Cellular Automaton experiments on local galactic structure. I. Model assumptions

¹ Institute of Astronomy, Madingley Road, Cambridge CB3 OHA, UK, and Institut d'Astrophysique, 5, Avenue de Cointe, B–4000 Liège, BelgiumPermanent address
² Institut de Physique, Sart–Tilman, B–4000, Liège, Belgium

Send offprint request to: J. Perdang

Received: 27 October 1994
Accepted: 6 March 1996

Abstract

The purpose of the present paper, combined with the companion paper (Lejeune & Perdang 1995, hereinafter Paper II), is to demonstrate that a Cellular Automaton (CA) framework incorporating detailed physical evolutionary mechanisms of the galactic components provides a straightforward approach for simulating local structural features in galaxies (such as those of flocculent spiral galaxies). Conversely, and more important, the observed local irregularities may give information on the relevant timescales of the evolutionary processes operating in these galaxies. In this paper we start out with a critical review of the more standard methods in use in galactic modelling. We insist on the fact that these models do not lend themselves to a straightforward inclusion of both the galactic dynamics and the physical evolution of the galactic components. We show that the Cellular Automaton approach can combine both effects, on condition that the dynamics is approximated by a stationary, in general space–dependent velocity field of the galactic matter. The main part of the paper addresses an extension of the Stochastic Propagating Star Formation scheme originally devised by Mueller & Arnett (1976). The model consists in a multi–state CA specifically designed to deal with the evolutionary behaviour of an off-centre region of a galaxy, of an area of a few . The model incorporates a detailed sequence of in part parametrised stellar evolutionary processes. In the version discussed here it includes as dynamical effects the motions of galactic matter due to a stationary circulation and, to some extent, due to the proper motions of the stars. The model we present is a first nontrivial instance of a CA defined over a lattice lacking geometric symmetries (crystal symmetries of standard CA, or rotational symmetry of the Mueller–Arnett CA). The precise geometry of the CA network of cells is imposed in our model by the space–dependent stationary galactic velocity field. Numerical results are discussed in the companion paper.