WHY DO HEAD-TAIL SOURCES EXIST IN POOR CLUSTERS OF GALAXIES

In a continuing study of nearby (z similar to 0.02-0.05) radio sources in poor clusters of galaxies, we obtained VLA observations of four he ad-tail (HT) sources as probes of the intracluster environments: NGC 7 42, NGC 1044, NGC 4061, and NGC 7503. NGC 742 apparently has a compani on, NGC 741, in the midst of its extended tail structure. NGC 7503 and NGC 4061 have horseshoe shapes very similar to the archetypal HT radi o galaxy, NGC 1265. These structures are remarkable because the source s are found in poor groups, where both the average density of the intr acluster medium (ICM) and the velocities of the galaxies (thus the ram pressures) are supposedly much lower than in the rich clusters. Yet t hese poor groups have narrow-angle tail (NAT) sources with the same ge neral morphologies as those in rich clusters. There is not much differ ence between our poor-cluster NAT sources and rich-cluster NAT sources , in terms of jet radii of curvature, jet opening angles, internal ram pressures within the jets, jet luminosity as a fraction of total sour ce luminosity, and ICM densities. It appears that the HT phenomenon is remarkably similar between the poor clusters and the rich clusters be cause the local conditions near these sources within their clusters ar e similar. An ICM density typical of that found in poor clusters (simi lar to 10(-4) cm(-3)) and a galaxy velocity typical of the rich cluste rs (similar to 600 km s(-1)) provide sufficient ICM ram pressure to be nd radio jets into NAT morphologies. One explanation for the high rela tive velocities of the poor cluster HT galaxies is that these clusters are dynamically young and are still collapsing.