HIGH-DENSITY AND HIGH-TEMPERATURE CIRCUMNUCLEAR MOLECULAR DISK IN M51

We performed high-resolution (5.''9 x 5.''1) (CO)-C-13(1-0) imaging ob servations of the central region (1.5 kpc in radius) of M51 with the N obeyama Millimeter Array in order to investigate the physical conditio ns of the circumnuclear dense molecular clouds. The obtained (CO)-C-13 image looks very similar to the (CO)-C-12 image in the sense that the emission is distributed along the inner part of the spiral arms, and there is a conspicuous deficiency of emission at the nucleus, in contr ast with the centrally peaked HCN image. The intensity ratio of HCN/(C O)-C-13 obtained in the inner 200 pc is higher than 3 and is almost co mparable to those in the circumnuclear molecular gas in the Seyfert ga laxy NGC 1068 and in the nuclear region of the prototypical starburst galaxy M82. On the other hand, the (CO)-C-12/(CO)-C-13 intensity ratio is similar to 7 +/- 2 in almost all of the central region, with no en hancement in the nuclear region. The obtained HCN/(CO)-C-13 ratio in t he nucleus of M51 was used to derive the physical conditions of the de nse molecular gas using the large velocity gradient approximation. Ass uming the usual abundance ratio and the velocity gradient of HCN and ( CO)-C-13, the ''inverted'' HCN/(CO)-C-13 intensity ratio suggests that the density and the temperature of the circumnuclear molecular gas ar e as high as 10(5) cm(-3) and greater than 100 K, respectively. As pos sible origins for these physical conditions, we discuss the interactio ns between nuclear winds or jets and the interstellar material and com pression of interstellar material by supernova driven winds.