Gas Disks and Supermassive Black Holes in Nearby Radio Galaxies

most reliable is, of course, by the measurement of the proper motions and accelerations
of the stars in the closest orbits to the black hole. This has now been achieved in
the nucleus of our own galaxy (Ghez et al., 1998; Melia & Falcke, 2001; Reid et al.,
2003; Ghez et al., 2003), though this will be the only case where we can make these
measurements. Kinematics of water masers in the inner nuclei also provide what
appear to be very robust measurements of the black hole mass (Greenhill & Gwinn,
1997; Herrnstein et al., 1999), though cases with suitable maser organization and
orientation prove hard to find. For larger samples, one must use dynamical modeling
either of the stars or of the gas in the nuclear region of the galaxy.
A black hole of mass M• dominates the gravitational potential inside an angular
‘radius of influence’ given by
θ• ∼ 0. ′′ 1
� M•
10 6 M⊙
� � �
−1 2 �1Mpc �
100km s
σ
D
11
(1.1)
Where θ• is the angular size projected on the sky of the radius of influence of
the black hole, σ is a typical velocity dispersion of stars in the galaxy and D is the
distance to the galaxy in Mpc. For typical nearby galaxies, this radius of influence
will be less than an arcsecond, so that the Hubble Space Telescope is required to make
the necessary observations.
The mass of the black hole may be sought by modeling the observed line of sight