M87 and M81) occur when RBH ≥ R disk. Differential Keplerian rotation throughout the disk may then generate turbulence and shocks that ionize the gas. D-type disks (NGC 4261 and NGC 6251) occur when RBH ≪ R disk. The regions of a disk that are exterior to RBH will rotate at approximately constant angular velocity in the galaxy's stellar potential. In the absence of differential rotation, shocks will be suppressed, and the gas will remain cold and dusty. Intermediate D/I types (3C264) may occur when RBH is a significant fraction of the disk's radius. Comparison of RBH with the sizes of the ionized regions in M87, NGC 4261, and NGC 6251 supports these suggestions.