High resolution studies of mJy and mu Jy radio sources

Below 1 mJy the radio source population is dominated by star-forming galaxies, but with a significant minority of AGN and low-luminosity radio galaxies. High-resolution radio observations made with MERLIN and the VLA have already had a significant impact on the study of distant star-forming galaxies, since the radio emission is not affected by dust obscuration. Here we present the first EVN observations of sub-mJy sources in the Hubble Deep Field at a resolution of 15 mas. Our preliminary results suggest that one of the most compact ``dusty starbursts'', with a tentative redshift of 4.41, may harbour an AGN. In conjunction with MERLIN+VLA and WSRT observations, we can now study micro-Jy radio sources from milli-arcsec to arcminute scales. In addition, we discuss results from a small-area VLBI survey of mJy radio sources. These constrain the fraction of AGN at the mJy level and also reveal a significant population of compact (typically 10 mas) steep spectrum sources possibly associated with nascent low-luminosity radio galaxies at moderate redshifts. Both these surveys demonstrate the potential for VLBI observations of distant, faint radio galaxies and how they may play an important role in establishing the incidence and properties of distant, weak AGN.