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Astron. Astrophys. 358, 812-818 (2000)

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7. Conclusions

We have carefully analyzed the data obtained for NGC 5668 with the Fabry-Perot interferometer TAURUS II at the WHT in order seek a connection between the star formation processes and vertical motions in spiral galaxies. We have found that there is a clear correlation between the morphology of the regions with a high residual velocity (HRVR's) and the intensity of the [FORMULA] emission, showing that the HRVR's are indeed regions with important vertical motions associated with star formation processes. Although we are not able to calculate the ages and energetics of these features, comparison with observations in other galaxies strongly supports the hypothesis that the structures detected present a wide age range, from young expanding shells in a bright HII region, to evolved chimneys blowing out hot gas to the halo, surrounded by several bright HII regions. The formation of these regions was probably induced by the pressure exerted by the expanding shell/chimney on the ambient gas. An alternative explanation for these features is that they are produced by infall/collision of gas clouds with the disk (e.g. Saito et al. 1992) followed by induced active star formation. Although this scenario explains in a very natural and simple way the fact that the velocity offsets are one-sided, this is also quite normal in expanding shells. If they are formed not exactly in the equator but slightly off-plane, they will grow on mostly in the low density side, therefore showing only one-sided offsets in the velocity structure. In fact, with the present data there is no way to decide whether the HRVR's are moving into or out of the disk. On the other hand, the different structures found in the HRVR's (from compact HII regions to rings of HII regions) are fully compatible with an evolutionary pattern in the chimney model. Therefore, although we can not rule out the infalling hypotheses with the present observations, we find the chimney scenario followed by sequential star formation in the shell borders to be a more likely explanation for what is happening in NGC 5668. These observations provide in any case a clear link between the star formation processes in the disk with other observed phenomena in NGC 5668 like high HVC's of neutral hydrogen. Observations of this kind have been made for other galaxies with a lower star formation rate and without HVC's, and the residual velocity field is not reported to show these features (see for example Jiménez-Vicente et al. 1999, for a similar study of NGC 3938). This fact seems to suggest that these features and the existence of HVC's in the disk are closely related. High resolution HI observations of NGC 5668 would be desirable to confirm the connection between those phenomena.

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© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000