VLBA observations of z > 4 radio-loud quasars

Abstract

We present high resolution (< 20 mas) observations of the radio continuum emission at 1.4 GHz from three high-redshift quasars: J1053-0016 (z=4.29), 1235-0003 (z=4.69), and J0913+5919 (z=5.11), thereby doubling the number of z > 4 radio-loud quasars that have been imaged at mas resolution. The observations were carried out with the Very Long Baseline Array (VLBA) of the NRAO. All three sources are unresolved in these observations, with source size limits of a few mas. In all cases the flux densities measured by the VLBA are within 10% of those measured with the VLA, implying that the sources are not highly variable on yearly timescales. We find no indication for multiple images that might be produced by strong gravitational lensing on scales from 20 mas (VLBA) to a few arcseconds (VLA), to dynamic range limits of ~100.