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Emission-line variability data on NGC 5548 argue strongly for the existenceof a mass of order 7 x 10^7 solar masses within the inner few light days of thenucleus in the Seyfert 1 galaxy NGC 5548. The time-delayed response of theemission lines to continuum variations is used to infer the size of theline-emitting region, and these determinations are combined with measurementsof the Doppler widths of the variable line components to estimate a virialmass. The data for several different emission lines spanning an order ofmagnitude in distance from the central source show the expected V proportionalto r^{-1/2} correlation and are consistent with a single value for the mass.Comment: 9 pages, 2 Figures. accepted by ApJ Letter

Keplerian Motion of Broad-Line Region Gas as Evidence for Supermassive Black Holes in Active Galactic Nuclei