Unification scheme and the distribution of neutral gas in compact radio sources

Abstract

We examine the consistency of H I properties with the unification scheme for radio galaxies and quasars, and any correlation with the symmetry parameters for a sample of compact steep-spectrum (CSS) and gigahertz peaked-spectrum (GPS) sources. In our sample, 15 out of 23 galaxies exhibit 21-cm absorption as against one out of nine quasars, which is broadly consistent with the unification scheme. Also there is a tendency for the detection rate as well as the column density for galaxies to increase with core prominence, fc, a statistical indicator of the orientation of the jet axis to the line of sight. This can be understood in a scenario where radio sources are larger than the scale of the circumnuclear Hi disc so that the lines of sight to the lobes at very large inclinations do not intersect the disc. The sources in our sample also exhibit the known anticorrelation between Hi column density and source size. This suggests that small linear size, along with intermediate values of core prominence, is a good recipe for detecting 21-cm absorption in CSS and GPS objects. Some of the absorption could also be arising from clouds which may have interacted with the radio jet. The H I column density and velocity shift of the primary absorption component, however, show no dependence on the degree of misalignment and the separation ratio of the radio sources. © 2006 RAS.