Radio-loud AGN in the XMM-LSS field

II. A dichotomy in environment and accretion mode?

¹ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: tasse@strw.leidenuniv.nl
² SUPA, Institute for Astronomy, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
³ CEA/DSM/DAPNIA, Service d'Astrophysique, Saclay, 91191 Gif-sur-Yvette, France


Received: 20 December 2007
Accepted: 9 May 2008

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 (“Radio-mode”).
In this paper, we independently study the internal and environmental properties of the optical hosts of the sample of ∼110 radio sources with redshifts in the XMM-LSS Survey region. We do this by building a comoving-scale-dependent overdensity parameter, based on the photometric redshift probability functions, to constrain the small (~75 kpc) and large (~450 kpc) scale environments of radio sources independently from their stellar mass estimates. Our results support the picture in which the comoving evolution of radio sources in the redshift range 1 is caused by two distinct galaxy populations, whose radio source activity is triggered by two different mechanisms. The first population, which dominates at high stellar masses () is 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.