ar X iv : a st ro - p h / 99 07 44 1 v 1 3 0 Ju l 1 99 9 Cosmic evolution and unified models for radio AGN

A new method of deriving the cosmological evolution of radio AGN is described which is based on the premise of unified models for quasars and radio galaxies. The 'starburst vs monster' debate takes place on several levels, for example the relation between starburst and AGN nuclear activity[1] and between background contributions[2]. The original version of the Madau diagram [3] led several authors in early 1997 to note the similarity between the cosmic histories of AGN and star formation rate [4, 5]. The Madau diagram has evolved [6], as has our view of how to determine space densities of AGN [7, 8]. Here we briefly set out this new view of the cosmic history of radio AGN, a history which must be correctly described if the relation between AGN and star-formation activity is to be understood. Previous analyses assumed two radio-AGN populations (e.g. [9]) based on radio spectrum: 'flat-spectrum' (core-dominated, and predominantly quasars) and 'steep-spectrum' (extended emission, double structures, and predominantly radio galaxies). In view of unification models which posit that radio quasars and BL Lac objects are end-on versions of radio galaxies, such a dichotomy makes no sense; and the derivations of space densities have dubious interpretation. 1 The paradigm Two anisotropic effects give rise to the unified view of quasars and radio galaxies [10, 11]: relativistically-beamed twin jets feed the double lobes of powerful radio sources [12], and the black-hole/accretion disk system is shrouded in a dusty torus whose axis is aligned with the radio axis [13]. A radio galaxy with double lobes is seen when the system is viewed side-on. As lines-of-sight approach the axis, the torus opening reveals the light of the nuclear black-hole/accretion-disk system which comes to dominate the galaxy light to produce a quasi-stellar object. When lines-of-sight coincide closely with the axis, Doppler enhancement of the relativistically-approaching radio jet leads to its compact flat-spectrum radio emission dominating the extended emission. Such 'core-dominated' quasars show superluminal motions in the jet structures as revealed by repeated VLBI observations (e.g. [14]). Recognition of these two mechanisms has given rise to the two current paradigms of radio-source unification, based on FRI and FRII radio galaxies [15] as the two parent populations. The FRI radio galaxies show the two regions of highest surface brightness in radio emission