INTRINSIC FORMATION OF NARROW-LINE ABSORPTION SYSTEMS IN QSOS .1. THE PISTON PHASE

This paper presents an intrinsic model for the formation of narrow absorption-line systems in QSO spectra. The model supposes that a wind from a central source interacts with surrounding enriched material. The wind acts like a piston and sweeps up ambient gas. Initially, the swept-up material is too hot to form absorption lines. It is shown that at some characteristic time catastrophic radiative cooling occurs in this material which then condenses into a thin cool shell. The observed absorption lines originate in this shell. The model contains only two free parameters, n0 the ambient gas density, and Ė the mechanical power output in the wind. The gross features (radial distance from the central source, column density in neutral hydrogen, ionization state, etc.) derived from this model are in encouraging accord with the observations of 3C 191. Further they are insensitive to the choice of n0 and Ė. The model predicts a minimum velocity (~ 0.001c relative to the source) at which intrinsically produced absorption lines should be seen. This minimum velocity is extremely insensitive to n0 and Ė. It is shown that the cool shell will then be further accelerated by resonance-line absorption. The dynamics of the model require a high degree of spherical symmetry in the gas distribution about the central source. It therefore appears plausible to associate this formation mechanism with gas in elliptical or young (high z) uncollapsed galaxies. © Royal Astronomical Society.