Time development of the emission lines and continuum of NGC 4151

The Seyfert galaxy NGC 4151 was observed spectrophotometrically at least once a month from May 1980 through July 1981. The broad optical emission lines all responded to changes in the optical nonstellar continuum, including lines formed in both the H II and extended ionization regions. Minimum line response and decay times are definitely less than one month and probably less than one week, but some of the line flux responded gradually over the course of the monitoring period. Thus, most of the flux arises in regions small in light travel distance while a significant fraction lies at about one light-year from the central source. The observed anticorrelation of the Balmer decrement with continuum flux is qualitatively consistent with modern theories of the broad-line region.