Abstract
We present a detailed study of the environments of a sample of nine low-power radio galaxies, based on new and archival XMM-Newton observations. We report new detections of group-scale environments around three radio galaxies, 3C 296, NGC 1044 and 3C 76.1. As with previous studies, we find that FR-I radio galaxies inhabit group environments ranging over nearly two orders of magnitude in bolometric X-ray luminosity, but find no evidence for a tight relationship between large-scale X-ray environment and radio-source properties such as size, radio luminosity, and axial ratio. We confirm earlier work showing that equipartition internal pressures are typically lower than the external pressures acting on the radio lobes, so that additional non-radiating particles must be present or the lobes must be magnetically dominated. We present the first direct observational evidence that entrainment may provide this missing pressure, in the form of a relationship between radio-source structure and apparent pressure imbalance. Finally, our study provides further support for the presence of an apparent temperature excess in radio-loud groups compared to the group population as a whole. For five of eight temperature excesses, the energy required to inflate the radio lobes is comparable to the energy required to produce this excess by heating of the group gas; however, in three cases the current radio source appears too weak to produce the temperature excess. It remains unclear whether the temperature excess can be directly associated with the current phase of AGN activity, or whether it is instead either a signature of previous AGN activity or simply an indicator of the particular set of group properties most conducive to the growth of an FR-I radio galaxy.
Highlights
The dynamics of low-power (FR-I: Fanaroff & Riley 1974), twinjet radio galaxies are thought to be controlled by interactions between the initially highly relativistic jets and the surrounding hot-gas medium, which act to decelerate the jets significantly on scales of typically a few kpc (e.g. Bicknell 1994; Laing & Bridle 2002a)
Significant correlations were found between source size and external pressure and pressure gradient, and between axial ratio and both external pressure and pressure gradient, with long, narrow sources associated with lower pressures and steeper gradients; this could be a selection effect, as longer sources are able to probe the outer regions of groups, where the pressure gradient is steeper
In agreement with earlier work, we find that FR-I radio galaxies are acted on by external pressures that are significantly higher c 2008 RAS, MNRAS 000, 1–21 than their internal, equipartition pressures, with apparent pressure imbalances ranging from a factor of 70 times underpressured to rough pressure balance
Summary
The dynamics of low-power (FR-I: Fanaroff & Riley 1974), twinjet radio galaxies are thought to be controlled by interactions between the initially highly relativistic jets and the surrounding hot-gas medium, which act to decelerate the jets significantly on scales of typically a few kpc (e.g. Bicknell 1994; Laing & Bridle 2002a). In order to understand how such structures are produced, and to investigate the evolution of FR-I sources on large scales, it is necessary to have good constraints on the hot-gas environments that confine their radio lobes. With Chandra and XMM-Newton it is possible to study radio-galaxy environments and jet/environment interactions in more detail Blanton et al 2001; Croston et al 2003; Fabian et al 2003); while there have been many studies of the apparent impact of FR-I sources in clusters, to date there have been few detailed Chandra and XMM-Newton studies of the environmental properties of nearby, well-studied FR-I radio galaxies. The role of environmental properties such as group/cluster richness, gas density and temperature distribution in determining large-scale radio structure remains uncertain
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