Dense Molecular Gas around AGN: HCN/CO in NGC3227
  There is now convincing evidence that the intensity of HCN molecular line
emission is enhanced around active galactic nuclei. In this paper we examine
the specific case of the Seyfert galaxy NGC3227, for which there are
subarcsecond resolution data for the HCN (1-0) 88 GHz and CO (2-1) 230 GHz
rotational lines, enabling us to spatially separate a circumnuclear ring at a
radius of 140pc and an inner nuclear region within 40pc of the AGN. The
HCN(1-0)/CO(2-1) flux ratio differs by more than an order of magnitude between
these two regions. We carry out large velocity gradient (LVG) computations to
determine the range of parameters (gas temperature and density, HCN/CO
abundance ratio, column densities and velocity gradients) that yield physically
plausible solutions for the observed flux ratio in the central 100pc. The
observed HCN/CO intensity ratio in the nucleus is consistent with very
optically thick thermalized emission in very dense (>=10^5cm^{-3}) gas, in
which case the HCN/CO abundance ratio there is unconstrained. Alternatively,
the HCN/CO intensity ratio could be due to optically thinner emissions but with
very high (~10^{-2}) HCN/CO abundance ratios. This possibility is more
consistent with the CO and HCN emissions seen in the nuclei of the Seyfert
galaxies NGC1068 and NGC6951. It would imply the velocity gradients are large
and the clouds may be gravitationally unbound. We estimate that the X-ray
ionisation rate at radii less than 20pc in the centre of NGC3227 exceeds
10^{-13}s^{-1}. X-ray ionisation and heating may lead to high HCN/CO ratios in
warm gas in a high-ionisation molecular phase near the AGN.
