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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 ⋆ ≥ 1011 Mʘ) 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 ≤ rp ≤ 30 h-1 kpc and a relative velocity Δv ≤ 500 km s-1 in redshift space. We measured both major (stellar mass ratio μ ≡ M ⋆ ,2/M ⋆ ,1 ≥ 1/4) and minor (1/10 ≤ μ < 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, nMM = 1.4, and minor mergers showing little evolution, nmm ~ 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 (nMMLT ~ 4 ) than for ETGs (nMMET= 1.8). Conclusions. Our results show that massive ETGs have undergone 0.89 mergers (0.43 major and 0.46 minor) since z ~ 1, leading to a mass growth of ~30%. We find that μ ≥ 1/10 mergers can explain ~55% of the observed size evolution of these galaxies since z ~ 1. Another ~20% is due to the progenitor bias (younger galaxies are more extended) and we estimate that very minor mergers (μ < 1/10) could contribute with an extra ~20%. The remaining ~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 ≲ 1, accounting for ~50−75% of that evolution in the last 8 Gyr. Nearly half of the evolution due to mergers is related to minor (μ < 1/4) events. [ABSTRACT FROM AUTHOR]

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⋆ ⩾ 1011 M☉) 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 ⩽ rp ⩽ 30 h-1 kpc and a relative velocity Δv ⩽ 500 km s-1 in redshift space. We measured both major (stellar mass ratio μ ≡ M⋆,2/M⋆,1 ⩾ 1/4) and minor (1/10 ⩽ μ < 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, nMM = 1.4, and minor mergers showing little evolution, nmm ∼ 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 (nLTMM ∼ 4) than for ETGs (nETMM = 1.8). Conclusions. Our results show that massive ETGs have undergone 0.89 mergers (0.43 major and 0.46 minor) since z ∼ 1, leading to a mass growth of ∼30%.We find that μ ⩾ 1/10 mergers can explain ∼55% of the observed size evolution of these galaxies since z ∼ 1. Another ∼20% is due to the progenitor bias (younger galaxies are more extended) and we estimate that very minor mergers (μ < 1/10) could contribute with an extra ∼20%. The remaining ∼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 ⋆ 1, accounting for ∼50-75% of that evolution in the last 8 Gyr. Nearly half of the evolution due to mergers is related to minor (μ < 1/4) events [ABSTRACT FROM AUTHOR]

Context. Studying the evolution of galaxies located within groups may have important implications for our understanding of the global evolution of the galaxy population as a whole. The fraction of galaxies bound in groups at z ~ 0 is as high as 60% and therefore any mechanism (among the many suggested) that could quench star formation when a galaxy enters group environment would be an important driver for galaxy evolution. Aims. Using the group catalog obtained from zCOSMOS spectroscopic data and the complementary photometric data from the COSMOS survey, we explore segregation effects occurring in groups of galaxies at intermediate/high redshifts. Our aim is to reveal if, and how significantly, group environment affects the evolution of infalling galaxies. Methods. We built two composite groups at intermediate (0.2 ⩽ z ⩽ 0.45) and high (0.45 < z ⩽ 0.8) redshifts, and we divided the corresponding composite group galaxies into three samples according to their distance from the group center. The samples roughly correspond to galaxies located in a group's inner core, intermediate, and infall region. We explored how galaxy stellar masses and colors - working in narrow bins of stellar masses - vary as a function of the galaxy distance from the group center. Results. We found that the most massive galaxies in our sample (log(Mgal/M☉) ⩾ 10.6) do not display any strong group-centric dependence of the fractions of red/blue objects. For galaxies of lower masses (9.8 ⩽ log(Mgal/M☉) ⩽ 10.6) there is a radial dependence in the changing mix of red and blue galaxies. This dependence is most evident in poor groups, whereas richer groups do not display any obvious trend of the blue fraction. Interestingly, mass segregation shows the opposite behavior: it is visible only in rich groups, while poorer groups have a a constant mix of galaxy stellar masses as a function of radius. Conclusions. These findings can be explained in a simple scenario where color- and mass-segregation originate from different physical processes. While dynamical friction is the obvious cause for establishing mass segregation, both starvation and galaxy-galaxy collisions are plausible mechanisms to quench star formation in groups at a faster rate than in the field. In poorer groups the environmental effects are caught in action superimposed to secular galaxy evolution. Their member galaxies display increasing blue fractions when moving from the group center to more external regions, presumably reflecting the recent accretion history of these groups. [ABSTRACT FROM AUTHOR]