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 10(4) cm(-3) for a column density N(HCO+)/Delta V of 3 x 10(12) 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 10(4) 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.