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

Abstract

We present sensitive interferometric 12/supCO, 13/supCO 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 12/supCO/13/supCO line intensity ratios , which is facilitated by having more than 90% of the flux in our interferometer maps. The global (9 kpc by 2.5 kpc) value of is high at 20-40. On smaller scales ( pc), is found to vary dramatically, reaching values in large parts of the bar, but dropping to values , typical for galactic disks, at a 13/supCO 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 12/supCO(10) 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 12/supCO and the HCN intensity peaks coincide, while the 13/supCO peak is slightly offset. This can be explained in terms of high gas temperature at the 12/supCO peak position. Along the bar, the relation between the distribution of 12/supCO, 13/supCO, dust lanes and velocity gradient is complex. A southern 13/supCO condensation is found offset from the 12/supCO ridge that generally coincides with the most prominent dust lanes. It is possible that strong 13/supCO 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 13/supCO than by 12/supCO.