Annu. Rev. Astron. Astrophys. 1980. 18: 165-218 Copyright © 1980 by Annual Reviews. All rights reserved

5.2. Nuclear Activity

Do the structures of extended radio sources identified with QSOs differ from those associated with galaxies? Although from the radio maps alone one cannot state with certainty whether an individual radio source is a quasar or a radio galaxy there are several statistical differences between the two classes of objects.

As we saw in Section 2.1.1 quasars tend to have more pronounced hot spots and their structures are more asymmetric than sources associated with galaxies. However, because the radio luminosities of quasars are systematically larger than those of radio galaxies, it is impossible to state whether these effects are causally related to the nature of the parent optical objects or to the luminosity of the associated extended sources.

Nevertheless, a real difference is indicated by the fact that the radio cores of quasars are stronger than those of radio galaxies for sources that have extended emission of similar power. It seems reasonable to ascribe this difference to the existence of a correlation between the radio and optical core luminosities in extended radio sources. This is not a one-to-one relationship, however. Perhaps the optical core luminosity measures the instantaneous nuclear activity whereas the radio core represents the activity integrated over a longer period. A very likely possibility is that extended quasars are indeed radio galaxies undergoing transient active periods. The weak optical nonthermal emission seen in the nuclei of some radio galaxies (Yee & Oke 1978) could well be associated with the simmering remnant of a QSO. The more pronounced asymmetry between opposite lobes (Section 2.1.1) would then be a relativistic effect of orientation produced by a more energetic pumping of energy into the hot spots.

The most violent examples of ("present") nuclear activity are probably BL Lac objects. Although differences between their properties and those of quasars appear to be jects were thought to possess little or no extended radio emission, but a survey by Weiler & Johnson (1980) has shown that 22 out of 42 objects surveyed have radio emission with sizes 1". There is therefore no evidence that the properties of this emission are qualitatively different from those of extended radio emission in quasars and radio galaxies.

Emission lines in the parent nuclei of radio sources are strongly connected with their radio structure. Almost all extended quasars exhibit strong broad emission lines. The fraction is about half for the most luminous (edge-brightened-double) 3CR radio galaxies and only 10% for radio galaxies with P₁₇₈ 10²⁶ W Hz^-1 (Hine & Longair 1979). For the limb-brightened double radio galaxies the ratio of core to extended flux increases with the width of the emission lines in a fairly continuous sequence from the extended radio galaxies to the extended quasars. This is also consistent with models of radio sources in which a more active nucleus pumps more energy into the extended radio lobes.

Emission lines can also help differentiate quasars that possess extended radio structure from those that do not. A large proportion of all quasars have little or no associated extended structure; for a few of these, upper limits of ~ 1% have been set on the ratio of extended to compact structure at 610 MHz (Miley & Hartsuijker 1978). The QSOs with the broadest and most irregular emission lines seem to be associated with the extended radio sources, whereas those with relatively narrow emission lines have compact radio structures (Miley & Miller 1979). There is also a tendency for FeII emission lines to be found preferentially in quasars without extended radio structures (Setti & Woltjer 1977, Miley & Miller 1979). These relations, while not properly understood, indicate that the broad line emission regions are closely connected with the production of radio sources.