  We present high angular resolution observations of the HCN(1-0) emission (at
~1" or ~34 pc), together with CO J = 1-0, 2-1, and 3-2 observations, toward the
Seyfert 2 nucleus of M51 (NGC 5194). The overall HCN(1-0) distribution and
kinematics are very similar to that of the CO lines, which have been indicated
as the jet-entrained molecular gas in our past observations. In addition, high
HCN(1-0)/CO(1-0) brightness temperature ratio of about unity is observed along
the jets, similar to that observed at the shocked molecular gas in our Galaxy.
These results strongly indicate that both diffuse and dense gases are entrained
by the jets and outflowing from the AGN. The channel map of HCN(1-0) at the
systemic velocity shows a strong emission right at the nucleus, where no
obvious emission has been detected in the CO lines. The HCN(1-0)/CO(1-0)
brightness temperature ratio at this region reaches >2, a value that cannot be
explained considering standard physical/chemical conditions. Based on our
calculations, we suggest infrared pumping and possibly weak HCN masing, but
still requiring an enhanced HCN abundance for the cause of this high ratio.
This suggests the presence of a compact dense obscuring molecular gas in front
of the nucleus of M51, which remains unresolved at our ~1" (~34 pc) resolution,
and consistent with the Seyfert 2 classification picture.

Astrophysics of Galaxies