2010A&A...518A..20L

Recent work has shown that major mergers of disc galaxies can only account for ∼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. 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^–1kpc in the sky plane and with a difference Δv≤500km/s in redshift space. 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 N_c with redshift is faster in more luminous samples. We provide N_c of M_*≥10¹⁰M_☉ galaxies, finding that the number including minor companions (N_c^m, mass ratio µ≥1/10) is roughly two times the number of major companions alone (N_c^M, mass ratio µ≥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 ∼1.7 times the major merger rate. Only 30% to 50% of the M_*≥10¹⁰M_☉ 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.