Extragalactic sources with one-sided radio structure

Abstract

A list has been compiled of 49 extragalactic sources, most of them identified with quasars, that appear to have a one-sided (D2 type) radio structure characterized by a single outer component displaced from a compact central (nuclear) component coincident with the optical object. The observed properties of a subsample of 28 D2 quasars that have an overall angular size larger than 5 arcsec are briefly discussed and compared with those of normal (Dl type) double quasars. It is found that the central components in most D2 sources account for more than half the total flux density at high frequencies in contrast to the Dl quasars which generally have less than 20 per cent of their total flux density in a central component. This makes it very unlikely that D2 sources are just those Dls in which there is a large intrinsic difference in the flux densities or separations of the two outer components. The observed properties of D2 sources are easier to understand in the relativistic beaming interpretation in which their axes are inclined at smaller angles with the line of sight compared to Dl sources.