Kinematic interpretation of the centaurus a absorption‐line system

Abstract

The location of the gas responsible for the absorption-line system toward the nucleus of Centaurus A is a puzzle. It is generally accepted that the line features close to the systemic velocity originate in the disk. The redshifted line features in particular, however, are usually thought to be due to gas close to the nucleus or even falling toward it. We present new (CO)-C-12 (1-0), HCO+ (1-0) and HCN (1-0) absorption-line measurements, as well as an alternative interpretation of the line system. Previous papers have demonstrated that the distribution of line emission of the inner molecular and outer atomic hydrogen disk can be accounted for by a system of tilted rings with varying inclination. Using the same model, but assuming that corotating absorbing gas is located at high altitudes above the disk, one can account for all major features of the absorption-line system. In this model, the absorption takes place in high-altitude clouds that are up to about 160 pc above the molecular disk of Centaurus A at radii between 1.7 and 1.9 kpc, accounting for the two strongest central line features. In our model, the redshifted line features are due to gas associated with disk material up to about 300 pc above the disk at radii of 0.4-0.6 kpc orbiting in the non-spherically symmetric potential of the Centaurus A galaxy. In this model, the systemic velocity is at 546 km s(-1), which is the velocity of the sharp spectral feature about 6 km s(-1) to the blue of the deepest absorption line. Our new model provides a natural explanation for the general structure of the complex absorption-line system, based on a tilted-ring model that already explains the disk line emission. In this model, no significant absorbing gas component closer than 200 pc to the nonthermal radio continuum nucleus is required, and the peculiar velocity structure of the absorption-line system is due to the kinematics of the molecular gas disk.