VLBI observations of the radio quasar J2228+0110 at z = 5.95 and other field sources in multiple-phase-centre mode

¹ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 200030 Shanghai, PR China
e-mail: hmcao@shao.ac.cn
² University of Chinese Academy of Sciences, 100049 Beijing, PR China
³ FÖMI Satellite Geodetic Observatory, PO Box 585, 1592 Budapest, Hungary
⁴ Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁵ Department of Astrodynamics and Space Missions, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands
⁶ Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 210008 Nanjing, PR China
⁷ ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁸ Department of Astronomy, University of Washington, Seattle WA 98195, USA

Received: 23 December 2013
Accepted: 24 January 2014

Abstract

A patch of sky in the SDSS Stripe 82 was observed at 1.6 GHz with Very Long Baseline Interferometry (VLBI) using the European VLBI Network (EVN). The data were correlated at the EVN software correlator at JIVE (SFXC). There are fifteen known mJy/sub-mJy radio sources in the target field defined by the primary beam size of a typical 30-m class EVN radio telescope. The source of particular interest is a recently identified high-redshift radio quasar: J222843.54+011032.2 (J2228+0110) at redshift z = 5.95. Our aim was to investigate the milli-arcsecond (mas) scale properties of all the VLBI-detectable sources within this primary beam area with a diameter of 20′. The source J2228+0110 was detected with VLBI with a brightness temperature T_b > 10⁸ K, supporting the active galactic nucleus (AGN) origin of its radio emission, which is conclusive evidence that the source is a radio quasar. In addition, two other target sources were also detected, one of them with no redshift information. Their brightness temperature values (T_b > 10⁷ K) measured with VLBI suggest a non-thermal synchrotron radiation origin for their radio emission. The detection rate of 20% is broadly consistent with other wide-field VLBI experiments carried out recently. We also derived the accurate equatorial coordinates of the three detected sources using the phase-referencing technique. This experiment is an early attempt of a wide-field science project with SFXC, paving the way for the EVN to conduct a large-scale VLBI survey in the multiple-phase-centre mode.