  NGC 4261 (3C 270) is a low-luminosity radio galaxy with two symmetric
kiloparsec-scale jets. Earlier Hubble Space Telescope observations indicated
the presence of a hundred-parsec scale disk of cool dust and gas surrounding a
central, supermassive ($\sim 4.9\times 10^8\msun$) black hole. The recent
detection of free-free radio absorption by a small, geometrically-thin disk,
combined with earlier studies of the disk's large scale properties, provide the
strictest constraints to date on the nature of the accretion process in this
system. We show here that a supersonic disk, illuminated by the active galactic
nucleus (AGN), can not only account for the observed radio shadowing, but can
also produce the optical broad lines emitted from this region. Beyond a
critical radius $r_c$, line cooling dominates over gravitational dissipation
and the gas is effectively cooled down to temperatures below $10^4$ K. Within
$r_c$, however, heating due to the release of gravitational energy overwhelms
line cooling and the plasma is heated very quickly to a temperature close to
its virial value as it falls toward the central engine. The disk is
optically-thin to UV and X-ray radiation within $r_c$, so the ionizing
radiation from the AGN is preferentially absorbed near $r_c$, affecting the
disk structure significantly. To include the ensuing photoionization effect, we
have used the algorithm Cloudy with additional heating introduced by
gravitational dissipation to calculate the temperature profile and line
emission from the disk in a self-consistent manner. The results of our model
calculation are consistent with current multiwavelength observations of the
disk in this source.
