On the X-ray emission of z ∼2 radio galaxies: IC scattering of the CMB and no evidence for fully-formed potential wells

Abstract

We present the results of 20 ks Chandra observations for each of 5 radio galaxies in the redshift range 2.0<z<2.6. The goals were to (i) study the nature of their non-thermal X-ray emission; (ii) investigate the presence and amount of hot gas; and (iii) look for active galactic nuclei (AGN) overdensities in fields around high redshift radio galaxies. For 4 of the 5 targets we detect unresolved X-ray components coincident with the radio nuclei. From spectral analysis of one of the cores and comparison to the empirical radio to X-ray luminosity ratio (L_R/L_X) correlation for AGN, we find that the cores are underluminous in the X-rays indicating that obscuring material (n(HI)∼10²² cm^-2) may be surrounding the nuclei. We detect X-ray emission coincident with the radio hotspots or lobes in 4 of the 5 targets. This extended emission can be explained by the Inverse-Compton (IC) scattering of photons that make up the cosmic microwave background (CMB). The magnetic field strengths of ∼100{-}200 μ G that we derive agree with the equipartition magnetic field strengths. The relative ease with which the lobe X-ray emission is detected is a consequence of the (1+z)⁴ increase in the energy density of the CMB. For one of the lobes, the X-ray emission could also be produced by a reservoir of hot, shocked gas. An HST image of the region around this radio component shows bright optical emission reminiscent of a bow-shock. By co-adding the 5 fields we created a deep, 100 ks exposure to search for diffuse X-ray emission from thermal intra-cluster gas. We detect no diffuse emission and derive upper limits of ∼10⁴⁴ erg s^-1, thereby ruling out a virialized structure of cluster-size scale at z∼2. The average number of soft X-ray sources in the field surrounding the radio sources is consistent with the number density of AGN in the Chandra Deep Fields, with only one of the fields showing a marginally statistically significant factor 2 excess of sources with f_{0.5-2 ~keV}>3×10^-15 erg s^-1 cm^-2. Analysis of the angular distribution of the field sources shows no evidence for large-scale structure associated with the radio galaxies, as was observed in the case of PKS 1138-262 by [CITE].