Shadowing of the Nascent Jet in NGC 4261 by a Line‐emitting Supersonic Accretion Disk

Abstract

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 100 pc scale disk of cool dust and gas surrounding a central, supermassive (~4.9 × 108 M☉) 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, provides 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), not only can account for the observed radio shadowing but can also produce the optical broad lines emitted from this region. At large radii, the gas is optically thin because the ram pressure due to turbulence is much larger than the thermal pressure of the gas. At smaller radii, but beyond a critical radius rc, line cooling dominates over gravitational dissipation and the gas is effectively cooled down to temperatures below 104 K. Within rc, however, heating due to the release of gravitational energy overwhelms line cooling and the gas falls onto the unstable portion of the cooling curve. Because cooling is quite inefficient under these conditions, the plasma is heated very quickly to a temperature close to its virial value as it falls toward the central engine. Thermal pressure of the gas dominates the turbulent ram pressure at a radius ~ rc, below which the flow probably becomes advection dominated. The disk is optically thin to UV and X-ray radiation within rc, so the ionizing radiation from the AGN is preferentially absorbed near rc, 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.