Constraints on the merging channel of massive galaxies since z~1
  (Abridged) 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 >1E11Msun, from the deep (m<26.5AB, 3 sigma) SHARDS survey.
SHARDS provides medium band photometry equivalent to low-resolution optical
spectra (R~50), allowing us to obtain extremely accurate photometric redshifts
(|Dz|/(1+z)~0.55%) and to improve the constraints on the age distribution of
the stellar populations. A strong correlation is found between the age
difference of central and satellite galaxy and stellar mass ratio, from
negligible age differences in major mergers to age differences ~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=Msat/Mcen>0.3, followed by a
decrease in the fractional mass growth rate linearly proportional to log mu, at
least down to mu~0.01, suggesting a decreasing role of mergers involving
low-mass satellites, especially if dynamical friction timescales are taken into
account. A simple model results in an upper limit for the average mass growth
rate of massive galaxies of DM/M/Dt~ 0.08+-0.02 per Gyr, over the z<1 range,
with a ~70% fractional contribution from (major) mergers with mu>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.
