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Astron. Astrophys. 353, 92-96 (2000)

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1. Introduction

The redshift dependence of the interaction and merger rate is an important test of the current models for the formation and evolution of galaxies. It is now well established that galaxy interactions play a major role in galaxy formation and evolution (e.g. Schweizer 1998, Combes 1999 for recent reviews). Toomre (1977) demonstrated that the universe's higher density in the past ( [FORMULA](1+z)3) suggests a higher past merger rate, increasing back in time as [FORMULA] (with time t) if the binding energies of binary galaxies had a flat distribution. If the galaxy merger rate is parametrized in the power-law form [FORMULA], then the exponent has been found to be [FORMULA] from Toomre's (1977) approach (assuming [FORMULA]). Statistics of close galaxy pairs from faint-galaxy redshift surveys (e.g. Yee & Ellingson 1995, Le Fevre et al. 1999) and morphological studies of distant galaxies support a large value of the exponent m for [FORMULA]. For instance, Abraham (1998) concluded that current best estimates for the merger rate are consistent with [FORMULA]. Preliminary studies of distant peculiar objects representing distinct results of interactions/mergers (collisional ring galaxies, polar-ring galaxies, mergers) also support [FORMULA] (Lavery et al. 1996, Reshetnikov 1997, Remijan et al. 1998, Le Fevre et al. 1999), although statistics are still insufficient. Many other surveys, including IRAS faint sources, or quasars, have also revealed a high power-law (e.g. Warren et al. 1994, Springel & White 1998). However, some recent works have suggested a moderate ([FORMULA]) (e.g. Neuschaefer et al. 1997, Wu & Keel 1998) or intermediate ([FORMULA]) (Burkey et al. 1994, Im et al. 1999) density evolution of merging systems with z.

From analytical formulation of merging histories (e.g. Carlberg 1990, 1991; Lacey & Cole 1993), it is possible to relate the dark haloes merger rate to the parameters of the universe (average density [FORMULA], cosmological constant [FORMULA]). The merging rates for visible galaxies should follow, although the link is presently not well known (Carlberg 1990, Toth & Ostriker 1992). Theoretical models based on Press-Schechter formalism (Carlberg 1990, 1991) predict a redshift evolution of the merger rate with [FORMULA] (the exponents must be somewhat changed if the average halo mass decreases with z - Carlberg et al. 1994). This conclusion is confirmed by numerical simulations within the CDM scenario - [FORMULA] ([FORMULA]) and [FORMULA] ([FORMULA]) for [FORMULA] (Governato et al. 1997).

Tidal tails originate in close encounters of disk galaxies (e.g. Toomre & Toomre 1972). The purpose of this note is to show that statistics of galaxies with extended tidal tails (tidal bridges have, on average, fainter surface brightnesses - Schombert et al. 1990) is a useful tool to study evolution of interaction rate to [FORMULA]. The simulations by Hibbard & Vacca (1997) - they showed that extended tidal features remain readily visible in the long exposures typical of the Hubble Deep Fields out to [FORMULA] - is the theoretical base for our work. We found that current statistics of such objects in the North and South Hubble Deep Fields (HDF-N and HDF-S correspondingly) leads to [FORMULA]. (Preliminary results based on the HDF-N only are presented in Reshetnikov 1999 - Paper fI.)

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© European Southern Observatory (ESO) 2000

Online publication: December 8, 1999