The dominant role of mergers in the size evolution of massive early-type galaxies since z ∼ 1

Aims. The role of galaxy mergers in massive galaxy evolution, and in particular to mass assembly and size growth, remains an open question. In this paper we measure the merger fraction and rate, both minor and major, of massive early-type galaxies (M-star >= 10(11) M-circle dot) in the COSMOS field, and study their role in mass and size evolution. Methods. We used the 30-band photometric catalogue in COSMOS, complemented with the spectroscopy of the zCOSMOS survey, to define close pairs with a separation on the sky plane 10 h(-1) kpc <= r(p) <= 30 h(-1) kpc and a relative velocity Delta v <= 500 km s(-1) in redshift space. We measured both major (stellar mass ratio mu = M-star,M-2/M-star,M-1 >= 1/4) and minor (1/10 <= mu < 1/4) merger fractions of massive galaxies, and studied their dependence on redshift and on morphology (early types vs. late types). Results. The merger fraction and rate of massive galaxies evolves as a power-law (1 + z)(n), with major mergers increasing with redshift, n(MM) = 1.4, and minor mergers showing little evolution, n(mm) similar to 0. When split by their morphology, the minor merger fraction for early-type galaxies (ETGs) is higher by a factor of three than that for late-type galaxies (LTGs), and both are nearly constant with redshift. The fraction of major mergers for massive LTGs evolves faster (n(MM)(LT) similar to 4) than for ETGs (n(MM)(ET) = 1.8). Conclusions. Our results show that massive ETGs have undergone 0.89 mergers (0.43 major and 0.46 minor) since z similar to 1, leading to a mass growth of similar to 30%. We find that mu >= 1/10 mergers can explain similar to 55% of the observed size evolution of these galaxies since z similar to 1. Another similar to 20% is due to the progenitor bias (younger galaxies are more extended) and we estimate that very minor mergers (mu < 1/10) could contribute with an extra similar to 20%. The remaining similar to 5% should come from other processes (e. g., adiabatic expansion or observational effects). This picture also reproduces the mass growth and the velocity dispersion evolution of these galaxies. We conclude from these results, and after exploring all the possible uncertainties in our picture, that merging is the main contributor to the size evolution of massive ETGs at z less than or similar to 1, accounting for similar to 50-75% of that evolution in the last 8 Gyr. Nearly half of the evolution due to mergers is related to minor (mu < 1/4) events.