Quasar Emission Line Profile Modelling

Abstract

We present fits of models for the C IV λ 1549, N V λ1240 and L α lines to observed profile data. Since these lines depend sensitively on the cloud ionization parameter (defined as the ratio of the illuminating flux to the cloud pressure; c.f. Krolik, McKee, and Tarter, 1981) and pressure, their relative strengths constrain the distribution of clouds in quasar broad line regions. We make the following assumptions: That the distribution of clouds around the continuum source is spherical and that the cloud motion relative to the continuum source is radial; that the clouds are in pressure equilibrium with hot intercloud medium (ICM) whose mass flux is conserved; and that the line fluxes emitted by the clouds as a consequence of the continuum illumination is given by “standard” (e.g. Kwan and Krolik, 1981) photoionization models. The cloud mass flux is not assumed to be constant, but is allowed to vary with velocity in order to fit the data. For the purposes of illustration, we have chosen a specific dynamical model: isothermal freefall with T ICM = 10 8 K. The cloud column density is assumed to be fixed at 10 23 cm −2 .