A representative survey of the dynamics and energetics of FRII radio galaxies

Abstract

We report the first large, systematic study of the dynamics and energetics of a representative sample of FRII radio galaxies with well-characterized group/cluster environments. We used X-ray inverse-Compton and radio synchrotron measurements to determine the internal radio-lobe conditions, and these were compared with external pressures acting on the lobes, determined from measurements of the thermal X-ray emission of the group/cluster. Consistent with previous work, we found that FRII radio lobes are typically electron-dominated by a small factor relative to equipartition, and are over-pressured relative to the external medium in their outer parts. These results suggest that there is typically no energetically significant proton population in the lobes of FRII radio galaxies (unlike for FRIs), and so for this population, inverse-Compton modelling provides an accurate way of measuring total energy content and estimating jet power. We estimated the distribution of Mach numbers for the population of expanding radio lobes, finding that at least half of the radio galaxies are currently driving strong shocks into their group/cluster environments. Finally, we determined a jet power-radio luminosity relation for FRII radio galaxies based on our estimates of lobe internal energy and Mach number. The slope and normalisation of this relation are consistent with theoretical expectations, given the departure from equipartition and environmental distribution for our sample.

RAiSE III: 3C radio AGN energetics and composition

“…The dynamical model based equipartition factors are also in good agreement with the 3C observations of Ineson et al (2017) on a source-by-source basis. These measurements are all consistent to within the 2σ uncertainties, as shown in Figure 12.…”

“…The general consensus is thus of FR-II lobes with field strengths approximately a factor two to three less than the equipartition value, or equivalently magnetic field energy densities approximately a factor of 40 less than that in the electrons. The similarity between our dynamical model equipartition factor estimates and those of Ineson et al (2017) is encouraging, however all but a single source (3C427.1) have dynamical equipartition factors fitted in the narrow range 0.32 < B/B eq < 0.42. The veracity of our fitting algorithm is further tested against observations of Pictor A, which has a measured equipartition factor of ∼0.7 (Hardcastle et al 2016).…”

“…w > 2) is inconsistent with our estimates at the 2σ level. The comparison of our dynamical model based jet power and equipartition factor estimates to independent measurements from Hardcastle et al (2016) and Ineson et al (2017) have yielded the same outcome: a proton-to-lepton ratio of w < 2 at the 2σ level. There is therefore strong evidence against an energetically dominant population of relativistic protons or thermal plasma (i.e.…”

“…The magnetic field strength is derived by comparing the observed X-ray emissivity to model predictions derived from the level of synchrotron emission. Hardcastle et al (2002), Croston et al (2004Croston et al ( , 2005 and Ineson et al (2017) all find sub-equipartition magnetic field strengths in the lobes of FR-IIs. Spectral ages calculated assuming fields in equipartition will thus generally be greatly underestimated.…”

“…Croston et al (2005) and Ineson et al (2017) found the energy density in powerful FR-IIs peaks at B/B eq ∼ 0.5 (corrected to observed spectral index s = 2α inj + 1) and 0.4 respectively based on X-ray inverseCompton emission from the synchrotron emitting electron Figure 11. Fitted distribution for the equipartition factor using the Full Fit algorithm (see Section 6), both with (blue) and without (grey) the use of the prior probability distributions.…”