5. The residual velocity field and the shell/chimney candidates (HRVR's)
The residual velocity field is calculated by subtracting the rotation model from the observed velocity field. Fig. 5 shows the resulting map. Although the map is quite noisy, a close inspection allows us to detect some regions which have a systematic deviation from rotational velocity greater than 10 km/s (which is 1.5 times the dispersion of the residual velocity map). We call these regions high residual velocity regions (HRVR's). In order to distinguish between HRVR's with real important vertical motions from regions whose residual could be an effect of noise or random motions in the disk, we also use the size and shape of the region. We therefore label as a HRVR only to those regions with a clear symmetry in the shape and with a mean diameter between 3 (which is our spatial resolution) and 13 arcsec (which corresponds to about 1.5 kpc). The regions have been marked with a circle and named with a letter in Fig. 5. The shape of the HRVR's (which is in many cases nearly circular) makes it very unlikely that those regions are related to streaming motions. It then seems logical to interpret the residuals as true vertical motions in the disk. This interpretation is strongly reinforced if we look at the intensity map in the locations where the HRVR's are found. We can see that the regions are clearly associated with regions of star formation which fall in their centres or immediate surroundings. In most cases the structure of the velocity dispersion map in the HRVR's is also strongly correlated with both intensity and residual velocity (reaching values up to 34 km/s in the centre of the HRVR), which supports the idea of regions of great activity (high temperature or violent motions). These facts strongly support the hypothesis that the HRVR's are regions with real vertical motions of the ionized gas which are related to star forming processes in the disk. The most straightforward interpretation of these features (although certainly not the only one) is that they are shells or chimneys (depending on their size and age and on whether they have been able to break the disk and to blow gas out of it) formed around highly active star-forming regions by the strong stellar winds or correlated SN explosions of the multiple massive young stars found whithin those regions. This scenario perfectly matches the chimney model proposed by Norman & Ikeuchi (1989) for the ISM and is also supported by observations in other galaxies (see for example T98). The fact that NGC 5668 has a great amount of star formation and that HVC's of neutral hydrogen have been detected in it (S96) makes the whole scenario fully coherent.
© European Southern Observatory (ESO) 2000
Online publication: June 20, 2000