To study accretion, a broad range of scales must be taken into account : from less than 3000 light-years, to distinguish between accretion from the diffuse medium and merger with a dwarf galaxy, to more than 30 million light-years so that the simulated cube represents a representative fraction of the universe without the results being biased by the periodic conditions imposed on the system. To this end, was developed the N-body numerical technique called "multi-zoom" to cover the necessary range of scales in a reasonable computing time. The results of this technique are illustrated on Figure 1. The principle is to realize successively, a series of simulations in cubes whose size is divided by 2 at each step, and where the mass resolution is increased by 8 at each stage. In the simulation n, is recorded at each time step the tidal gravitational field created in the zone to be treated in simulation n+1 by the matter located outside this zone. The matter flow entering this zone is also recorded. The same data recorded at the stage n-1 are used to take into account the influence of the matter located outside the simulation n zone. At the first step n=1, periodic conditions are applied to the system. This procedure permits to apply the maximum resolution only in a restricted fraction of the initial volume, in a zone where a galaxy or a galaxy cluster is formed. The multi-zoom method is coupled to a dynamical N-body code taking into account dark matter, two gas phases (hot-diffuse and cold with fractal structure) and a stellar phase as well as matter and energy exchanges between these phases. The calculation of the gravitational interactions, of the star formation, the cooling/heating of the gas allows a detailed study of gas accretion in galaxy formation.
While zooming on 4 levels in various areas of the initial simulation cube (60 million light-years) 10 galaxies could be isolated, containing from 3.7 1010 to 1.9 1012 solar masses ( 5 103 to 2.5 105 particles). The growth of these galaxies was followed step by step, which allowed to show that gas accretion provides a larger fraction of the final mass (45% to 95%) then mergers with other galaxies. The accretion rate, from 1 to 100 solar masses per year, is comparable to the rate of star formation : the accretion fills the galactic cold gas reservoir which fuels the stellar formation. Finally accretion maps for each galaxy could be traced in galactocentric co-ordinates. The analysis of these maps (see Figure 2) reveals that the geometry of accretion is rather variable. Certain trends however emerge. The accretion is not isotropic, it is prevalent in the galactic plane between 150 and 300 000 light-years from the galactic center. This prevalence is more marked for gas than for dark matter. This directivity of accretion will have a consequence on the rate of rotation and the size of the discs formed.