The energetics of giant radio galaxy lobes from inverse Compton scattering observations

Abstract

Context. Giant radio galaxy (GRG) lobes are excellent laboratories for studying the evolution of the particle and magnetic field energetics and the past activity of radiogalaxy jets, as indicated by recent results of X-ray observations with Suzaku. However, these results are based on assumptions of the shape and extension of the GRG lobe electron spectrum. Aims. We re-examine the energetics of GRG lobes as derived by inverse Compton scattering of cosmic microwave background (CMB) photons (ICS-CMB) by relativistic electrons in RG lobes to assess the realistic physical conditions of RG lobes, their energetics, and their radiation regime. We consider the steep-spectrum GRG DA 240 recently observed by Suzaku as a reference case and we also discuss other RG lobes observed with Chandra and XMM. Methods. We model the spectral energy distribution (SED) of the GRG DA 240 East lobe to obtain constraints on the shape and on the extension of the electron spectrum by using multi-frequency information from radio to gamma-rays. We use radio and X-ray data to constrain the shape and normalization of the electron spectrum and we then calculate the Sunyaev-Zel'dovich (SZ) effect expected in GRG lobes that is sensitive to the total electron energy density. Results. We show that the electron energy density U e derived form X-ray observations can yield only a rough lower limit to its actual value and that most of the estimates of U e based on X-ray measurements have to be increased even by a large factor by considering realistic estimates of the lower electron momentum p 1 . This moves RG lobes away from the equipartition condition toward a particledominated and Compton power dominance regime. We propose to use the distribution of RG lobes in the U e /U B vs. U e /U CMB plane as another divide divide between the different physical regimes of particle and field dominance and radiation mechanism dominance in RG lobes. Conclusions. We conclude that the SZ effect produced by ICS-CMB mechanism observable in RG lobes provides reliable estimate of p 1 and U e and is the best tool to determine the total energy density of RG lobes and to assess their physical regime. This observational tool is available with the sensitive high-frequency radio and mm experiments.

A multi-frequency study of the SZE in giant radio galaxies

“…∼ E −αr ), which is expected to have the same value of α X (see, e.g., Longair 2003). Observations of the ICCMB spectrum at high energies (X-rays or gamma-rays) can also set the absolute normalization (assuming a unique spectral slope α) of the electron spectrum used for the SZE calculations (see Colafrancesco & Marchegiani 2011). Given the shape of the electron spectrum, the total intensity and the spectral shape of the SZE in RG lobes depend on the minimum en-ergy of the electron distribution (see, e.g., Colafrancesco 2008, Colafrancesco & Marchegiani 2011.…”

“…Observations of the ICCMB spectrum at high energies (X-rays or gamma-rays) can also set the absolute normalization (assuming a unique spectral slope α) of the electron spectrum used for the SZE calculations (see Colafrancesco & Marchegiani 2011). Given the shape of the electron spectrum, the total intensity and the spectral shape of the SZE in RG lobes depend on the minimum en-ergy of the electron distribution (see, e.g., Colafrancesco 2008, Colafrancesco & Marchegiani 2011. In this context, it has been shown that the ICCMB X-ray emission provides only marginal constraints to the minimum energy of the electron population in the lobes (Colafrancesco 2008).…”

The correlation between radio power and Mach number for radio relics in galaxy clusters

“…Erler et al 2015), and possible detection of the nonthermal SZE can give information on the properties of the electrons which are not depending on the magnetic field properties (see e.g. Colafrancesco & Marchegiani 2011 for the case of radio galaxies lobes). In the hard X-ray and gamma rays, the possible detection of the Inverse Compton emission from relativistic electrons in relics can constrain their high-energy spectra, allowing to discriminate between the acceleration and adiabatic compression scenario (see Paulo, Colafrancesco & Marchegiani 2017).…”

To the problem of the secondary CMB anisotropy separation

“…For an average GRG in the microwave range, the area around which is built by stacking, it was found that 1) there is a positive signal in the millimeter range; 2) the spectrum in the millimeter range is flat; 3) the signal in the submillimeter range is absent; 4) the topological properties in the average CMB fluctuations map possess special features: a weak insignificant maximum (< 1σ) is located in the minimum zone; 5) a significant SZ-effect, related with the scattering of CMB photons by the electrons of the GRG jet plasma [18] is not observed. A comparison with other populations of objects has revealed that given the average microwave properties, the distant radio galaxies with redshift ranges 0.3 ≤ z < 0.7.…”