Mathematics – Logic
Scientific paper
Dec 1977
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1977jrasc..71..443v&link_type=abstract
J. Roy. Astron. Soc. Canada, 71, 443-468
Mathematics
Logic
4
Scientific paper
The 10 most important points in this review of head-tail galaxies (defined as an optical galaxy near one end of a highly elongated radio structure) are: 1) Since January 1968, objects, in clusters of galaxies, showing a tail at radio wavelengths have been reported, with an average of 2 or 3 head-tail galaxies being discovered per year in the ensuing decade. 2) Morphologically, such a tail can be interpreted in terms of a trail along the orbital trajectory, within the cluster, of the associated galaxy. 3) All detailed theories proposed so far involve an intracluster medium between the member galaxies of a cluster. 4) In-situ particle acceleration outside the confines of the optical galaxies are dictated by observations. 5) Median values of physical parameters of head-tail galaxies are often similar to corresponding values for normal double radio galaxies (not necesarily in clusters of galaxies) for this similarity to be a pure chance effect. 6) This fact and other observations indicate that any detailed theory of head-tail galaxies must also account for normal double sources, and vice-versa. 7) Several head-tail galaxies have a pronounced curvature far along their trails, suggesting that mechanoisms other than siple orbital trajectories may play a role in the distant parts of trails. 8) Observations, especially of polarization, indicate that the trail density near the end of the tail is of the same order of magnitude as the density of the intracluster medium surrounding the trail. 9) The problem of a pronounced curvature seems to lead to an interpretation of the distant parts of trails in terms of buoyancy forces; considering the energies derived from turbulent motions associated with these forces, one would predict that the longest trails should be emitting more radio power than the average trail. This, however, may not be the case (see Fig. 14), possibly implying that additional forces are at work here. 10) Lastly, a very small number of trails may possess a radio source virtually at the end of the tr! ail, but the physical significance of this still has to be determined, and more observations are needed.
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