The environments of FRII radio sources

Abstract

Using ROSAT observations, we estimate gas pressures in the X-ray-emitting medium surrounding 63 FRII radio galaxies and quasars. We compare these pressures with the internal pressures of the radio-emitting plasma estimated by assuming minimum energy or equipartition. In the majority of cases (including 12/13 sources with modelled, spatially resolved X-ray emission) radio sources appear to be underpressured with respect to the external medium, suggesting that simple minimum-energy arguments underestimate the sources' internal energy density. We discuss possible departures from the minimum energy condition and the consequences of our result for models of the dynamics of radio galaxies, in particular self-similar models (Kaiser & Alexander 1997).

The Karl G. Jansky Very Large Array Sky Survey (VLASS). Science Case and Survey Design

“…The details of the mechanism(s) of interaction between radio-loud AGN and their environments, on all scales, remain unclear. Basic questions, such as whether the most powerful sources are expanding supersonically throughout their lifetimes (e.g., Begelman & Cioffi 1989;Hardcastle & Worrall 2000), or what provides the pressure supporting the lobes of low-power objects (e.g., Birzan et al 2008;Croston et al 2008) remain unanswered. These questions can only be addressed by the accumulation of large, statistically complete samples of radio sources with imaging capable of resolving them on scales ∼10kpc, combined with excellent multi-wavelength data.…”

Radio galaxies and feedback from AGN jets

“…Thus it appears that LERGs, while still possessing active jets, have no sign of a radiatively efficient accretion disk, torus, corona, or accretion-driven emission lines, while HERG behave like radio-quiet AGN with the addition of a jet. The nuclear optical and Xray emission seen from some LERGs (Hardcastle and Worrall, 2000b) is consistent with coming from the jet only. The situation is confused by the existence of remnant sources (Section 3.4), where the jet has recently switched off -distinguishable from active LERGs by the absence of any nuclear emission associated with the jet -and by a very few peculiar objects that lack one or more of the standard AGN radiative components (Ramos Almeida et al, 2011), but these do not change the basic picture.…”

“…The thermal X-ray radiation from the hot phase of the ambient medium of the RLAGN has been of huge importance to constraining dynamical models of these sources since its first discovery (Longair and Willmore, 1974;Hardcastle and Worrall, 2000b). More recently, sensitive X-ray imaging has revealed deficits of X-ray emission ('cavities') associated with the kpcscale lobes of many RLAGN (e.g., Bîrzan et al, 2004), and also found a small number of unambiguous shock features, demonstrating supersonic bulk motion of the lobes through the medium (e.g., Kraft et al, 2003;Croston et al, 2009Croston et al, , 2011.…”

“…These can be derived from deprojection (normally spherical deprojection) of the observed X-ray surface brightness profile if we assume that the X-rayemitting gas dominates density and pressure -an assumption that is more valid on larger scales than smaller ones -and from X-ray spectroscopy to estimate the temperature (profile) of the hot gas. Bulk inference of radio galaxy environments from X-ray data is now relatively routine (see e.g., Worrall and Birkinshaw (2000); Hardcastle and Worrall (2000b); Croston et al (2008c); Ineson et al (2015)) though systematic uncertainties may arise through the assumption of spherical symmetry. It is far more difficult to estimate the environmental density and pressure profiles based on optical information alone.…”

“…The conclusions relating to large-scale environments discussed in the previous Section have also been derived primarily for RLAGN populations in the local Universe (typically z < 0.5). At z > 1 environmental studies have only been possible to date for the rare objects at the high-luminosity tail of the population (e.g., Hardcastle and Worrall, 2000b;Belsole et al, 2007), leading to difficulties in making comparisons with local populations or examining relationships with radio properties. Luminous, high-redshift radio galaxies appear to be strongly associated with cluster and (at z > 2) protocluster environments, and have proved a useful tracer of the richest overdensities at z > 2 (e.g., Venemans et al, 2007;Miley and De Breuck, 2008;Wylezalek et al, 2013;Hatch et al, 2014).…”