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

Abstract

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 >∼ 1011M⊙, from the SHARDS survey. SHARDS provides medium band photometry equivalent to low-resolution optical spectra (R∼50), allowing us to obtain extremely accurate photometric redshifts (median |∆z|/(1 + z) ∼ 0.55%) and to improve the constraints on the age distribution of the stellar populations. Our dataset is volume-limited, probing merger progenitors with mass ratios 1:100 (µ ≡ Msat/Mcen = 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 ∼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 µ >∼ 0.3, followed by a decrease in the fractional mass growth rate linearly proportional to log µ, at least down to µ ∼ 0.01, suggesting a decreasing role of mergers involving lowmass companions, 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 (∆M/M)/∆t ∼ 0.08 ± 0.02 Gyr−1 , over the z ∼ 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.