Previous abstract Next abstract

Session 4 - Low Luminosity AGN and Starburst Galaxies.
Display session, Monday, June 10
Tripp Commons,

[4.11] Locally Optimally Emitting Clouds and the Origin of AGN Broad Emission Lines

K. T. Korista (Univ. Ky), J. A. Baldwin (CTIO), G. J. Ferland, D. A. Verner, J. W. Ferguson (Univ. Ky)

A recent paper by Baldwin et al.\ (1995, ApJ, 455, L119) suggested that the observed spectra of AGNs and QSOs are dominated by powerful selection effects. Namely, that for each emission line only a narrow range of gas density and distance from the ionizing continuum source results in maximum reprocessing efficiency, corresponding to ``locally optimally emitting clouds.'' These parameters depend on the ionization and excitation potentials of the line, and its thermalization density. The mean QSO line spectrum can be reproduced by simply summing the contributions from clouds having a wide range of gas densities and distances from the ionizing source. Here we investigate the dependence of the predicted spectrum on the expected presence of a range of cloud column densities, and on simple cloud distribution functions in each of the three axes: radius from ionizing source, gas density, and column density. Also investigated are the effects of the shape of the ionizing continuum, the gas metallicity, and the presence of non-thermal line widths. A number of observational characteristics in QSOs/AGNs are predicted qualitatively by this model, since the line formation radius naturally depends on the ionization potential, modulated by the thermalization density. Among these are emission line profile differences, the reverberation of the emission lines, and the scaling of the characteristic size of the BLR with luminosity. Possible origins of the observed Baldwin Effect are also discussed.

Program listing for Monday