VLBI observations of the radio quasar J2228+0110 at z=5.95 and other
  field sources in multiple-phase-centre mode
  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 arcmin. The source
J2228+0110 was detected with VLBI with a brightness temperature T_b>10^8 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 >10^7 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.
