Compact radio emission from z  ~  0.2 X-ray bright AGN^⋆

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
e-mail: zuther@ph1.uni-koeln.de
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 4 October 2011
Accepted: 13 April 2012

Abstract

Context. The radio and X-ray emission of active galctic nuclei (AGN) appears to be correlated. The details of the underlying physical processes, however, are still not fully understood, i.e., to what extent the X-ray and radio emission is originating from the same relativistic particles, or from the accretion-disk, or corona, or both.

Aims. We study the cm radio emission of an SDSS/ROSAT/FIRST matched sample of 13 X-raying AGN in the redshift range 0.11 ≤ z ≤ 0.37 at high angular resolution with the goal of searching for jet structures or diffuse, extended emission on sub-kpc scales.

Methods. We used MERLIN at 18 cm for all objects and Western EVN at 18 cm for four objects to study the radio emission on scales of  ~500 pc and  ~40 pc for the MERLIN and EVN observations, respectively.

Results. The detected emission is dominated by compact nuclear radio structures. We find no kpc collimated jet structures. The EVN data indicate compact nuclei on 40 pc scales, with brightness temperatures typical for accretion-disk scenarios. Comparison with FIRST data shows that the 18 cm emission is resolved out up to 50% by MERLIN. Star-formation rates based on large aperture SDSS spectra are generally too low to produce considerable contamination of the nuclear radio emission. We can, therefore, assume the 18 cm flux densities to be produced in the nuclei of the AGN. Together with the ROSAT soft X-ray luminosities and black-hole mass estimates from the literature, our sample objects closely follow the Merloni et al. fundamental plane relation, which appears to trace the accretion processes. Detailed X-ray spectral modeling from deeper hard X-ray observations and higher angular resolution at radio wavelengths are required to make more progress in separating jet and accretion related processes.