Extended Inverse-Compton Emission from Distant, Powerful Radio Galaxies

Abstract

We present Chandra observations of two relatively high redshift FR II radio galaxies, 3C 432 and 3C 191 (z = 1.785 and z = 1.956 respectively), both of which show extended X-ray emission along the axis of the radio jet or lobe. This X-ray emission is most likely to be due to inverse-Compton scattering of Cosmic Microwave Background (CMB) photons. Under this assumption we estimate the minimum energy contained in the particles responsible. This can be extrapolated to determine a rough estimate of the total energy. We also present new, deep radio observations of 3C 294, which confirm some association between radio and X-ray emission along the NE-SW radio axis and also that radio emission is not detected over the rest of the extent of the diffuse X-ray emission. This, together with the offset between the peaks of the X-ray and radio emissions may indicate that the jet axis in this source is precessing.

0.5 Mpc-scale extended X-ray emission in thez= 2.48 radio galaxy 4C 23.56

“…Converting our measured 0.5–8 keV luminosity to 2–10 keV with Γ= 3, we find a lower limit to the energy stored in relativistic electrons in the lobes of 4C 23.56 of E e ≃ 8.0 × 10 58 erg; adopting a more typical Γ= 2, E e ≃ 2.2 × 10 59 erg. Comparing these estimates with those of Erlund et al (2006) for three z ∼ 2 H z RGs, we note that while the X‐ray lobes in 4C 23.56 are at least twice as large as the lower‐redshift sources, the minimum energy stored in IC‐emitting electrons appears to be roughly comparable. Considering only losses through IC emission, the radiative lifetime of the 4C 23.56 X‐ray lobes is ∼10 7 yr.…”

“…We believe that in 4C 23.56, as in most previously reported H z RGs with X‐ray‐bright radio lobes, IC scattering of CMB photons is the most likely emission mechanism for the majority, if not the entirety, of the extended X‐ray flux. Fourteen z > 1 H z RGs currently in the literature (Carilli et al 2002; Fabian et al 2003a,b; Scharf et al 2003; Yuan et al 2003; Belsole et al 2004; Overzier et al 2005; Blundell et al 2006; Erlund et al 2006) have X‐ray lobes on scales ranging from a few tens to a few hundreds of kpc with typical luminosities of a few times 10 44 erg s −1 . The X‐ray lobes in 4C 23.56 are the second largest discovered to date, 1 and the first ≳0.5 Mpc‐scale lobes discovered at the z > 2 epoch suspected to be the predominant era of massive galaxy formation.…”

“…If we assume that all electrons responsible for the X‐ray lobes have this Lorentz factor, we can use the observed X‐ray luminosity and the known CMB photon energy density to estimate the energy stored in the IC‐emitting electron population. Following Erlund et al (2006), the energy in the scattering electron population is then given by where L 44 is the 2–10 keV X‐ray luminosity in units of 10 44 erg s −1 .…”

“…In recent years, observations with the Chandra and XMM–Newton satellites have detected extended X‐ray emission in an increasing sample of H z RGs (Carilli et al 2002; Fabian, Celotti & Johnstone 2003a; Fabian et al 2003b; Scharf et al 2003; Yuan et al 2003; Belsole et al 2004; Overzier et al 2005; Blundell et al 2006; Erlund et al 2006). Generally associated with the radio structure, the extended X‐ray emission is believed to arise in most cases partly or wholly from inverse‐Compton (IC) scattering of cosmic microwave background (CMB) photons by relativistic electrons in the jets and/or lobes of the radio galaxy.…”

“…Fabian et al 2003b; Blundell et al 2006). X‐ray measurements have allowed estimates of the minimum Lorentz factors, γ min , in radio hotspots (Blundell et al 2006) and the total energy contained within X‐ray luminous radio lobes (Erlund et al 2006) at z ∼ 2. Scharf et al (2003), noting the relative positions of X‐ray and Lyman α (Lyα) flux in 4C 41.17 at z = 3.8, suggested that the IC‐scattered X‐ray photons could themselves give rise to a secondary feedback mechanism by photoionizing the surrounding medium.…”