The double broad-line emitting regions in NGC 5548 as possible evidence for a supermassive binary

Abstract

The Seyfert 1 galaxy NGC 5548 is shown to have two distinct broad-line emitting regions at different radial velocities. The more luminous region is redshifted relative to the galaxy's systemic velocity by about 275 km/s and undergoes flux changes that are correlated with the continuum variability with a lag of 15 + or - 9.4 days. The weaker component is blueshifted relative to the systemic velocity by about 2475 km/s, it also varies with time, but the variability of this region is uncorrelated with the continuum variability. Although there is at this time no direct evidence that the components are gravitationally bound, it is suggested that these observations might be explained by a supermassive binary system, with each binary component having its own associated broad-line region. The period of this system is expected to be at least of order 110 yr, and the semimajor axis of the orbit must be at least 10 to the 17th/sin i cm (i.e., 48/sin i lt-days); these parameters lead to total masses for the system which are consistent with other estimates.