The Difference between Radio-loud and Radio-quiet Active Galaxies

The recent development of unified theories of active galactic nuclei (AGNs) has indicated that there are two physically distinct classes of these objects-radio-loud and radio-quiet. The primary observational distinctions between the two types are the following. (1) The radio-loud objects produce large-scale radio jets and lobes with the kinetic power of the jets being a significant fraction of the total bolometric luminosity. On the other hand, the weak radio ejecta of the radio-quiet objects are energetically insignificant. (2) The radio-loud objects are associated with elliptical galaxies which have undergone recent mergers, while the radio-quiet objects prefer spiral hosts. (3) The space density of the radio-loud objects at a given optical luminosity is ~10 times lower than that of the radio-quiet objects. Despite these differences, the (probable) thermal emissions from the AGNs (continua and lines from X-ray to infrared wavelengths) are quite similar to the two classes of object. We argue that this last result suggests that the black hole masses and mass accretion rates in the two classes are not greatly different, and that the difference between the classes is associated with the spin of the black hole. We assume that the normal process of accretion through a disk does not lead to rapidly spinning holes and propose that galaxies (e.g., spirals) which have not suffered a recent major merger event contain nonrotating or only slowly rotating black holes. When two such galaxies merge, the two black holes are known to form a binary and we assume that they eventually coalesce. In the small fraction of mergers in which the two parent" galaxies contain very massive holes of roughly equal mass, a rapidly spinning, very massive hole results. It is proposed that such mergers are the progenitors of powerful radio sources, in which the radio jets are powered by the spin energy of the merged hole. We calculate the distributions of mass and spin for the merged holes from the parent hole mass distribution, which is derived from the optical luminosity function of radio-quiet AGNs adopting different activity patterns. The ratio of the number of radio-loud to radio-quiet AGNs at a given thermal (e.g., optical) luminosity is determined by the galaxy merger rate. The required fraction of galaxies which merge during the average lifetime (~10^8^ yr) of a radio-loud AGN is found to be 10-1, i.e., a merger rate of 1 in ~10^9^ yr. The Blandford-Znajek formalism is then used to predict the radio luminosity and radio luminosity function of the merged population. Comparisons between the predicted and observed radio luminosity functions constrain the efficiencies with which jet power is extracted from the spinning hole and radio emission is produced by the jet. The cosmological evolution of the radio properties of the radio-loud objects is related to the increased frequency of merger events at earlier epochs.