  Due to its proximity, the mass of the supermassive black hole in the nucleus
of Andromeda galaxy (M31), the most massive black hole in the Local Group of
galaxies, has been measured by several methods involving the kinematics of a
stellar disk that surrounds it. We report here the discovery of an eccentric
Halpha emitting disk around the black hole at the center of M31 and show how
modeling this disk can provide an independent determination of the mass of the
black hole. Our model implies a mass of 5.0_{-1.0}^{+0.8} x 10^7 Mo for the
central black hole, consistent with the average of determinations by methods
involving stellar dynamics, and compatible (at 1-sigma level) with measurements
obtained from the most detailed models of the stellar disk around the central
black hole. This value is also consistent with the M-sigma relation. In order
to make a comparison, we applied our simulation on the stellar kinematics in
the nucleus of M31 and concluded that the parameters obtained for the stellar
disk are not formally compatible with the parameters obtained for the Halpha
emitting disk. This result suggests that the stellar and the Halpha emitting
disks are intrinsically different from each other. A plausible explanation is
that the Halpha emission is associated with a gaseous disk. This hypothesis is
supported by the detection of traces of weaker nebular lines in the nuclear
region of M31. However, we cannot exclude the possibility that the Halpha
emission is, at least partially, generated by stars.
