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

摘要

Aims. 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. 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 10~(10) M? 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 10~(10) M? 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.