A complete sample of intermediate-strength radio sources selected from the GB/GB2 1400-MHz surveys. II - High-resolution maps and optical identifications of sources with very steep spectra

A complete sample of GB/GB2 sources with α (l400,5000) ≥ +0.95 and stronger than 550 mJy at 1400 MHz, were observed at about 3-arcsec resolution with the VLA. Optical identifications were made using the PSS prints; seven are new. A large fraction of the program sources have a classical double structure. Maps made at 1465 MHz show that the identification rate increases sharply from 25% for narrow doubles to 90% for triple and complex structures. Extended sources are mostly identified with distant galaxies, compact (< 1 arcsec) sources with stellar objects. No difference was found between the double morphologies of empty field (EF) sources and galaxies. About 20% of the EF's in the sample have compact, unresolved structures, and rather flat, self-absorbed low-frequency spectra, and could be luminous extended cores of distant objects. This fraction is significantly lower than the fraction of compact EF sources with S1400 ≥ 2 Jy, and suggests that the intermediate-strength (GB/GB2) sample contains a larger fraction of strongly evolved objects than do strong-source samples. Differing spatial distributions of these objects may account for the above results.