Consistency of spherical, gravity-dominated dynamics with quasar high-ionization emission-line profiles

Abstract

Active galactic nucleus (AGN) emission-line profiles contain much information about the kinematics of the emission-line region. However, despite the large existing spectral data base, relatively little has been done to exploit this potential source of information fully because all existing analytic methods are strongly model-dependent and often involve large numbers of free parameters. In this paper we use line profile data to test a simple kinematic model-spherically symmetric gravitational free fall-in which we limit the number of free parameters by requiring physical self-consistency. The predictions of this model are fitted to high-resolution (1 A) spectra of the stronger rest-frame ultraviolet emission lines in 12 quasars with z ~ 2. We find that if all the lines are radiated predominantly from the illuminated faces of the emission- line clouds, the profiles of Lα, N V λ1240, and C IV λ1549 can be simultaneously well fitted with very similar parameters for all 12 quasars. However, three points cast doubt on whether this model actually describes the broad emission line regions of these quasars: in some cases, there are no parameter choices which lead to acceptable fits if C III] λ1909 is added to the list of predicted line profiles; the best-fit parameters found for the fits to Lyα, N V λ1240, and C IV λ1549 are likely not to permit acceptable fits to other lines, once comparable profile data for them become available; finally, our results imply gravitational masses comparable to the most massive known galaxies, together with accretion rates which are likely to exceed known supply mechanisms. We therefore believe that spherically symmetric gravitational free fall does not correctly describe the dynamics of quasar broad emission line regions. Similar efforts applied to other dynamical scenario are likely to result in similarly severe constraints.