2. Observations, calibration and data reduction
Five sources (0004+139, 0830+101, 0906+041, 0938+119 and 1500+045) were observed during a single 24 hour observing run using a global VLBI array on 27/28 September 1992. Another five sources (0046+063, 0243+181, 1338+381, 1428+423 and 1557+032) were observed with the European VLBI Network (EVN) and the Hartebeesthoek Radio Astronomical Observatory 26 m antenna in South Africa on 25/26 and 27/28 October 1996. Source coordinates, redshifts and total flux densities at 6 cm are given in Table 1. The parameters of the radio telescopes used in the two experiments are shown in Table 2. The observations were made at 5 GHz in left circular polarization. Data were recorded using the Mk III VLBI system in Mode B with 28 MHz total bandwidth, and correlated at the MPIfR correlator in Bonn, Germany.
Table 1. Source parameters.
Table 2. VLBI telescopes in the September 1992 (top) and October 1996 experiment (bottom) and their characteristics at 5 GHz.
Initial calibration was done using the NRAO AIPS package (Cotton 1995; Diamond 1995). Clock offset and instrumental delay errors were corrected using the strong sources 0804+499 and 0235+164 in the global and the EVN experiments, respectively. Data were fringe-fitted using AIPS using 5 minute solution intervals. We used the system temperatures measured during the observations and previously determined gain curves for each telescope for the initial amplitude calibration, which was then adjusted using amplitude calibrator sources, based on total flux density values measured nearly contemporaneously to our observations with the Effelsberg telescope. For the September 1992 experiment, this was also checked using VLA data obtained in parallel with our VLBI observation. Total flux densities determined from VLBI images were typically 10-15% smaller than those determined from the VLA observations, which may indicate either the presence of extended structures undetectable with VLBI or residual calibration errors.
The Caltech DIFMAP program (Shepherd et al. 1994) was used for self-calibration and imaging, starting with point source models with flux densities consistent with the zero-spacing values. RMS image noises (3 ) were 0.2-0.4 and 0.6-1.0 mJy/beam (depending on the telescopes' performance and integrated on-source time) for the global and the EVN experiments, respectively. Plots of self-calibrated correlated flux densities as a function of projected baseline length, as well as clean images resulting from the DIFMAP imaging process are shown in Fig. 1 for both experiments. Image parameters are listed in Table 3. All sources but the most distant one, 1428+423, appear to be well resolved and most of them show asymmetric structure.
Table 3. Parameters of VLBI maps in Fig. 1a-j.
We performed model fitting in DIFMAP using self-calibrated uv-data in order to quantitatively compare these sources with other extremely high redshift quasars. The results of model fitting are listed in Table 4. In all cases we fixed the first component at the phase center. While we searched for the simplest possible model (i.e. the smallest possible number of Gaussian components), not all components can be distinguished as separate features on the maps. In the case of 0004+139, we kept only one component for the extended emission because the position angle of the beam lies close to the source structure direction and the correlated flux density versus uv-distance plot indicates the presence of a large component.
Table 4. Fitted elliptical Gaussian model parameters of the source structures.
We also made resolution VLA D configuration images of the five sources observed in the 1992 global experiment. VLA data were obtained at the same time as the phased array data used for the global VLBI experiment. These images were made using the NRAO AIPS package with typically 3-6 iterations of self-calibration and imaging. We show VLA images of 0830+101 and 1500+045 in Fig. 2a and 2b, respectively. The other three sources appeared unresolved with the VLA in our observations.
© European Southern Observatory (ESO) 1999
Online publication: March 10, 1999