Abstract
How do galaxies form? One of the first hypotheses was to imagine a sudden or progressive collapse of a single system on itself, which would rapidly form stars, resulting in an elliptical galaxy, or a halo of old stars and a disk of gas and young stars, depending on the relative speed of collapse and star formation. Another scenario, the hierarchical scenario, which is favoured today, consists in thinking that the first galaxies are all disks (because of the gas), and that the merging of galaxies cancels out the angular momentum and destroys the disks, to form early-type galaxies. Add to this the fact that most baryons are still outside galaxies, so much matter is accreted from cosmic filaments, even today.
To quantify the hierarchical scenario, we need to be able to accurately estimate the dynamic friction that causes galaxies to merge together. The process is delicate and chaotic, especially friction at a distance. And Chandrasekhar's formula, widely used for analytical estimation, is often not valid. We must try to estimate friction with numerical simulations, but these are generally far from reality, due to limited spatial and mass resolutions.
Friction calculations can be used to estimate the speed of bulge formation by mergers, the speed of bars in dark matter halos, the lifetime of compact groups of galaxies, depending on the distribution of dark matter, and so on. We can see that the main friction is due to dark matter, and yet this is the most unknown component. This may help constrain dark matter models. At present, the standard cold dark matter model (CDM) has difficulty explaining the high frequency of bulge-free galaxies around us.
