High-density and high-temperature molecular gas around the AGN in M51

Abstract

We investigated the physical properties of molecular gas in the nuclear region of M51 (Seyfert 2). We obtained an aperture synthesis 13CO(J = 1 − 0) image using the Nobeyama Millimeter Array (NMA), and compared it with NMA 12CO(J = 1 − 0) and HCN(J = 1 − 0) maps at similar spatial resolutions. Within a radius of 180 pc from the center, the 13CO(1 − 0) integrated intensity was found to be 3 times weaker than that of HCN(1 − 0). Large-Velocity-Gradient (LVG) calculations suggest that the observed high HCN(1 − 0)/ 13CO(1 − 0) intensity ratio would arise from dense ( n H 2 ∼ 10 5 cm −3) and hot ( T kin ≳ 300 K) molecular clouds in the nuclear molecular disk. We also observed in the 12CO(1 − 0), (3 − 2), 13CO(1 − 0), and (3 − 2) lines using the Nobeyama 45m and JCMT 15m telescopes. We detected weak 13CO lines as well as strong 12CO lines. The LVG calculations assuming a two-component model suggest that there is a large amount of low-density ( n H 2 ∼ 3 − 6 × 10 2 cm −3), low-temperature ( T kin ∼ 20 – 50 K) gas, and a small amount of high-density ( n H 2 ≳ 10 4 cm −3), high-temperature ( T kin ≳ 500 K) gas. The existence of the high-density and high-temperature component, although having a quite small beam filling factor, supports the aperture synthesis observation results mentioned above. Since this dense, hot gas is located in the nuclear molecular disk around the Active Galactic Nucleus (AGN), it may be heated by the strong X-ray radiation and/or by the shock induced by the radio jet.