Kinematics of the Broad Emission Line Region in NGC 5548

We derive both total flux and velocity-resolved response functions for the C IV λ1549 emission line from the data obtained in the 1993 NGC 5548 monitoring campaign. These response functions imply that (1) the emission region stretches from inside 1 lt-day to outside 10 lt-day and is probably better described as round than flat and (2) the velocity field is dominated by a red/blue symmetric component (e.g., two- or three-dimensional random motions or rotation in a disk), but there is also significant radial infall. Quantitative modeling indicates that the random speeds are typically a few times as large as the radial speed. However, no simple model gives a completely acceptable fit to the data. These inferences rule out numerous simple and otherwise plausible models for broad-line region dynamics, including out-flowing winds, radial free- fall, rotation in a disk, or collisionless orbital motion.