2000A&A...363...93H

We present sensitive interferometric ¹²CO, ¹³CO and HCN observations of the barred spiral galaxy NGC 7479, one of the few barred galaxies known to have a continuous gas-filled bar. We focus on the investigation and interpretation of ¹²CO/¹³CO line intensity ratios R_12/13, which is facilitated by having more than 90% of the flux in our interferometer maps. The global (9kpc by 2.5kpc) value of R_12/13 is high at 20-40. On smaller scales (∼750pc), R_12/13 is found to vary dramatically, reaching values >30 in large parts of the bar, but dropping to values ∼5, typical for galactic disks, at a ¹³CO condensation in the southern part of the bar. We interpret these changes in terms of the relative importance of the contribution of a diffuse molecular component, characterized by unbound gas that has a moderate optical depth in the ¹²CO(1-0) transition. This component dominates the ¹²CO along the bar and is also likely to play an important role in the center of NGC 7479. In the center, the ¹²CO and the HCN intensity peaks coincide, while the ¹³CO peak is slightly offset. This can be explained in terms of high gas temperature at the ¹²CO peak position. Along the bar, the relation between the distribution of ¹²CO, ¹³CO, dust lanes and velocity gradient is complex. A southern ¹³CO condensation is found offset from the ¹²CO ridge that generally coincides with the most prominent dust lanes. It is possible that strong ¹³CO detections along the bar indicate quiescent conditions, downstream from the major bar shock. Still, these condensations are found close to high velocity gradients. In the central region, the velocity gradient is traced much more closely by ¹³CO than by ¹²CO.