Kinematic interpretation of the Centaurus A absorption-line system

The location of the gas responsible for the absorption-line system toward t he nucleus of Centaurus A is a puzzle. It is generally accepted that the li ne features close to the systemic velocity originate in the disk. The redsh ifted line features in particular, however, are usually thought to be due t o 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 a s an alternative interpretation of the line system. Previous papers have de monstrated that the distribution of line emission of the inner molecular an d outer atomic hydrogen disk can be accounted for by a system of tilted rin gs with varying inclination. Using the same model, but assuming that corota ting absorbing gas is located at high altitudes above the disk, one can acc ount for all major features of the absorption-line system. In this model, t he 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 t o about 300 pc above the disk at radii of 0.4-0.6 kpc orbiting in the non-s pherically symmetric potential of the Centaurus A galaxy. In this model, th e 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 lin e. Our new model provides a natural explanation for the general structure o f the complex absorption-line system, based on a tilted-ring model that alr eady explains the disk line emission. In this model, no significant absorbi ng gas component closer than 200 pc to the nonthermal radio continuum nucle us is required, and the peculiar velocity structure of the absorption-line system is due to the kinematics of the molecular gas disk.