DYNAMICAL MODELS OF EMISSION-LINE GAS IN RADIO GALAXIES Tidal Interactions / Merger Scenarios A. M. KOEKEMOER Institut d Astrophysique de Paris 98bis Bd. Arago, 75014 Paris, France AND G. V. BICKNELL ANU Astrophysical Theory Centre Australian National University, Canberra, ACT 0200, Australia 1. Introduction Extended emission-line regions (EELRs) in radio ellipticals are generally thought to trace gas acquired externally, eg. through interaction with a gas-rich disk galaxy (Athanassoula and Bosma 1985, Barnes and Hern- quist 1992, Hernquist and Mihos 1995). We examine here the dynamical evolution of gas in mergers, focussing on the conditions required for colli- sions between streams of gas. We find that such collisions can occur over a relatively wide range of encounter geometries, producing large-velocity- amplitude kinematic signatures characteristic of those observed in EELRs. This is relevant to the formation of shocks, which can account for the ion- ization properties of EELRs (Koekemoer and Bicknell, this conference). 2. Model Results The evolution of a gas-rich galaxy encountering a large spherical system was modelled using Smoothed Particle Hydrodynamics (SPH, Koekemoer and Bicknell 1995); the results presented here are for a spherical halo mass M = 1O 12 M0, velocity dispersion = 250 km s" 1 , and core radius r c = lkpc. A gas disk of mass 1O 9 M0, radius 5 kpc and temperature 5000 (warm, partially ionized ISM) was settled in an isothermal halo with M = 5 10 1 , = 100 km s -1 , r c = 0.5 kpc. We investigated pericentric distances in the range r p = 2 — 10 kpc, pericentric velocities v p 471 R. Ekers et al. (eds.), Extragalactic Radio Sources, 471-472. 1996 IAU. Printed in the Netherlands. available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0074180900081523 Downloaded from https://www.cambridge.org/core. IP address: 172.245.147.148, on 14 Jun 2019 at 01:23:50, subject to the Cambridge Core terms of use,