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

Abstract

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 < 200 pc) where CO emission is depressed. The obtained 4'' resolution HCN maps suggest the existence of a rotating dense molecular disk (the nuclear disk) whose rotation axis (P.A. ~ 160°) is just aligned to the direction of the nuclear jet (P.A. ~ 165°), not to the rotation axis of the galactic plane. The radius of the nuclear disk is estimated to be 70 pc, and the rotation velocity of the nuclear disk is 16(sin i)^-1 km s^-1 at r = 70 pc, where i is the inclination angle of the nuclear disk. The HCN/CO integrated intensity ratio exceeds 0.3 inside r = 140 pc, which is a factor of 10 larger than those of the spiral arm 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 (n_{H2 ~ 10⁵ cm^-3). The column density of molecular hydrogen toward the nucleus was estimated to be NH2 > 1.5 × 10²³ 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.}