On the nature of emission-line clouds of quasars and active galactic nuclei

It is proposed that the clouds responsible for the emission lines (broad and maybe narrow) in active galactic nuclei (AGN) and quasars (QSOs) are stratified and not uniform in density, as usually assumed to date. Specifically, it is suggested that they are winds from evolved stars, in particular red giants. This assumption appears to "solve," in a straightforward fashion, several problems concerning the origin, dynamics, and response of these clouds to changes in the continuum luminosity; at the same time it preserves the most successful feature of the current two-phase equilibrium photoionization models: the narrow range in the values of the ionization parameter ({XI}) needed to reproduce the observed line ratios, which is also identical with that required for the coexistence of the two phases in equilibrium. More specifically, the existence of the winds' density gradient guarantees, for a given continuum luminosity L and distance r from the continuum source, the existence of the proper density n_*_ such that the value of the ionization parameter is {XI} = {XI}_c_^*^ at that density (Krolik, McKee, and Tarter). All higher densities (the inner part of the wind) will be at {XI} < {XI}_c_^*^ and hence in the cool (T ~ 10^4^ K) phase, emitting the observed line radiation. The part of the wind at density lower than n_*_ will be at {XI} > {XI}_c_^*^ and hence in the hot (T ~ 10^8^ K) phase; it will presumably evaporate and may provide the gas that powers the central engine. Under these conditions and assuming mass loss M^dot^ = 10^-6^ M_sun_ yr^-1^ per red giant, the size of an individual cloud (i.e., radius at which the wind evaporates) is ~ 7 x 10^13^ cm and the density at its edge n =6 x 10^8^ cm^-3^, while its column density N_H_ ~ 4 X 10^22^ cm^-2^, in agreement with the requirements of the standard model (Kwan and Krolik). The number of such clouds needed to provide the required covering in a high-luminosity QSO (L ~ 10^46^ ergs s^-1^) is ~ 10^6^-10^7^, in reasonable agreement with the number of stars expected around a supermassive (M ~ 10^8^ M_sun_) black hole (assuming 1% are red giants). In addition, this model can provide the necessary fuel to power the active nucleus (Shull) in terms of the required number of red giants and their assumed mass loss and implies the gravitational origin of the observed line widths. It can also account for a number of correlations observed between the covering factor, column density, and absolute luminosity (Wu et al.; Wamstecker and Colina; Reichert et al.) and also between the line widths of forbidden lines and their critical densities (Filippenko and Halpern). Finally it can account for the observed logarithmic line profiles in terms of virial (isotropic) motions around the central black hole and provides certain new insights concerning the morphological and evolutionary aspects of active galaxies.