Astron. Astrophys. 363, 93-107 (2000)

5. Conclusions

We have mapped the barred spiral galaxy interferometrically in the transitions of ¹²CO, ¹³CO and HCN. Our main results are as follows:

1. The ¹²CO map shows a continuous gas distribution all allong the bar. Comparison to single dish observation shows that the interferometer does not miss a significant amount of flux. The velocity field derived from the ¹²CO map is complex, showing the S-shaped isovelocity contours typical of noncircular gas orbits in a strong bar.

2. A high velocity feature is identified close to the center. This may be a ring associated with an Inner Lindblad resonance, a tilted rotating disk fed directly by mass infall along the bar or even an inner bar decoupled from the main bar.

3. ¹³CO emission is detected in the central condensation, the southern part of the bar and a single location in the northern part of the bar. HCN emission is only detected from the center. The ¹³CO central emission is offset by from the ¹²CO and HCN intensity maxima, which are coincident. Along the bar, the most prominent peak of southern ¹³CO condensation is also clearly offset from the ¹²CO distribution.

4. The ¹²CO/¹³CO line intensity ratio, , varies dramatically. Globally (9 kpc 2.5 kpc), is 20-40, a high value compared to typical ratios found in the disk component of galaxies or even central regions of normal galaxies. This indicates a prominent contribution of diffuse, unbound molecular gas with a moderate optical depth in the ¹²CO (10) transition in both the bar and the center of NGC 7479.

5. On smaller sizescales of  pc, exceeds 30 in large parts of the bar, reaching values usually found in starburst mergers. Since values as low as 5 are also found in the bar, close to the ¹³CO condensation, and since a bar environment is very well mixed, we discard an underabundance of the ¹³C isotope as a possible explanation of the very high found in many places. Instead, this is explained by a dominant component of diffuse gas, readily produced by either tidal disruption or cloud collisions in the bar potential. The large variation in is reflected by large changes of likely values of the conversion factor from ¹²CO intensity ro H₂ column density. In the central 1.5 kpc, the Galactic `standard' conversion factor (SCF) overestimates the gas mass by a factor of up to 10; in ¹²CO peaks along the bar the discrepancy is even larger. Only in a ¹³CO complex in the bar we find the SCF to be correct.

6. The offset in the central HCN (and ¹²CO) peak from the ¹³CO peak can be attributed to a gradient in kinetic temperature in which the highest gas kinetic temperature is at the position of the HCN peak. This leads to the prediction that the ¹³CO (21)/(10) intensity ratio should be higher at the HCN peak than at the ¹³CO peak.

7. The region along the bar where is small might be an area where the conditions are more quiescent, which is also indicated by the narrowness of the lines, both in ¹²CO and ¹³CO, found here. If the ¹²CO ridge along the bar, which coincides closely with the dust lanes, is taken to be the location of the bar shock, the ¹³CO condensation is behind or downstream this shock, possibly in a region where the disrupted (but also compressed) gas emerging from the shock can form bound molecular complexes. However, the region where the velocity gradient is steepest along the bar does not coincide exactly with either the ¹²CO or the ¹³CO distribution. In the center, ¹³CO traces the steepest velocity gradient much more closely than ¹²CO. Thus, the relation between the molecular tracers and the shock is complex.

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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