,
Another non-statistical approach is to actually draw Monte Carlo realizations of the merging paths of the halos and follow the merging history of individual halo from high redshift until the present (Kauffmann & White 1993; Cole & Kaiser 1988). During each merging, gas that has not cooled is shock-heated to the virial temperature of the new halo, then cools on to the center of the new halo, which is usually identified with the central galaxy of the largest predecessor. The other galaxies are disrupted of their gas. The star forming history in a galaxy now also depends on the merging history of the surrounding halos.
The more difficult part is to model the merging of individual galaxies, which has been considered to be much less frequent than those of their parent halos. Galaxy merging is more likely to happen if the ratio of their approaching speed and their internal velocity dispersion is less than some critical value. Following the argument in Cole et al. (1994), the ratio is proportional to the ratio of the halo mass and the galaxy mass. The merger time scale can be modeled as:
tmerge=tmerge,0(Mhalo/Mgalaxy),
where > 0 and is, again, a free parameter.
Kauffmann et al. (1993) also considered a merger timescale as the timescale
of the dynamical friction. Similarly in their model, galaxies merge when
(Mgalaxy/Mhalo) exceeds some critical value.
One thing that is lacking in modeling galaxy mergers is to figure out how the merger dynamics can actually induce gas inflows and thus affect the star-forming rate.
References page
Introduction page