PREDICTING DARK-MATTER HALO EVOLUTION WITH A MODIFIED PRESS-SCHECHTER MODEL
The conventional Press-Schechter (PS) model for halo clustering evolution (see, e.g., Lacey \& Cole~\cite{lc}) treats any halo merger as equally significant when defining halo formation and destruction. The usual definition states that a halo of mass $M$ at $t$ was formed when half of its mass was assembled in a single halo, while its destruction time is defined as the time when it doubles its mass. This does not take into account that the halo mass could change either by a single major merger or by the continuous accretion of tiny halos. A consistent definition of halo formation time should rely on the relevance of the merger event experienced by the halo. In this sense, only major mergers can produce a substantial rearrangement of the halo internal structure and, hence, they are responsible of the formation of new halos and the destruction of the old ones. A modified PS clustering model has been developed in Salva\-dor-Sol\'e {\it et al.}~\cite{ssm} and Raig {\it et al.}~\cite{rgs} which is based on the distinction between halo major mergers and captures of small-mass halos. The latter are regarded as contributing to mass accretion. In the present work, the predictions of such modified PS model are compared with the results from N-body simulations of scale-free universes. A new semi-analytic model of galaxy formation based on the modified PS model is presently under construction in Barcelona.