6.1. Physical processes
Numerical simulations have enlightened the dynamical processes (Barnes & Hernquist 1992). They represent galaxies as 3-components systems, disks, bulges, and dark haloes. The latter play an important role, although they do not share most of the perturbations; they are essential to provide the dynamical friction that makes the baryonic systems decay and merge. It is the dark matter that takes most of the angular momentum away, allowing the luminous mass to fall towards the center (Barnes 1988).
Gas dissipation is also a key factor in merger simulations. The actual equivalent viscosity of the ISM is not well known, and it is not possible to reproduce the full multiphase medium. Two extreme modelisations are commonly used: the ISM is represented either as a continuous fluid, submitted to pressure forces, shock waves and artificial viscosity (finite difference scheme, SPH), or as colliding clouds, with no pressure forces and no shocks (sticky particles). Both can reproduce the main characteristics of gas flow in galaxies, provided that true viscous torques are negligible. The star-formation rate, global laws and corresponding feed-back are also not known in most perturbed dynamical situations, and present modelisations are rough approximations. Note that the star formation is itself fundamental for the dynamics, since it can lock the gas into the non-dissipative medium and halt for a while the mass transfer towards the center. Since stars are observed to be formed inside giant molecular clouds in our Galaxy, the latter being the result of agglomerations of smaller entities, one process could be to relate star formation to cloud-cloud collisions, in the sticky particles modelisation (Noguchi & Ishibashi 1986). Another more widely used is to adopt a Schmidt law for the SFR, i.e. the rate is proportional to a power n of the gas volumic density, n being between 1 and 2 (Mihos et al 1992). In both cases, it was shown that interacting galaxies were the site of strong starbursts, that could be explained both by the orbit crowding in density waves triggered by the tidal interactions, and by the gas inflow and central concentrations, accumulating the gas in small and very dense regions. This depends of course on the non- linearity of the Schmidt law, and SF-efficiency strongly depends on the power n (see e.g. Mihos et al 1992). Mihos & Hernquist (1994) use a hybrid-particles techniques, within SPH, to describe the effects of gas depletion and formation of a young star population.
Another peculiar feature of merging galaxies is the ability of forming giant complexes, that will soon become dense stellar clusters. This can be understood, given the larger velocity dispersion of perturbed systems. The disks are heated in tidal interactions, since the relative orbital energy is transformed into internal disordered motions. This has the consequence to increase the critical Jeans scale for gravitational instabilities, and to create giant complexes (e.g. Rand 1993). The global Jeans length is 2 / g, where is the velocity dispersion and g is the gas surface density of the galactic disk. The corresponding growth time is ff / g, and the instabilities will occur as soon as Q / g becomes lower than 1. For the same ratio / g, a perturbed system with elevated and g will see the condensations of larger complexes of mass M 4 / g, in the same time-scale . These complexes with larger internal dispersions, and larger gravitational support will be less easy to disrupt through star-formation, which enhances the star-formation efficiency (Elmegreen et al 1993). The thermal Jeans length is also larger, due to the hotter gas temperature induced by the larger number of stars in the clouds, and high mass stars are favored. This explains the existence of giant and dense star clusters observed with HST in merging galaxies (Schweizer & Seitzer 1998, Meurer 1996). These clusters are estimated of masses 107 M, and ages 3 108 yrs. They could evolve in typical globular clusters in about 15 Gyr.