DISENTANGLING THE CIRCUMNUCLEAR ENVIRONS OF CENTAURUS A. II. ON THE NATURE OF THE BROAD ABSORPTION LINE

We report on atomic gas (Hi) and molecular gas (as traced by CO(2-1)) redshifted absorption features toward the nuclear regions of the closest powerful radio galaxy, Centaurus A (NGC 5128). Our Hi observations using the Very Long Baseline Array allow us to discern with unprecedented sub-parsec resolution Hi absorption profiles toward different positions along the 21 cm continuum jet in the inner 0 ''.3 (or 5.4 pc). In addition, our CO(2-1) data obtained with the Submillimeter Array probe the bulk of the absorbing molecular gas with little contamination by emission, which was not possible with previous CO single-dish observations. We shed light on the physical properties of the gas in the line of sight with these data, emphasizing the still open debate about the nature of the gas that produces the broad absorption line (similar to 55 km s(-1)). First, the broad Hi line is more prominent toward the central and brightest 21 cm continuum component than toward a region along the jet at a distance similar to 20 mas (or 0.4 pc) further from the nucleus. This indicates that the broad absorption line arises from gas located close to the nucleus, rather than from diffuse and more distant gas. Second, the different velocity components detected in the CO(2-1) absorption spectrum match well with other molecular lines, such as those of HCO+(1-0), except the broad absorption line that is detected in HCO+(1-0) (and most likely related to that of the HI). Dissociation of molecular hydrogen due to the active galactic nucleus seems to be efficient at distances r less than or similar to 10 pc, which might contribute to the depth of the broad Hi and molecular lines.