Astron. Astrophys. 323, 363-373 (1997)

7. Conclusions

The main aim of this paper was to improve our understanding of the connection between star formation activity and the presence of bars in spiral galaxies located in regions of low galaxy density. In particular, we intended to explain for what reasons only a fraction of SBs show indices of enhanced star formation activity. Our main results can be summarized as follows:

1) The non-interacting late-type galaxies most active in forming stars have both strong and long bars. However, not all strong and long bars are actively creating stars. Weak bars do not display any significant excess of star formation activity. In general strong bars are long as well. Although the sample is too poor to draw definite and more detailed conclusions, the trend outlined here is unquestionable.

2) The selected galaxies have been shared in four distinct classes according to their respective bar strength, , and relative SFR. During the dynamical evolution, SBs probably go through these classes according to various specific tracks. Numerical simulations enlight possible scenarios. Very young strong bars are first characterized by a vigorous episode of star formation and two different radial gaseous abundance gradients, one steep in the bar and one shallow in the disc. Then, the galaxies progressively fall back in a more quiescent state with a nearly flat abundance gradient across the whole galaxy. On the contrary, weak bars are unable to induce significant star formation or flat abundance gradients.

3) The slope profile of the radial abundance gradient is monitored by: i) The strength of the bar (weak bar slow and moderate modifications of the initial gradient; strong bar quick flattening of the initial gradient). ii) The age of the bar (young bar spatially distinct gradients; old bar single gradient). iii) The spatially-dependent star formation efficiency.

4) For late-type spirals the controversy concerning the role of bars in enhancing/reducing star formation may be resolved if in fact only young and strong bars enhance star formation. Larger samples would however be necessary to fully confirm this assertion.

From these conclusions, it clearly appears that the formation of a spontaneous strong bar in an isolated gas-rich Sc-like disc is a major event in the dynamical history of the galaxy. This results in a significant and specific alteration of the spatial distribution and intensity of star formation, as well as of the radial abundance gradients. In particular, some of these remarkable characteristics could help finding young bars.

Finally, some caveats must be mentioned: a) These results only concern isolated late-type spirals. b) We do not take into account possible accretions or interactions which could play a role in the subsequent internal evolution and consequently modify the star formation efficiency, the morphology of bars, and even the Hubble type (Pfenniger 1993). c) The IRAS data do not have a sufficient resolution to allow detailed studies of the star formation morphology, e.g. along the bar or in the nucleus. Generally, the most intense star formation tends to occur in the circumnuclear regions near the ILRs when they exist or in the nuclei when ILRs are absent (see Telesco et al. 1993). Studies of individual galaxies are still too scarce to allow statistical investigations in such a context. No doubt that ISO data will be most valuable to improve the present situation.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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