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Astron. Astrophys. 327, 921-929 (1997)

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1. Introduction

In recent years, evolution of galaxy clusters has attracted ever more attention. On the one hand, it has become accessible for observations, both with ground-based and space instruments. On the other hand, discovery of Butcher-Oemler effect testifies to an epoch of fast evolution in clusters, associated with galaxy interaction. Mergers of galaxies 1are considered to be one of the most probable explanations for the change of colour which accompanies appearance of ellipticals and lenticulars instead of early-type spirals at [FORMULA] (see, e.g., Dressler et al. 1994 and cited there). Comparatively fast evolution of clusters and groups, caused by mergers, is also confirmed by the results of direct numerical simulation (see, e.g., Barnes 1996).

One of the effects associated with mergers is rapid evolution of the galaxy mass and angular momentum distribution function [FORMULA], related to appearance of massive galaxies. Calculation of [FORMULA] is of great interest both by itself and in the context of a merger model of activity suggested by Kats & Kontorovich (1990, 1991). Given [FORMULA], one can predict the luminosity function of active galactic nuclei in this model.

The mass function of merging galaxies [FORMULA] can be described in terms of the Smoluchowski kinetic equation. As Cavaliere et al. (1991, 1992) and, independently, Kontorovich et al. (1992), Kats et al. (1992) have shown, mergers result in an analog of phase transition (or "explosive evolution"), which implies fast formation of a distribution tail (corresponding to massive galaxies) and a "new phase": cD-galaxies 2.

The joint distribution function which takes into consideration both mass and angular momentum can be described by a generalized Smoluchowski equation (see Kats & Kontorovich 1990, 1992, where this equation was solved in the simplest case of a constant merger probability - an analog of phase transition does not take place in this variant - and without allowance for the orbital angular momentum). In this paper we present the results of simulation of galaxy mass and momentum evolution in clusters, with the orbital momentum and more realistic mass dependence of the merger probability taken into account.

In Sect. 2 we discuss Monte Carlo simulation of mergers. In Sect. 3 we compare the results for [FORMULA] with a direct numerical solution of the Smoluchowski equation. Sect. 4 contains discussion of the results.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998