  We present sensitive interferometric 12CO, 13CO and HCN observations of the
barred spiral galaxy NGC7479, known to be one of the few barred galaxies with a
continuous gas-filled bar. We focus on the investigation and interpretation of
12CO/13CO line intensity ratios, which is facilitated by having more than 90%
of the flux in our interferometer maps. The global (9kpc by 2.5kpc) value of
the 12CO/13CO ratio is high at 20 - 40. On smaller scales (~ 750 pc), the ratio
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 NGC7479. 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 and density 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.
