Why do head-tail sources exist in poor clusters of galaxies?

Abstract

In a continuing study of nearby (z~0.02-0.05) radio sources in poor clusters of galaxies, we obtained VLA observations of four head-tail (HT) sources as probes of the intracluster environments: NGC 742, NGC 1044, NGC 4061, and NGC 7503. NGC 742 apparently has a companion, NGC 741, in the midst of its extended tail structure. NGC 7503 and NGC 4061 have horseshoe shapes very similar to the archetypal HT radio galaxy, NGC 1265. These structures are remarkable because the sources are found in poor groups, where both the average density of the intracluster medium (ICM) and the velocities of the galaxies (thus the ram pressures) are supposedly much lower than in the rich clusters. Yet these poor groups have narrow-angle tail (NAT) sources with the same general morphologies as those in rich clusters. There is not much difference 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 source luminosity, and ICM densities. It appears that the HT phenomenon is remarkably similar between the poor clusters and the rich clusters because the local conditions near these sources within their clusters are similar. An ICM density typical of that found in poor clusters (~10^-4^ cm^-3^) and a galaxy velocity typical of the rich clusters (~600 km s^-1^) provide sufficient ICM ram pressure to bend radio jets into NAT morphologies. One explanation for the high relative velocities of the poor cluster HT galaxies is that these clusters are dynamically young and are still collapsing.

Why do head-tail sources exist in poor clusters of galaxies?

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Abstract

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