Mon. Not. R. Astron. Soc. 311, 565±575 (2000) Hubble Space Telescope imaging of the CFRS and LDSS redshift surveys ± IV. Influence of mergers in the evolution of faint field galaxies from z , 1 O. Le Fe Ávre, 1w R. Abraham, 2 S. J. Lilly, 3 R. S. Ellis, 2 J. Brinchmann, 2 D. Schade, 6 L. Tresse, 1,4 M. Colless, 5 D. Crampton, 6 K. Glazebrook, 7 F. Hammer 8 and T. Broadhurst 9 1 Laboratoire d'Astronomie Spatiale - CNRS, Traverse du Siphon, B.P.8, 13376 Marseille Cedex 12, France 2 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA 3 Department of Astronomy, University of Toronto, 60 St George Street, Toronto, Ontario, M5S 3H8, Canada 4 Istituto di Radioastronomia ± CNR, Via P. Gobetti, 101 40129 Bologna, Italy 5 Mount Stromlo & Siding Spring Observatories, Australian National University, Weston Creek, Canberra, ACT 2611, Australia 6 Dominion Astrophysical Observatory, 5071 West Saanich Road, Victoria, BC, V8X 4M6, Canada 7 Anglo-Australian Observatory, P.O. Box 296, Epping, NSW 1710, Australia 8 DAEC, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France 9 Department of Astronomy, University of California, 601 Campbell Hall, University of California, Berkeley CA 94720-3411, USA Accepted 1999 September 1. Received 1999 September 1; in original form 1998 August 12 ABSTRACT Hubble Space Telescope images of a sample of 285 galaxies with measured redshifts from the Canada±France Redshift Survey (CFRS) and Autofib±Low Dispersion Spectrograph Survey (LDSS) redshift surveys are analysed to derive the evolution of the merger fraction out to redshifts z , 1. We have performed visual and machine-based merger identifications, as well as counts of bright pairs of galaxies with magnitude differences dm # 1:5 mag. We find that the pair fraction increases with redshift, with up to ,20 per cent of the galaxies being in physical pairs at z , 0:75±1. We derive a merger fraction varying with redshift as /1  z 3:2^0:6 , after correction for line-of-sight contamination, in excellent agreement with the merger fraction derived from the visual classification of mergers for which m  3:4 ^ 0:6. After correcting for seeing effects on the ground-based selection of survey galaxies, we conclude that the pair fraction evolves as /1  z 2:7^0:6 . This implies that an average L* galaxy will have undergone 0.8±1.8 merger events from z  1 to z  0, with 0.5 to 1.2 merger events occuring in a 2-Gyr time-span at around z , 0:9. This result is consistent with predictions from semi-analytical models of galaxy formation. From the simple coaddition of the observed luminosities of the galaxies in pairs, physical mergers are computed to lead to a brightening of 0.5 mag for each pair on average, and a boost in star formation rate of a factor of 2, as derived from the average [O ii] equivalent widths. Mergers of galaxies are therefore contributing significantly to the evolution of both the luminosity function and luminosity density of the Universe out to z , 1. Key words: galaxies: evolution ± galaxies: interactions ± cosmology: observations. 1 INTRODUCTION Mergers of galaxies have long been known to play an important role in the evolution of galaxies. Detailed case studies in the local Universe have shown the powerful effect mergers can have on galaxy morphologies and on the triggering of star formation (eg. Kennicutt 1996; Schweizer 1996). The importance of mergers has also been emphasized by computer simulations involving either major mergers of galaxies with comparable mass, or the merger of dwarf galaxies with a more massive galaxy (Mihos & Hernquist 1994a,b; Hernquist & Mihos 1995; Mihos 1995, hereafter M95). If merging is at work at all during the lifetime of galaxies, then the space density, mass, luminosity, and morphology of galaxies must change with epoch. Several authors suggest that merging systems can explain part (or all) of the excess number of galaxies observed in deep photometric galaxy counts (Rocca-Volmerange & Guiderdoni 1990; Broadhurst, Ellis & Glazebrook 1992). Mergers can influence such surveys either by the separate counting of individual systems ultimately destined to merge, or through more subtle effects. For example, dissipative mergers are expected to trigger some degree of star formation, so in q 2000 RAS w E-mail: lefevre@astrsp-mrs.fr