The environment of active galactic nuclei – I. A two-component broad emission line model

Abstract

The flowing interstellar medium in the central parsec of an active nucleus plays a crucial role in determining its observed emission line features. Mass loss from the central stellar cluster acts as one source of material in this flow. Since the flow is likely to be hypersonic, shock waves will be created in it. Gas in thermal equilibrium with the central radiation field and at the stagnation pressure of the flow has an ionization parameter corresponding to that deduced from observations of the high-ionization broad emission lines (HIL). The thermal equilibrium is obtained as the shocked gas cools rapidly by Compton scattering of the central continuum. The entire flow back-scatters X-rays which can illuminate the outer regions of an accretion disc thus providing the deep X-ray heating needed to produce the low-ionization broad emission lines (LIL). The outer region where this disc becomes optically thin radiates most of these LIL and perhaps the observed near-infrared bump. This two-component model resolves the well-known cloud confinement problem by gravitational confinement for the LIL and by advocating a transient cloud population for the HIL. It also leads to a simple explanation for the systematic velocity shifts observed between high- and low-ionization lines.