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 mill iarcsec (mas), corresponding to linear scales of similar to 0.5-1.5 kp c at a redshift of 0.5. The principal aim of the study was to search f or multiple gravitational imaging of compact radio components with ima ge separations smaller than that of the smallest galaxy lens system kn own (335 mas) and corresponding to masses appropriate to compact/dwarf galaxies and to the bulges of spiral galaxies. A secondary aim was th e morphological classification of a sample of flat-spectrum sources wi th kpc-scale structure. We particularly wanted to find out the frequen cy of occurrence of compact symmetric objects (CSOs: linear size < 1 k pc) and medium symmetric objects (MSOs: linear size < 15 kpc) which ar e believed to be the precursors of the large classical double radio so urces. The parent sample consisted of 1665 flat-spectrum sources selec ted from two VLA surveys made with similar to 200 mas resolution (the Jodrell Bank-VLA Astrometric Survey - JVAS - and the Cosmic Lens All-S ky Survey - CLASS), from which we used visibility data to select sourc es (55 in total) with significant structure in the size range 90-300 m as. Sources with multiple compact components having a flux density rat io < 8:1 could be high-magnification lens systems, CSO/MSOs or objects with strong jets. A step-by-step observational filtering process at s uccessively higher angular resolution was employed to classify the 55- source subsample, Initial MERLIN observations at 5 GHz (50 mas resolut ion) 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 resolut ion) 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 k pc-scale structure are asymmetric core-jets, The remaining sources inc lude 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 unk nown population: bright core CSO/MSOs. No definite high-magnification gravitational lenses were found. The implication is that the optical d epth to lensing with image separations in the range 90-300 mas is seve ral times less than on arcsec scales.