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 di stinctions between the two types are the following. (1) The radio-loud objects produce large-scale radio jets and lobes with the kinetic pow er of the jets being a significant fraction of the total bolometric lu minosity. On the other hand, the weak radio ejecta of the radio-quiet objects are energetically insignificant. (2) The radio-loud objects ar e associated with elliptical galaxies which have undergone recent merg ers, while the radio-quiet objects prefer spiral hosts. (3) The space density of the radio-loud objects at a given optical luminosity is alm ost-equal-to 10 times lower than that of the radio-quiet objects. Desp ite these differences, the (probable) thermal emissions from the AGNs (continua and lines from X-ray to infrared wavelengths) are quite simi lar to the two classes of object. We argue that this last result sugge sts that the black hole masses and mass accretion rates in the two cla sses are not greatly different, and that the difference between the cl asses is associated with the spin of the black hole. We assume that th e normal process of accretion through a disk does not lead to rapidly spinning holes and propose that galaxies (e.g., spirals) which have no t suffered a recent major merger event contain nonrotating or only slo wly rotating black holes. When two such galaxies merge, the two black holes are known to form a binary and we assume that they eventually co alesce. In the small fraction of mergers in which the two ''parent'' g alaxies contain very massive holes of roughly equal mass, a rapidly sp inning, very massive hole results. It is proposed that such mergers ar e the progenitors of powerful radio sources, in which the radio jets a re powered by the spin energy of the merged hole. We calculate the dis tributions of mass and spin for the merged holes from the parent hole mass distribution, which is derived from the optical luminosity functi on 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 (almost-equal-to 10(8) yr) of a radio-loud AGN is found to be 10(-1), i.e., a merger rate of 1 in congruent-to 10(9) yr. The Blandford-Znaje k formalism is then used to predict the radio luminosity and radio lum inosity function of the merged population. Comparisons between the pre dicted and observed radio luminosity functions constrain the efficienc ies with which jet power is extracted from the spinning hole and radio emission is produced by the jet. The cosmological evolution of the ra dio properties of the radio-loud objects is related to the increased f requency of merger events at earlier epochs.