Aperture Synthesis CO and HCN Observations of M51: Dense Molecular Disk around a Low-Luminosity Active Galactic Nucleus

We performed aperture synthesis CO (J = 1 ? 0) and HCN (J = 1 ? 0) observations of M51 using the Nobeyama Millimeter Array with spatial resolutions of 4'' (corresponding to 190 pc at D = 9.6 Mpc) and 6'', respectively. Higher resolution (4'') HCN images were also made to resolve the structure of the circumnuclear region. The CO map shows clumpy structures that are mainly distributed along a pair of inner spiral arms. In contrast, intense HCN emission was detected in the central region (r 300 pc). The high HCN/CO ratio means that most of the molecular gas is dense enough to excite HCN (J = 1 ? 0) emission (nH2 ~ 105 cm-3). The column density of molecular hydrogen toward the nucleus was estimated to be NH2 > 1.5 ? 10 -->23 cm -->?2, which plausibly accounts for the large amount of material absorbing X-rays from the nucleus. These results suggest that the 50-100 pc scale molecular disk would play an important role for the obscuration of a weak Seyfert nucleus in M51.