Steep-spectrum radio cores in high-redshift galaxies

We have made high-resolution radio images, using the Very Large Array (VLA), of a sample of radio galaxies at z > 2 selected from the MRC/1 Jy complete sample. These are also the first detailed observations of radio galaxies at the high rest-frame frequencies of 15-30 GHz. Compact (<0.2 arcsec) cores at the mJy and sub-mJy level were detected in most of the objects, coincident with the optical galaxies. In sharp contrast to the flat spectra of radio cores in galaxies at low redshift, we find that most of the cores at high redshift have a steep spectral index (alpha > 0.5) between 4.7 and 8.3 GHz (observed). We identify this steepening with the optically thin part of a synchrotron self-absorbed spectrum at frequencies above the turnover. Both core- and lobe-dominated quasars have also been reported to show such a turnover from flat to steep spectra but at much higher observed frequencies. We suggest that the cores in galaxies and quasars are intrinsically similar (in the rest frame of the emitting plasma) and that the observed differences can be explained in terms of Doppler shifting by different amounts in quasar and galaxy cores. This model is consistent with the unification scheme for radio galaxies and quasars. Using the theory of synchrotron self-absorption we estimate sizes of similar to 1 pc, magnetic fields of similar to 1 G and electron densities of similar to 1000 cm(-3) for the cores of both quasars and radio galaxies. The similar values are perhaps an indication of similar physical processes and parameters in their cores. The spectrum at turnover provides information on scales much smaller than the telescope resolution. It is therefore a useful tool for probing the cores of galaxies which are usually too weak for very long-baseline interferometry studies.