Constraints on the merging channel of massive galaxies since z ∼ 1

We probe the merging channel of massive galaxies over the z = 0.3-1.3 redshift window by studying close pairs in a sample of 238 galaxies with stellar mass greater than or similar to 10(11) M-circle dot, from the SHARDS (Survey for High-z Absorption Red and Dead Sources) survey. SHARDS provides medium-band photometry equivalent to low-resolution optical spectra (R similar to 50), allowing us to obtain extremely accurate photometric redshifts (median |Delta z|/(1 + z) similar to 0.55 per cent) and to improve the constraints on the age distribution of the stellar populations. Our data set is volume limited, probing merger progenitors with mass ratios 1: 100 (mu = M-sat/M-cen = 0.01) out to z = 1.3. A strong correlation is found between the age difference of host and companion galaxy and stellar mass ratio, from negligible age differences in major mergers to age differences similar to 4 Gyr for 1: 100 minor mergers. However, this correlation is simply a reflection of the mass-age trend in the general population. The dominant contributor to the growth of massive galaxies corresponds to mass ratios mu greater than or similar to 0.3, followed by a decrease in the fractional mass growth rate linearly proportional to log mu, at least down to mu similar to 0.01, suggesting a decreasing role of mergers involving low-mass companions, especially if dynamical friction time-scales are taken into account. A simple model results in an upper limit for the average mass growth rate of massive galaxies of (Delta M/M)/Delta t similar to 0.08 +/- 0.02 Gyr(-1), over the z less than or similar to 1 range, with an similar to 70 per cent fractional contribution from (major) mergers with mu greater than or similar to 0.3. The majority of the stellar mass contributed by mergers does not introduce significantly younger populations, in agreement with the small radial age gradients observed in present-day early-type galaxies.