The Dominant Role of Mergers in the Size Evolution of Massive Galaxies since z ∼ 1

We estimate the merger rate, both major (stellar mass ratio it mu M-star,2/M-star,1 >= 1/4) and minor (1/10 <= mu 1/4), of massive (M-star >= 10(11) M-circle dot) early-type galaxies (ETGs) in the COSMOS field by close pairs statistics. The merger rate of massive ETGs evolves as a power-law (1+z)(n), showing the minor merger little evolution with redshift, n(mm) similar to 0, in contrast with the increase of major mergers, n(MM) = 1.8. Our results shows 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%. In addition, it 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 20%. The remaining similar to 5% should come from other processes (e.g., adiabatic expansion or observational effects). These results suggest that mergers are the main contributor to the size evolution of massive ETGs, accounting for similar to 55%-75% of that evolution in the last 8 Gyr. Nearly half of this merging evolution is related with minor (mu < 1/4) events.