Radio-loud AGN in the XMM-LSS field

Abstract

In recent years, several authors have argued that low luminosity radio-loud active galactic nuclei (AGN) have a different mode of accretion, triggered by different physical mechanisms, than “normal” optically- or X-ray-selected AGN. The latter have a radiatively efficient nucleus (sometimes called “Quasar-mode”), which according to the unified scheme may be obscured from direct view at optical wavelengths, whereas essentially all of the energetic output of the low-luminosity radio-loud AGN is in their radio jets (“Radiomode”). In this paper, we independently study the internal and environmental properties of the optical hosts of the sample of ∼110 radio sources with redshifts 0.1 10 10.5−10.8 M� )i s that of massive elliptical galaxies, lying in galaxy groups or clusters, where the radio source is triggered by the cooling of the hot gas in their atmosphere. At these stellar masses, we find that the fraction of galaxies that host radio-loud AGN is essentially the same as that in the local Universe. The second population of radio sources have lower stellar masses, lie in large scale underdensities, and show excess mid-IR emission consistent with a hidden radiatively efficient active nucleus. The radio-loud fraction at these masses is increased relative to the local Universe. We argue that galaxy mergers and interactions may play an important role in triggering the AGN activity of this second population.