Hubble Space Telescope imaging of the CFRS and LDSS redshift surveys - IV.Influence of mergers in the evolution of faint field galaxies from z similar to 1

Hubble Space Telescope images of a sample of 285 galaxies with measured red shifts from the Canada-France Redshift Survey (CFRS) and Autofib-Low Disper sion Spectrograph Survey (LDSS) redshift surveys are analysed to derive the evolution of the merger fraction out to redshifts z similar to 1. We have performed visual and machine-based merger identifications, as well as count s of bright pairs of galaxies with magnitude differences delta m less than or equal to 1.5 mag. We find that the pair fraction increases with redshift , with up to similar to 20 per cent of the galaxies being in physical pairs at z similar to 0.75-1. We derive a merger fraction varying with redshift as proportional to(1+z)(3.2 +/- 0.6), after correction for line-of-sight co ntamination, in excellent agreement with the merger fraction derived from t he visual classification of mergers for which m=3.4 +/- 0.6. After correcti ng for seeing effects on the ground-based selection of survey galaxies, we conclude that the pair fraction evolves as proportional to(1+z)(2.7 +/- 0.6 ). This implies that an average L* galaxy will have undergone 0.8-1.8 merge r 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 similar to 0.9. This result is consistent with predi ctions from semi-analytical models of galaxy formation. From the simple coa ddition of the observed luminosities of the galaxies in pairs, physical mer gers are computed to lead to a brightening of 0.5 mag for each pair on aver age, and a boost in star formation rate of a factor of 2, as derived from t he average [O II] equivalent widths. Mergers of galaxies are therefore cont ributing significantly to the evolution of both the luminosity function and luminosity density of the Universe out to z similar to 1.