Dense molecular gas associated with the circumnuclear star-forming ring inthe barred spiral galaxy NGC 6951

We present high-resolution (3 "-5 ") observations of CO (1-0) and HCN (1-0) emission from the circumnuclear star-forming ring in the barred spiral gal axy NGC 6951, a host of a type 2 Seyfert nucleus, using the Nobeyama Millim eter Array and the Nobeyama 45 m telescope. We find that the distribution o f the HCN emission is different from that of CO in the circumnuclear region of NGC 6951; it is confirmed that CO emission is dominated by "twin peaks" morphology with two spiral arms, which are connected to the dust lanes, as reported by Kenney et al. On the other hand, although the HCN emission als o shows a twin peaks morphology, the HCN peaks are spatially shifted downst ream compared with the CO peaks. Most of the HCN emission is associated wit h the circumnuclear ring, where vigorous star formation occurs. The HCN to CO integrated intensity ratio in the brightness temperature is also enhance d in the star-forming ring. The peak value of the R-HCN/CO is about 0.16-sc ale, R-HCN/CO 0.18, which Is comparable to the R-HCN/CO in the starburst re gions of NGC 253 and M82. Consequently, the HCN emission spatially correlat es better with the massive star-forming regions than the CO emission in the circumnuclear region of NGC 6951. The formation mechanism of dense molecul ar gas has been investigated next. No significant enhancement of R-HCN/CO i s observed at the CO peaks, which are interpreted as x(1)/x(2) orbit-crowdi ng regions. This suggests that the shocks along the orbit crowding do not p romote the formation of dense molecular gas effectively but enhance the pre sence of low-density giant molecular clouds in NGC 6951. Instead, gravitati onal instability can account for the dense molecular gas formation in the c ircumnuclear star-forming ring because Toomre's Q value is below unity ther e. The R-HCN/CO toward the type 2 Seyfert nucleus of NGC 6951 is 0.086 aver aged over central r < 120 pc region. This is a rather normal value compared with nonactive galaxies such as the Milky Way and quite different from oth er type 2 Seyfert galaxies NGC 1068 and M51 where extremely high R-HCN/CO v alues, similar to 0.5, have been reported. The variety of R-HCN/CO values i n these Seyfert nuclei would be attributed to the different physical condit ions of the molecular gas around the nuclei.