Superdisks in radio galaxies

Abstract

We present evidence for a striking new feature of powerful radio galaxies: gigantic disklike structures of thermal gas and dust oriented roughly perpendicular to the radio jets and situated around the elliptical host galaxy. These superdisks, or fat pancakes, appear to have a typical diameter of at least 75 kpc and a width of ~25 kpc. Quite plausibly, they are tidally stretched remnants of gas rich disk galaxies, or damped Lyman-a clouds, captured by the massive elliptical, and heated by its hot coronal gas. Observational manifestations of the superdisks include the sharp, quasi-linear, edges of the radio lobes on the side facing the central elliptical, detected in at least a dozen well mapped radio galaxies at low to moderate redshifts. For radio galaxies with z > 1.8, evidence for superdisks is based on the apparent asymmetry of diffuse Lya emission associated with the radio lobes, which is a very sensitive tracer of dust. This asymmetry can be understood if the brighter Lya emission is associated with the radio lobe on the near side of the nucleus and hence not obscured by the dust in the superdisk. Superdisks can provide more consistent explanations for some of the best established correlations among the radio source properties: namely, the Laing-Garrington effect and the correlated radio-optical asymmetry. They are also likely to be responsible for part of the low-frequency flux variability of compact radio sources. We argue they may provide an improved explanation of Lya absorption dips in the emission profiles of distant radio galaxies. Additional methods of searching for superdisks and testing this scenario are proposed.