2004ApJ...616L..55M

We present the first interferometric ¹²CO (J=3-2) observations (beam size of 3".9x1".6 or 160x65 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/cm3). 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/pc) 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/pc) 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.