Hco and Hcn J=3-2 Absorption toward the Center of Centaurus A

We have investigated the presence of dense gas toward the radio source Cen A by looking at the absorption of the HCO and HCN (3-2) lines in front of the bright continuum source with the Submillimeter Array. We detect narrow HCO (3-2) absorption, and tentatively HCN (3-2), close to the systemic velocity. For both molecules, the J = 3 − 2 absorption is much weaker than for the J = 1− 0 line. From simple excitation analysis, we conclude that the gas density is on the order of a few 10 cm for a column density N(HCO)/∆V of 3×10 cm km s and a kinetic temperature of 10 K. In particular, we find no evidence for molecular gas density higher than a few 10 cm on the line of sight to the continuum source. We discuss the implications of our finding on the nature of the molecular gas responsible for the absorption toward Cen A. Subject headings: galaxies: individual (NGC5128, Cen A) — galaxies: ISM — ISM: molecules — radio lines: galaxies