High-resolution observations of faint radio sources and the angular size-flux density relation

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Abstract
High-resolution radio observations are presented of 40 5C sources which make up two samples complete to 15 mJy at 1407 MHz and 55 mJy at 408 MHz respectively. The observations were made with the NRAO Very Large Array and the Cambridge 5-km telescope, and are used to define the angular size distribution of radio sources at low flux densities. We find median angular sizes of 5–10 arcsec at 1407 MHz and 10–15 arcsec at 408 MHz, although the distributions extend to significantly larger angular sizes at both frequencies. These small values are compared with predictions of evolution models from the literature. Although linear size evolution can be invoked to explain the small median angular sizes, we suggest that these result because many of the 5C sources are similar to those observed in high-frequency surveys rather than to 3CR sources. In particular, the spectral index distribution is broader and the sample contains a larger fraction of compact sources (both steep- and flat-spectrum) than is seen in 3CR. New flux density measurements at 2.7, 5 and 10.6 GHz are used to define the spectra of some sources over a wide range of frequency. The positions determined from the present observations place limits on any systematic errors in the 5C surveys and these, together with the detailed morphologies, have enabled the continuation of a deep optical identification programme.

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