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

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6. Some shell/chimney properties

We were able to find eight HRVR's in NGC 5668. When we look at the positions of the HRVR's in the intensity and velocity dispersion map, we find that they are always related to regions of active star formation. Whether the star formation regions lie in the centre or the surroundings of the HRVR's can probably be explained as an age effect. The youngest, yet small expanding shells show the star-forming regions at their centre and are therefore roughly circular in the intensity map. Fig. 6 shows an example of this type. The more evolved stalled chimneys are surrounded by a more or less complete ring-like structure of star-forming regions which are probably formed by sequential star formation (see T98). Fig. 7 shows one of these cases. Both figures show clearly that there is a strong correlation between residual velocity, intensity and velocity dispersion in the HRVR's. In the case of region B (Fig. 6) the peak in the residual velocity perfectly matches the position of a bright HII region and of a minimum in the velocity dispersion. In the case of region A (Fig. 7) the residual velocity peaks roughly at the centre of a ring-like structure formed by HII regions, and the velocity dispersion peaks right in the centre of this ring-like structure in the low intensity part. The structure of the [FORMULA] emission around region A can be seen better in Fig. 8 where this region, extracted from the image of NGC 5668 taken with the WFPC/2 of the HST 2 using the F658N filter is shown. This filter perfectly matches the wavelength of the [FORMULA] line for NGC 5668, and although it also contains the NII line and continuum emision, it mostly traces the ionized gas. Fig. 8 shows that there are not only compact HII regions, but also a large amount of diffuse gas emission around this region. This can also be seen in most of the other HRVR's detected in NGC 5668 which are visible in the HST image. Although the structure of most of the HRVR's do not show this aspect (for example many of the ring-like structures are not as complete as the shown example, but show just half a ring or even less), there is always a clear correlation between the structure of residual velocity, intensity and velocity dispersion. Moreover the HST image shows that around these HRVR's (seven of the eight detected are visible in the HST image) there are not only numerous HII regions, but also a large amount of difuse ionized gas. Table 3 gives some of the properties of the shell/chimneys detected in NGC 5668. It is worth noting that all the HRVR's (except region G) have an average velocity dispersion above the average of the galaxy (which is around 16.5 km/s throughout the whole disk as shown in Fig. 9). As we noted previously, the HRVR's are always in or surrounded by regions of high velocity dispersion.

[FIGURE] Fig. 6. Example of a circular region: Region B. Left panel shows a greyscale map of [FORMULA] intensity with the residual velocity contours (from 0 (outer) to 15 (inner) km/s in steps of 5 km/s). Right panel shows a greyscale map of [FORMULA] intensity with the velocity dispersion contours (13 (inner) and 18 (outer) km/s in steps of 5 km/s).

[FIGURE] Fig. 7. Example of a ring-like region: Region A. Left panel shows a greyscale map of [FORMULA] intensity with the residual velocity contours (from -15 (inner) to 0 (outer) km/s in steps of 5 km/s). Right panel shows a greyscale map of [FORMULA] intensity with the velocity dispersion contours (from 15 (outer) to 30 (inner) km/s in steps of 5 km/s).

[FIGURE] Fig. 8. Region A as seen by the WFPC/2 of the HST with the F658N filter.

[FIGURE] Fig. 9. Radial distribution of the corrected velocity dispersion for NGC 5668. Error bars represent the dispersion within a ring. The dashed horizontal line represents the average value of 16.5 km/s.


[TABLE]

Table 3. Shell/chimney properties. Columns show 1) The HRVR's ID according to Fig. 5, 2) Estimated radius in pc, 3) Maximum residual velocity, 4) Average velocity dispersion around region centre, 5) Type: R=Ring-like, C=circular, U=undefined.


Although it is difficult to make an estimation for the ages of the shells, previous studies (T98) show that they range from a few Myr for the small expanding shells up to some tens of Myr for the largest stalled chimneys.

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© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000
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