The Redshift Evolution of Wet, Dry, and Mixed Galaxy Mergers from Close Galaxy Pairs in the DEEP2 Galaxy Redshift Survey

Abstract

We study the redshift evolution of galaxy pair fractions and merger rates for different types of galaxies using kinematic pairs selected from the DEEP2 Redshift Survey, combined with other surveys at lower redshifts. By parameterizing the evolution of the pair fraction as -->(1 + z)m, we find that the companion rate increases mildly with redshift with -->m = 0.41 ± 0.20 for all galaxies with – -->21 m = 1.27 ± 0.35, while the red companion rate of red galaxies is better fitted with the negative slope -->m = − 0.92 ± 0.59. For the chosen luminosity range, we find that at low redshift the pair fraction within the red sequence exceeds that of the blue cloud, indicating a higher merger probability among red galaxies compared to that among the blue galaxies. With further assumptions on the merger timescale and the fraction of pairs that will merge, the galaxy major merger rates for -->0.1 10−3 h3 Mpc −3 Gyr −1 with a factor of 2 uncertainty. At -->z ~ 1.1, 68% of mergers are wet, 8% of mergers are dry, and 24% of mergers are mixed, compared to 31% wet mergers, 25% dry mergers, and 44% mixed mergers at -->z ~ 0.1. Wet mergers dominate merging events at -->z = 0.2–1.2, but the relative importance of dry and mixed mergers increases over time. About 22%-54% of present-day -->L* galaxies have experienced major mergers since -->z ~ 1.2, depending on the definition of major mergers. Moreover, 24% of the red galaxies at the present epoch have had dry mergers with luminosity ratios between $1t{:}t 4$ --> and $4t{:}t 1$ --> since -->z ~ 1. Our results also suggest that the wet mergers and/or mixed mergers may be partially responsible for producing red galaxies with intermediate masses, while a significant portion of massive red galaxies are assembled through dry mergers at later times.