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 of the order of a few 104 cm−3 for a column density N(HCO+)/ΔV of 3 × 1012 cm−2 km−1 s and a kinetic temperature of 10 K. In particular, we find no evidence for molecular gas density higher than a few 104 cm−3 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.