  According to the hierarchical model, small galaxies form first and merge
together to form bigger objects. In parallel, galaxies assemble their mass
through accretion from cosmic filaments. Recently, the increased spatial
resolution of the cosmological simulations have emphasised that a large
fraction of cold gas can be accreted by galaxies. In order to compare the role
of both phenomena and the corresponding star formation history, one has to
detect the structures in the numerical simulations and to follow them in time,
by building a merger tree.
