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Astron. Astrophys. 344, 51-60 (1999)

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1. Introduction

There are about fifty known radio loud quasars at redshift [FORMULA] with a total flux density at 5 GHz [FORMULA] mJy. Some of them have been imaged at 5 GHz with VLBI (Gurvits et al. 1992, 1994; Xu et al. 1995; Taylor et al. 1994; Frey et al. 1997; Udomprasert et al. 1997). Here we present first epoch VLBI images of a further ten [FORMULA] quasars. We show that their structural properties are similar to those of other known sources at [FORMULA]. The present sample includes the most distant radio loud quasar known to date, 1428+423 at [FORMULA] (Hook & McMahon 1998).

Our interest in studying the milliarcsecond radio structures in high redshift quasars is motivated in part by their potential usefulness for cosmological tests (e.g. Kellermann 1993; Gurvits et al. 1999). Recent analysis of a sample of 151 quasars imaged at 5 GHz with milliarcsecond resolution has led to the conclusion that a simple assumption about the spectral properties of "cores" and "jets" can explain the apparent greater compactness of the sources at higher redshift (Frey et al. 1997; Gurvits et al. 1999). However, this result is based on a sample with considerable spread of structural properties on milliarcsecond scale. More data on the milliarcsecond radio structures, especially at high redshift are needed to study the structural properties of the quasars as well as various cosmological models to test.

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© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999
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