Cellular Automaton experiments on local galactic structure. II. Numerical simulations

Abstract

This paper is a step towards demonstrating that the multi{parameter Cellular Automaton framework designed for the simulation of local galactic structure, developed in a companion paper (Perdang & Lejeune 1996, Paper I), is capable of duplicating the local irregularities observed in the structure of flocculent spiral galaxies. The numerical simulations exhibit the development of fractured galactic arms and the formation of fractal geometries associated with the matter distribution (fractal structure of the arms, of bulk dimension 1.7 and border dimension 1.3; distribution of dierent stellar components on fractal supports, of dimension 1.6, for reasonable estimates of the free model parameters). The prediction of fractional values for the dierent dimensions specifying the simulated structures can be exploited as a qualitative test of adequacy of the proposed model. The precise quantitative values of the observed dimensions, in conjunction with the observable global mass fractions of the dierent galactic components, play the parts of constraints for the free model parameters. We show that the currently theoretically inaccessible values of the free parameters of the formulation can be recovered from observation. In this Paper we report on an extended series of nu- merical experiments carried out in the framework of a 2{dimensional Cellular Automaton (CA) model for local galactic structure described in the companion paper (Pa- per I). The model takes account of stellar evolutionary eects not dealt with in current CA models, as well as of quasi-stationary motions more complex than usual dier- ential rotation. Our results are indicative that the floc- cular character of galaxies can be simulated in the CA framework. For technical details of the implementation of the galactic physics we refer to Paper I. Our results are given in a relative reference frame F in uniform rotation with respect to the galaxy to be simulated.