Dense Molecular Gas and Star Formation in Nearby Seyfert Galaxies

An imaging survey of CO(1-0), HCN(1-0), and HCO${}^{+}$(1-0) lines in the centers of nearby Seyfert galaxies has been conducted using the Nobeyama Millimeter Array and the RAINBOW interferometer. Preliminary results reveal that 3 Seyferts out of 7 show abnormally high HCN/CO and HCN/HCO${}^{+}$ ratios, which cannot occur even in nuclear starburst galaxies. We suggest that the enhanced HCN emission originated from X-ray irradiated dense obscuring tori, and that these molecular line ratios can be a new diagnostic tool to search for pure'' AGNs. According to our HCN diagram, we suggest that NGC 1068, NGC 1097, and NGC 5194 host pure’’ AGNs, whereas Seyfert nuclei of NGC 3079, NGC 6764, and NGC 7469 may be ``composite’’ in nature.