The Merger-Driven Evolution of Massive Galaxies

We explore the rate and impact of galaxy mergers on the massive galaxy population using the amplitude of the two-point correlation function on small scales for M * > 5 × 1010 M sun galaxies from the COSMOS and COMBO-17 surveys. Using a pair fraction derived from the Sloan Digital Sky Survey as a low-redshift benchmark, the large survey area at intermediate redshifts allows us to determine the evolution of the close-pair fraction with unprecedented accuracy for a mass-selected sample: we find that the fraction of galaxies more massive than 5 × 1010 M sun in pairs separated by less than 30 kpc in three-dimensional space evolves as F(z) = (0.0130 ± 0.0019) × (1 + z)1.21± 0.25 between z = 0 and z = 1.2. Assuming a merger timescale of 0.5 Gyr, the inferred merger rate is such that galaxies with mass in excess of 1011 M sun have undergone, on average, 0.5 (0.7) mergers involving progenitor galaxies both more massive than 5 × 1010 M sun since z = 0.6 (1.2). We also study the number density evolution of massive red sequence galaxies using published luminosity functions and constraints on the M/L B evolution from the fundamental plane. Moreover, we demonstrate that the measured merger rate of massive galaxies is sufficient to explain this observed number density evolution in massive red sequence galaxies since z = 1.