Evidence for a Supermassive Black Hole in the Nucleus of the Seyfert Galaxy NGC 5548
Crenshaw, D.M., and Blackwell, J.H. 1990, ApJ, 358, L37

The international campaign to monitor the variable Seyfert 1 galaxy NGC 5548 with the IUE has provided an extensive and well-sampled set of spectroscopic observations. These observations are used to study the response of the C IV 1550 Å emission-line profile to changes in the photoionizing continuum. Near the end of the IUE campaign, the continuum flux at 1440 Å and the total C IV flux dropped by factors of 2.9 and 1.8, respectively, in 16 days. The red wing of the C IV profile responded more rapidly to the sharp continuum drop than the blue wing, indicating that clouds in the inner broad-line region (BLR) are undergoing gravitational infall. These results provide direct evidence that the central engine is a supermassive object, presumably a black hole, with a mass on the order of 10⁷ solar masses. Analysis of the profile variations also demonstrates that excess emission in the blue wing of C IV is from a component that is physically distinct from the bulk of the BLR.