Enhanced HCN (1-0) emission in the type-1 Seyfert galaxy NGC 1097

The central kpc region of the low-luminosity type-1 Seyfert galaxy NGC 1097 has been observed in the CO (1-0) and HCN (1-0) lines with the Nobeyama Millimeter Array and the NRO 45 m telescope. We have found a striking enhancement of the HCN emission toward the active nucleus of NGC 1097; a large fraction of the CO emission comes from a circumnuclear starburst ring (r similar to 10 or 700 pc at D = 14.5 Mpc), whereas the HCN emission is dominated by a strong unresolved peak at the nucleus. The HCN-to-CO integrated intensity ratio on the brightness temperature scale, R-HCN/CO, is about 0.11 in the aperture of the 45 m telescope beams (15 for CO and 19 for HCN), but reaches 0.34 in the smaller aperture (10 x 4.4). These CO and HCN properties in NGC 1097 are similar to those in the type-2 Seyfert galaxies NGC 1068 and NGC 5194 (M 5 1).