Locally optimally emitting clouds and the variable broad emission line spectrum of NGC 5548

In recent work Baldwin et al, proposed that in the geometrically extended b road-line regions (BLRs) of quasars and active galactic nuclei, a range in line-emitting gas properties (e.g., density, column density) might exist at each radius and showed that under these conditions the broad emission line spectra of these objects may be dominated by selection effects introduced by the atomic physics and general radiative transfer within the large pod o f line-emitting entities. In this picture, the light we see originates in a vast amalgam of emitters but is dominated by those emitters best able to r eprocess the incident continuum into a particular emission line. We test this "locally optimally emitting clouds" (LOC) model against the ex tensive spectroscopic database of the Seyfert 1 galaxy NGC 5548. The time-a veraged, integrated-light UV broad emission line spectrum from the 1993 Hub ble Space Telescope (HST) monitoring campaign is reproduced via the optimiz ation of three global geometric parameters: the outer radius, the index con trolling the radial cloud covering fraction of the continuum source, and th e integrated cloud covering fraction. We make an ad hoc selection from the range of successful models, and for a simple spherical BLR geometry we simu late the emission-line light curves for the 1989 IUE and 1993 HST campaigns , using the respective observed UV continuum light curves as drivers. We fi nd good agreement between the predicted and observed light curves and lags- a demonstration of the LOC picture's viability as a means to understanding the BLR environment. Finally, we discuss the next step in developing the LO C picture, which involves the marriage of echo-mapping techniques with spec tral simulation grids such as those presented here, using the constraints p rovided by a high-quality, temporally well-sampled spectroscopic data set.