The two-phase model: The basic paper is by Krolik, McKee and Tarter (1981). Other detailed papers on this topic are by Lepp et al. (1985), and Kallman and Mushotzky (1985). Krolik (1988) has discussed transient cool clouds embedded in hot gas. The difficulties with the model, especially in relation to the hot medium temperature, are discussed by Fabian et al. (1986) and Mathews and Ferland (1987). The last reference addresses the dynamical implications in great detail. For cloud formation, in relation to thermal instability, see Mathews and Doane (1990).
Other models: Magnetic confinement is discussed in Rees (1987). Cloud formation in winds is discussed by Smith and Raine (1985), Perry and Dyson (1985, winds and obstacles) and Shlosman et al (1985, radiation pressure driven wind from the surface of a disk). Collin-Souffrin, Dyson, Dowell and Perry (1988) suggested a two component model for the BLR, including high ionization broad line clouds, and the outer parts of a central accretion disk. More on accretion disks as a source of line emission can be found in a series of papers by Collin-Souffrin, Dumont and collaborators (See Dumont and Collin-Souffrin 1990 and references therein). The orbiting cloud model has been suggested by Kwan and Carroll (1982), Carroll and Kwan (1985) and Carroll (1985). The implications to the NLR are discussed in Carroll and Kwan (1983). Critical discussions of these and other models are given in review articles by Mathews and Capriotti (1985) and Osterbrock and Mathews (1986). BLR models involving stars are discussed by Voit and Shull (1988), Penston (1988), Kazanas (1989) and Scoville and Norman (1988).
Line profiles: For line profile observations see chapter 2. The basic work on acceleration by radiation pressure is by Mathews (1974, 1982a) and Blumenthal and Mathew (1975, 1979). These authors have also investigated the shape of the clouds, in particular "pancake" clouds. Different radial dependences are discussed by Capriotti et al (1980, 1981), Carroll and Kwan (1983, 1985), Bradley and Puetter (1986) and Penston et al. (1990). Shields has investigated disk line profiles in a number of papers; see Shields (1989) for references. More work on disk-type profiles is in Mathews (1982b). Application of disk profiles to the case of Arp 102B are discussed in Chen et al. (1989). For reviews see Mathews and Capriotti (1985), Osterbrock and Mathews (1986) and Shields (1989).
Line asymmetry and wavelength shifts: The first theoretical discussion of line asymmetry is by Ferland, Netzer and Shields (1979). Further work is by Capriotti et al. (1981) and Puetter and Hubbard (1987). Kallman and Krolik (1986) investigated line profiles in a presence of a Compton thick medium. For the observations, see chapter 2.
Cross-correlation tests of radial motion: See Maoz et al. (1991) and references therein.