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Issue A&A
Volume 433, Number 1, April I 2005
Page(s) 87 - 100
Section Extragalactic astronomy
DOI 10.1051/0004-6361:20041657



A&A 433, 87-100 (2005)
DOI: 10.1051/0004-6361:20041657

On the X-ray emission of z $\mathsf{\sim2}$ radio galaxies: IC scattering of the CMB and no evidence for fully-formed potential wells

R. A. Overzier1, D. E. Harris2, C. L. Carilli3, L. Pentericci4, H. J. A. Röttgering1 and G. K. Miley1

1  Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
    e-mail: overzier@strw.leidenuniv.nl
2  Smithsonian Astronomical Observatory, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
3  National Radio Astronomy Observatory, New Mexico Array Operations Center (VLA, VLBA), PO Box O, 1003 Lopezville Road, Socorro, NM 87801, USA
4  Dipartimento di Fisica, Università degli Studi Roma Tre, Rome, Italy

(Received 13 July 2004 / Accepted 30 November 2004)

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 ( $L_R/L_{\rm X}$) correlation for AGN, we find that the cores are underluminous in the X-rays indicating that obscuring material ( $n(\hbox{H\rm I})\sim10^$ 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 ${\sim}100{-}200$ $\mu $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 ${\sim}10^$ erg s-1, thereby ruling out a virialized structure of cluster-size scale at $z\sim2$.

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 $f_{0.5-2~\rm ~keV}>3\times10^$ 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 Pentericci et al. (2002).


Key words: galaxies: high-redshift -- galaxies: active -- X-rays: galaxies: clusters -- X-rays: general

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© ESO 2005


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