On Nulling Interferometers and the Line-emitting Regions of Active Galactic Nuclei

The nulling interferometers proposed to study planets around other stars aregenerally well suited for studying small-scale structures surrounding otherbright pointlike objects such as the nuclei of active galaxies. Conventionalinterferometric techniques will produce useful maps of optical/IR line andcontinuum emission within active galaxies on scales of 10 milliarcseconds(mas), but similar studies of broad-line regions will require baselines longerthan those currently envisaged. Nevertheless, nulling interferometers currentlyunder development will be able to constrain quasar velocity fields onmilliarcsecond scales, as long as they are equipped with spectrographs capableof resolving lines several hundred km/s wide. This Letter describes howanalyses of line emission leaking through the edges of the null in such aninstrument can reveal the size, shape, and velocity field of nebular gas on theoutskirts of a quasar broad-line region. If this technique proves effective, itcould potentially be used to measure the mass function of quasar black holesthroughout the universe.Comment: Latex, 9 pages, 2 figures, to appear in 1 October ApJ Letter