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 10^4 cm^-3 for a column density N(HCO+)/dV of 3x10^12 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.
