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.