Submillimeter Array 12CO (J = 3-2) Interferometric Observations of the Central Region of M51

We present the first interferometric 12CO (J = 3-2) observations (beam size of 3.″9 × 1.″6 or 160 × 65 pc) with the Submillimeter Array (SMA) toward the center of the Seyfert 2 galaxy M51. The image shows a strong concentration at the nucleus and weak emission from the spiral arm to the northwest. The integrated intensity of the central component in 12CO (J = 3-2) is almost twice as high as that in 12CO (J = 1-0), indicating that the molecular gas within an ~80 pc radius of the nucleus is warm (≳100 K) and dense (~104 cm-3). Similar intensity ratios are seen in shocked regions in our Galaxy, suggesting that these gas properties may be related to active galactic nucleus or starburst activity. The central component shows a linear velocity gradient (~1.4 km s-1 pc-1) perpendicular to the radio continuum jet, similar to that seen in previous observations and interpreted as a circumnuclear molecular disk/torus around the Seyfert 2 nucleus. In addition, we identify a linear velocity gradient (0.7 km s-1 pc-1) along the jet. Judging from the energetics, the velocity gradient can be explained by supernova explosions or energy and momentum transfer from the jet to the molecular gas via interaction, which is consistent with the high-intensity ratio.