Flat-spectrum radio sources with kpc-scale structure

We have carried out the first systematic study of flat-spectrum radio sources which have significant structure on angular scales 90-300 milliarcsec (mas), corresponding to linear scales of similar to 0.5-1.5 kpc at a redshift of 0.5. The principal aim of the study was to search for multiple gravitational imaging of compact radio components with image separations smaller than that of the smallest galaxy lens system known (335 mas) and corresponding to masses appropriate to compact/dwarf galaxies and to the bulges of spiral galaxies. A secondary aim was the morphological classification of a sample of flat-spectrum sources with kpc-scale structure. We particularly wanted to find out the frequency of occurrence of compact symmetric objects (CSOs: linear size < 1 kpc) and medium symmetric objects (MSOs: linear size < 15 kpc) which are believed to be the precursors of the large classical double radio sources. The parent sample consisted of 1665 flat-spectrum sources selected from two VLA surveys made with similar to 200 mas resolution (the Jodrell Bank-VLA Astrometric Survey - JVAS - and the Cosmic Lens All-Sky Survey - CLASS), from which we used visibility data to select sources (55 in total) with significant structure in the size range 90-300 mas. Sources with multiple compact components having a flux density ratio < 8:1 could be high-magnification lens systems, CSO/MSOs or objects with strong jets. A step-by-step observational filtering process at successively higher angular resolution was employed to classify the 55-source subsample, Initial MERLIN observations at 5 GHz (50 mas resolution) enabled us to classify similar to 75 per cent of the sources. The remaining sources were observed with the VLBA at 5 GHz (2 mas resolution) and in a few cases with MERLIN at 22 GHz (10 mas resolution). The resulting maps show that the majority of flat-spectrum sources with kpc-scale structure are asymmetric core-jets, The remaining sources include 23 new CSO/MSOs, a much smaller fraction of the parent sample (1.4 per cent) than is found in VLBI surveys of flat-spectrum sources (5-10 per cent). About half of the new CSO/MSOs constitute a hitherto unknown population: bright core CSO/MSOs. No definite high-magnification gravitational lenses were found. The implication is that the optical depth to lensing with image separations in the range 90-300 mas is several times less than on arcsec scales.