Changing molecular gas properties in the bar and center of NGC 7479

We present sensitive interferometric (CO)-C-12, (CO)-C-13 and HCN observati ons of the barred spiral galaxy NGC 7479, one of the few barred galaxies kn own to have a continuous gas-filled bar. We focus on the investigation and interpretation of (CO)-C-12/(CO)-C-13 line intensity ratios R-12/13, which is facilitated by having more than 90% of the flux in our interferometer ma ps. The global (9 kpc by 2.5 kpc) value of R12/13 is high at 20-40. On smal ler scales (similar to 750 PC), R12/13 is found to vary dramatically, reach ing values > 30 in large parts of the bar, but dropping to values similar t o 5, typical for galactic disks, at a (CO)-C-13 condensation in the souther n part of the bar. We interpret these changes in terms of the relative impo rtance of the contribution of a; diffuse molecular component, characterized by unbound gas that has a moderate optical depth in the (CO)-C-12(1-->0) t ransition. This component dominates the (CO)-C-12 along the bar and is also likely to play an important role in the center of NGC 7479. In the center, the (CO)-C-12 and the HCN intensity peaks coincide, while the (CO)-C-13 pe ak is slightly offset. This can be explained in terms of high gas temperatu re at the (CO)-C-12 peak position. Along the bar, the relation between the distribution of (CO)-C-12, (CO)-C-13, dust lanes and velocity gradient is c omplex. A southern (CO)-C-13 condensation is found offset from the (CO)-C-1 2 ridge that generally coincides with the most prominent dust lanes. It is possible that strong (CO)-C-13 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 vel ocity gradient is traced much more closely by (CO)-C-13 than by (CO)-C-12.