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Galaxies associated with powerful radio sources are among the most frequently used cosmological probes, because they can be observed at very large distances. The study of these objects using optical, infrared, and multi-frequency VLA and VLBI observations will be of use in developing a comprehensive picture of the cosmological properties of this class of sources and in restricting various cosmological scenarios.
Here we present the results of VLBI observations at 18 cm of two high redshift radio galaxies, 4C41.17 (z=3.8) and 4C23.56 (z=2.419). These Mk3 Mode A observations (total banwidth of 56 MHz) were made using 5 EVN telescopes, (Bonn, Jodrell Bank, Noto, Onsala, and Westerbork) in May 1993. The data were correlated at the Max Planck Institute for Radio Astronomy in Bonn (Germany).
The radio source 4C41.17 represents one of the most distant galaxies known to date. Its core has a flux density of 25 mJy at 18 cm. It has been observed at a variety of wavelengths including optical (Chambers 1989,) infrared (McLean 1987,) and radio (VLA 20, 6 and 2 cm, Chambers et al. 1990.) We present a map of the brightest spot in the inner part of the radio jet. The interpretation of these data allow us to derive important parameters for the physical processes taking place within the central 100 pc of the galaxy's core. The source 4C23.56 is less studied thus far. We have imaged the brightest feature in its radio structure on a scale of tens of milliarcseconds. Its flux density is about 15 mJy at 18 cm. The feature is only slightly resolved.
The reported observations are part of a project to study the evolution of the compact radio structure of the most distant radio galaxies. Radio galaxies at this distance tend to exhibit an alignment between the optical continuum and the radio axis which is contrary to the morphology of the nearer objects (Chambers et al. 1987). The images we have obtained provide more information for studying the effect of this alignment.
