Spectro-photometric close pairs in GOODS-S: major and minor companions of intermediate-mass galaxies

Aims. Recent work has shown that major mergers of disc galaxies can only account for similar to 20% of the growth of the galaxy red sequence between z = 1 and z = 0. Our goal here is to provide merger frequencies that encompass both major and minor mergers, derived from close pair statistics. We aim to show that reliable close pair statistics can be derived from galaxy catalogues with mixed spectroscopic and photometric redshifts. Methods. We use B-band luminosity-and mass-limited samples from a Spitzer/IRAC-selected catalogue of GOODS-S. We present a new methodology for computing the number of close companions, N-c, when spectroscopic redshift information is partial. The methodology extends the one used in spectroscopic surveys to make use of photometric redshift information. We select as close companions those galaxies separated by 6 h(-1) kpc < r(p) < 21 h(-1) kpc in the sky plane and with a difference Delta v <= 500 km s(-1) in redshift space. Results. We provide N-c for four different B-band-selected samples. It increases with luminosity, in good agreement with previous estimations from spectroscopic surveys. The evolution of Nc with redshift is faster in more luminous samples. We provide Nc of M-star >= 1010 M-circle dot galaxies, finding that the number including minor companions (N-c(m), mass ratio mu = 1/10) is roughly two times the number of major companions alone (N-c(M), mass ratio mu = 1/3) in the range 0.2 = z < 1.1. We compare the major merger rate derived by close pairs with the one computed by morphological criteria, finding that both approaches provide similar merger rates for field galaxies when the progenitor bias is taken into account. Finally, we estimate that the total (major+minor) merger rate is similar to 1.7 times the major merger rate. Conclusions. Only 30% to 50% of the M-star = 1010 M-circle dot early-type (E/S0/Sa) galaxies that appear between z = 1 and z = 0 may have undergone a major or a minor merger. Half of the red sequence growth since z = 1 is therefore unrelated to mergers.