Dense molecular gas around AGN: HCN/CO in NGC 3227

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 NGC 3227, 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 140 pc and an inner nuclear region within 40 pc 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 density, and velocity gradient) that yield physically plausible solutions for the observed flux ratio in the central 100 pc. The observed HCN/CO intensity ratio in the nucleus is consistent with very optically thick thermalized emission in very dense (≈ 10 ⁵ cm ^-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 emission but from gas with a very high (∼10 ^-2) HCN/CO abundance ratio. This possibility is more consistent with the CO and HCN emission seen in the nuclei of the Seyfert galaxies NGC 1068 and NGC 6951. 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 20 pc in the centre of NGC 3227 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.