Dynamical consequences of CDM merger trees

Within the standard structure formation scenario, galaxies form as the result of an extended merger process. The first structures to form are small bound objects occurring at z ∼ 10, which then merge repeatedly to form increasingly larger objects. In this context, elliptical galaxies are generally seen as the result of a major merger of spiral systems. Through extensive SPH simulations of merging spirals, we have explored these processes with the aim of quantifying their relaxation times. This is important, as it sets a minimum time interval between the onset of a merger, and the appearance of an elliptical galaxy. We then compare this constraint with predictions of the hierarchical scenario, computed through Press-Schechter merger trees. We find evidence for small elliptical systems which appear not to have been formed by a recent merger process.