Too Few to be Relevant: Major Merger Statistics up to z ∼ 1

We study the evolution of galaxy structure and of the major merger fraction since z ∼ 1 to present in GOODS-S field for M_B ≤ -20 and M_* ≥ 10¹⁰ M_sun selected galaxies. We segregate the galaxies into early-type galaxies (ET, E/S0/Sa), and late-type galaxies (LT, Sb-Irr) by their position in the concentration-asymmetry plane, while we pick up high asymmetric sources as disk-disk major merger remnants. We find that the early-type fraction (f_ET) rises with cosmic time in both samples. However, the number density evolution is very different: the decrease in the total number density of M_B ≤ -20 galaxies since z = 1 is due to the decrease in the late-type population, while the increase in the total number density of M_* ≥ 10¹⁰ M_sun in the same redshift range is due to early-type evolution. This suggests that we need a structural transformation between late-type galaxies that form stars actively and early-type galaxies in which the stellar mass is located. Comparing the observed evolution with the disk-disk major merger rate in GOODS-S, we infer that only ∼ 17% of the early-type galaxies that appear since z = 1 can be explained by these kinds of mergers, suggesting that minor mergers and secular evolution are the main processes in the structural evolution of M_* ≥ 10¹⁰ M_sun galaxies since z ∼ 1.