Astron. Astrophys. 346, 45-57 (1999)

5. Conclusions

The main results of our high resolution study of the stellar bar region of the SBc galaxy UGC 2855 and of its companion UGC 2866 are:

1. The interferometric image of the ¹²CO transition of UGC 2855 reveals a continuous molecular bar of length kpc. UGC 2855 thus becomes one of only a few galaxies known to have dense gas distributed all along its stellar bar. The molecular mass detected in the bar is M, if a standard CO-H₂ conversion factor is assumed. The intensity distribution appears strongly centrally peaked.

2. The velocity field of the gas along the bar is dominated by solid body rotation. Only slight perturbations, indicative of streaming motions across the bar, are found. The smooth changes and lack of discontinuities in the velocity field together with the narrow line widths suggest that the gas along the bar of UGC 2855 does not experience strong shocks.

3. The ¹²CO/¹³CO ratio along the bar and in the center is almost constant at 5-10. This ratio, which is typical for the disks (5) and centers (10) of normal galaxies, also indicates quiescent conditions along the bar.

4. Only weak H emission is detected along the bar of UGC 2855. The emission is stronger in the southeastern part of the bar, where the velocity field of the molecular gas is more disturbed. The center is clearly detected in H, but the strongest H peak is associated with a spiral arm. Higher extinction in the nucleus might change this ratio, but even then the central region would not dominate in H as it does in many starburst galaxies. Thus, while there is some central star formation activity, UGC 2855 is not (yet) a starburst galaxy, as is also indicated by the IRAS color index.

5. A high velocity structure dominates the inner radius of the bar. This structure is responsible for the strong central intensity peak. It breaks up into a number of possibly virialised GMAs, the velocity of none of which coincides with the systemic velocity. This structure may be a clumpy disk, a torus or even an inner bar, in all cases fed by gas infalling along the main bar. In the simple case of a rotating disk, more than 50% of the dynamical mass ( M, which is less than the mass in the corresponding inner region of the Galaxy) may be gaseous.

6. In models, shocks along a bar can be avoided if the central mass concentration is low. Then, the bar has no ILR. Even though the velocity structure seen can also be explained in terms of a bar with an ILR, this is an attractive model for UGC 2855 since it naturally explains the quiescent gas along found along the bar, as well as the absence of any feature along the bar minor axis expected from gas on orbits.

7. In contrast, the gas-rich bar of NGC 7479 shows a disturbed velocity field, varying ¹²CO/¹³CO line ratios and strong H emission both along the bar and in the center. This demonstrates that even among the small group of galaxies with gas-rich bars large differences are found.

8. In evolutionary terms, we suggest that the bar of UGC 2855 is a very young object, where the process of central concentration is just starting. Then, this galaxy is likely to develop into a starburst in the future, a state that NGC 7479 has already reached.

9. UGC 2866, the companion of UGC 2855, shows strong CO emission (corresponding to M with a standard conversion factor) in a continuous, possibly bar-like feature of about 1.5 kpc radius. Both the FIR colors and the very strong H emission concentrated in the inner 300 pc indicate that this galaxy is already experiencing a strong starburst.

10. The activity in both UGC 2866 and UGC 2855 may have been triggered by interaction between the two galaxies. The response time in the small, compact galaxy, UGC 2866, may be shorter than in the large, open spiral UGC 2855, thus explaining the different level of activity seen at present.

© European Southern Observatory (ESO) 1999

Online publication: May 6, 1999
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