© CAMBRIDGE UNIVERSITY PRESS 1999

1.4 Physical Characteristics: Quasars

Keeping in mind the serendipitous discovery of the first two quasars, we need to specify the broad criteria that would help distinguish a quasar from a star or a galaxy. In the classic book Quasi-Stellar Objects by Geoffrey and Margaret Burbidge, written in the early years of quasar searches, the following criteria specified by Maarten Schmidt were given:

• star-like object identified with a radio source;
• variable light;
• large ultraviolet flux of radiation;
• broad emission lines in the spectra with absorption lines in some cases;
• large redshift.

Later searches showed that the radio source property is not generic and only some 10 per cent of quasars may be radio-loud. We shall discuss this property of quasars in Chapter 9. In fact, X-ray emission seems more common with quasars, as became clear with the studies of the EINSTEIN observatory in 1980. Time variability has been found in several wavebands, e.g. visible, radio, X-ray etc. In the 1970s very long baseline interferometry revealed several cases of apparent superluminal separation of the components of radio-loud quasars on the scale of parsecs. We shall consider again in Section 6.1 the defining characteristics of quasars and other related objects.

The spectra of quasars have been used to get information on the physical environment in the universe at large redshifts. This environment can be in the quasar itself or in its vicinity or in the intermediate region between it and us (the observers). In particular, the absorption lines may arise from absorbing material in the quasar itself or in the intervening medium. With the discovery of high redshift galaxies this property is no longer the prerogative of quasars.

Finally, in 1979 the first case of the gravitational lensing of quasars came to light. Two quasar images named PHL 0957+561 A and B seemed so similar in physical properties that it became plausible to argue that they were virtual images of only one real object. The multiple imaging could be caused by the bending of light from the source to the observer by an intervening galaxy (or a cluster of galaxies, or some massive lump of dark matter), allowing more than one path. Gravitational lensing has since been proposed for several multiple quasar systems with members having very close angular separations ( 5-6 arcsec).

We will discuss the details and implications of all these physical characteristics in later chapters.