Of the thousands of sources which have been cataloged only a few hundred have been reliably optically identified with galaxies or with quasars. For a considerable number of sources accurate positions have been measured, but no optical object is found above the plate limit of the Palomar Sky Survey. For a much larger number of sources the position accuracy is not sufficient to distinguish between the two or more objects lying within the error rectangle.
Optical identifications are important for two reasons.
It is not possible from radio measurements alone to determine the distance to a radio source. Thus, only if the radio source is identified with a galaxy or quasar is it possible to measure the redshift and thus deduce the distance from the Hubble law. Distances are, of course, required to estimate the absolute radio luminosity and linear dimensions from measurements of radio flux density and angular size.
Optical studies of radio galaxies and quasars may give some insight into the problem of the origin of the intense radio emission.
For these reasons much of the earlier work on the discrete radio sources was concentrated on the determination of accurate positions to permit unambiguous optical identifications. Today, coordinates of at least the stronger sources may be routinely determined by interferometry with an accuracy of the order of 1 second of arc. Nevertheless, the identifications are difficult. Firstly, often there is no apparent optical counterpart of the radio position, so that any associated optical object is either subluminous or at such a great distance that it is not visible, even to large optical telescopes. Secondly, many radio sources have dimensions of the order of 1 minute of arc or more and a complex distribution of radio brightness so that more than one galaxy or quasar is found within the area covered by the source. Often, but not always, the identified galaxy lies near the centroid of radio emission, but it may also be coincident with one of the individual radio components, so that unambiguous identifications are difficult.