Intermittent Radio Galaxies and Source Statistics

Abstract

We suggest that extragalactic radio sources are intermittent on timescales of ~10⁴-10⁵ yr. With the use of a simple spherical model of a cocoon/shock system, it is found that inactive sources fade rapidly in radio luminosity but that the shock in the ambient medium continues to expand supersonically, thereby keeping the whole source structure intact during the inactive phases. The fading of inactive sources, and the effect of the intermittency on the expansion velocity, can readily explain the observed overabundance of small radio sources. In particular, the plateau in the observed distribution of sizes found by O'Dea & Baum can be interpreted as being due to intermittency. The model predicts that very young sources will be particularly radio luminous, once the effects of absorption have been accounted for. Furthermore, it predicts the existence of a significant number of faint "coasting" sources. These might be detectable in deep, low-frequency radio maps or via the X-ray and optical emission-line properties of the shock front.