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Astron. Astrophys. 322, 73-85 (1997)

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1. Introduction

Many studies of the origin of the excitation of the emission line gas surrounding active galaxy nuclei often aim at confirming photoionization by either counting the number of ionizing photons available (e.g. Binette et al. 1985) or by probing the anisotropy of the nuclear ionizing radiation (Wilson et al. 1994; Acosta-Pulido et al. 1990; Acosta-Pulido 1993) through the observation of structures like the ionization cones. On the other hand most of the objects with extended narrow line regions (ENLR) exhibit radio jets or plasmons (Wilson & Tsvetanov 1994 and references therein) which may contribute to the heating and ionization of the gas as well as to its spatial distribution. Extensive works on the interaction between these radio ejecta (jets, plasmons) and their surrounding gas have followed two radically different approach: on one hand, there were pure hydrodynamical simulations, essentially dedicated to radio-galaxies and addressing the problem of the radio emission (e.g. see Coleman & Bicknell 1985; Higgins et al. 1995; Steffen et al. 1996); on the other hand, there where the planar steady-state shock models intended to model the line ratios and incorporating an exhaustive handling of the ionization, emission and transfer processes (Contini & Aldrovandi 1983, 1986; Viegas-Aldrovandi & Contini 1989; Sutherland et al. 1993, Dopita & Sutherland 1995, 1996). As a result of their incomplete description of the atomic processes, the former do not provide maps of line fluxes, ratios or line profiles to compare with the observations. On the contrary, the latter provide very complete sets of line diagnostics but lack any structural or kinematic information.

Simple models of the interaction of a plasmon and the ISM gas, either in the expansion dominated stage (Pedlar et al. 1985) or in the bowshock dominated one (Taylor, Dyson & Axon 1992, hereafter TDA), have been developed to fill this gap. These models, however, considered only the [O III ] [FORMULA] 5007 line and provided only a few diagnostics (the [O III ] line profile and flux, the shift between radio and [O III ] emissions). To alleviate such limitations, we have undertaken to build a model of extragalactic bowshocks (e.g. such as expected in Seyfert galaxies) based on the hydrodynamical description of TDA but which can handle the presence of magnetic field as well as consider an exhaustive set of atomic processes such as those contained in the multipurpose shock/photoionization code MAPPINGS IC (see description in Appendix A). The pioneering work of TDA is now extended to cover most of the astrophysically interesting elements and lines. Our model includes a time dependent ionization balance, an updated atomic data set and a wide range of ionization processes, all of which has led to a much more reliable computation of the ionization state and line emissivities.

In this paper, we first give a detailed description of our model. We then describe the behavior of a test particle during its evolution, showing the effects of different hydrodynamical assumptions. Last, we illustrate the results produced by the complete model with three examples. The characteristics of MAPPINGS IC are reviewed in Appendix A. Grids of line ratios and profiles for various bowshocks will be presented in a second paper.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998