Bright extragalactic radio sources at 2.7 GHz – II. Observations with the Cambridge 5-km telescope

Abstract

This paper presents observations with the Cambridge 5-km telescope of 51 radio sources taken from a complete sample of 168 extragalactic sources having S (2.7 GHz) > 1.5 Jy. The measurements were made at a frequency of either 5 or 2.7 GHz. Of these 51 sources, only nine were resolved; the three most extended sources were also observed at 1400 and 408 MHz with the Cambridge One-Mile telescope. Since the other 117 sources have been studied previously with the 5-km telescope, structural information is now available on the whole 2.7-GHz sample and allows a comparison to be made with analogous samples selected at low frequencies. The large numbers of unresolved sources in the 2.7-GHz sample are not all flat-spectrum objects as is commonly thought; no less than 44 per cent have steep high-frequency spectra, with $$\alpha^5_{2.7}$$ ≽ 0.5. These unresolved steep-spectrum objects cannot be classified as either extended radio sources or ‘centimetre-excess’ compact objects; they form a distinct class of their own. Nevertheless, there is evidence that it may be useful to consider both compact and extended steep-spectrum sources together as a single population.