I hope that the above has indeed illustrated that studying the ISM in nearby galaxies via the 21-cm line of HI is not only feasible but very exciting and rewarding. Moreover, in order to put to the test the multitude of models which exist for the overall structure of the ISM and for the detailed evolution of an OB association, this kind of observations is indispensable. At present beam averaged values for several parameters describing the state of the neutral gas can be determined, typically sampling volumes with a diameter of a few hundred parsec. Values for the measured velocity dispersion, volume density and spin temperature are generally in line with values derived for the local Galaxy. There are indications that by employing yet higher resolutions these values might change. As mentioned above, Braun and Walterbos (1989) show that peak brightness temperatures might vary as a function of galaxy type and position in a galaxy. They argue that the peak brightness is a measure for the star formation rate. If this is correct then this should show up in their VLA high resolution HI survey of a dozen or so nearby galaxies. Another way to throw light on the intrinsic temperature of the gas is by HI absorption observations. A new survey, extending the observations reported by Dickey and Brinks (1988) is on its way.
It is evident from the high resolution HI imaging that all galaxies show a multitude of filaments, arcs, loops and shells. It seems as if most of these features can be explained by the effects which OB stars have on their environment. Some features, equivalent to Heiles' supershells and perhaps best exemplified by the observations of M101 (van der Hulst and Sancisi 1988) require much more energy and their most likely explanation is infall of material. One of the questions which can now be addressed is how this infall is related to the presence of high velocity clouds. A further step will be to try to decide where these clouds come from; are they tidal debris or is it material which has its origin in the disk and which has been ejected by supernovae upon breakthrough of a bubble blown by an evolving OB association. Alternatively, the HVCs could be disk material which has been shot into the halo by the impact of a small companion and which is falling back.
In this review I have restricted myself to HI observations which trace
the cool and warm neutral medium. It is clear that for a complete
picture one would like to include
H
and radio continuum data
which trace the WIM. One such project, which looks at the
smaller scale interaction of an OB association with the ISM and which
combines radio and optical data is underway
(Brinks et al. 1989).
The aim of this project is to study in detail a
number of OB associations in M31 and to provide constraints for the
various models which
have been proposed and to derive the energy balance for these
regions. Other obvious
components which should be added in order to gain a more complete
understanding are
the cool dust and the molecular phase whereas the last piece of the
puzzle will have to
come from the new generation of X-ray satellites such as ROSAT which
will eventually
supply information on the HIM, thus completing the picture of the ISM.
Acknowledgements
I would like to thank the organizing committee and especially Harley Thronson for having invited me to Wyoming and for reimbursing a substantial part of my expenses. The other part, not less substantial, was covered by the Royal Greenwich Observatory. I am grateful to all those who have supplied me with photographic material and verbal input which was used in the preparation of this manuscript, notably Erik Deul, Jurjen Kamphuis, Michael Rupen, Ed Salpeter, Thijs van der Hulst, and Jacqueline van Gorkom. It is a pleasure to thank Robert Braun, John Dickey, Carl Heiles, Colin Norman, Daniel Puche, Evan Skillman, Guillermo Tenorio-Tagle, Thijs van der Hulst, and David Westpfahl for their extensive comments on an earlier version of this manuscript.