1999ApJ...511..157K

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 galaxy NGC 6951, a host of a type 2 Seyfert nucleus, using the Nobeyama Millimeter Array and the Nobeyama 45 m telescope. We find that the distribution of 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 also shows a twin peaks morphology, the HCN peaks are spatially shifted downstream compared with the CO peaks. Most of the HCN emission is associated with the circumnuclear ring, where vigorous star formation occurs. The HCN to CO integrated intensity ratio in the brightness temperature scale, R_HCN/CO, is also enhanced in the star-forming ring. The peak value of the R_HCN/CO is about 0.16-0.18, which is comparable to the R_HCN/CO in the starburst regions of NGC 253 and M82. Consequently, the HCN emission spatially correlates better with the massive star-forming regions than the CO emission in the circumnuclear region of NGC 6951. The formation mechanism of dense molecular gas has been investigated next. No significant enhancement of R_HCN/CO is observed at the CO peaks, which are interpreted as x₁/x₂ orbit-crowding regions. This suggests that the shocks along the orbit crowding do not promote the formation of dense molecular gas effectively but enhance the presence of low-density giant molecular clouds in NGC 6951. Instead, gravitational instability can account for the dense molecular gas formation in the circumnuclear star-forming ring because Toomre's Q value is below unity there. The R_HCN/CO toward the type 2 Seyfert nucleus of NGC 6951 is 0.086 averaged 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 other type 2 Seyfert galaxies NGC 1068 and M51 where extremely high R_HCN/CO values, ∼0.5, have been reported. The variety of R_HCN/CO values in these Seyfert nuclei would be attributed to the different physical conditions of the molecular gas around the nuclei.