Annu. Rev. Astron. Astrophys. 1991. 29: 581-625 Copyright © 1991 by Annual Reviews. All rights reserved

9.4 Quasars

Many of the properties of the ultraluminous galaxies (luminosity nonthermal optical emission line ratios, and spectral energy distributions) are similar to those of previously identified optical quasars. The detection of CO emission in Mrk 231 (Sanders et al 1987) stimulated a search for CO emission in normal optical quasars, since Bokensburg et al (1977) had speculated on the basis of the optical emission lines that this galaxy might harbor a dust-embedded quasar. Most spectacular are the detections of CO emission in the optical quasars IZw1 and Mrk 1014 (Barvainis et al 1989, Sanders et al 1988b), the latter at a redshift of 47,000 km s^-1 z = 0.17). These detections are not just impressive in their own right, but also potentially of great importance to our understanding of the evolutionary circumstances that lead to AGNs and quasars. Recently, Alloin et al (1991) have reported the detection of CO in the quasar E1821 + 643 at a redshift of 0.30. If this detection is confirmed, the measured CO luminosity indicates a total H₂ content exceeding 10¹¹ M.

Scenarios elucidating a sequence of events whereby gas-rich galactic mergers promote the evolution of a nuclear starburst cluster to form a central AGN have been put forward by David et al (1987), Norman & Scoville (1988), and Hernquist (1989). In these models, the standard optical quasar represents a phase subsequent to the gas-rich nuclear starburst. In the early evolution of such mergers, the bulk of the luminosity is emitted in the far infrared due to the large abundance of interstellar dust: however, the high luminosity generated by the nuclear starburst and by accretion onto the AGN will eventually disperse the interstellar gas and dust, leaving a normal optical quasar.