Observational Constraints on the Merger History of Galaxies since z ≈ 6: Probabilistic Galaxy Pair Counts in the CANDELS Fields

Galaxy mergers are expected to have a significant role in the mass assembly of galaxies in the early universe, but there are very few observational constraints on the merger history of galaxies at z > 2. We present the first study of galaxy major mergers (mass ratios <1:4) in mass-selected samples out to z ≈ 6. Using all five fields of the Hubble Space Telescope/CANDELS survey and a probabilistic pair-count methodology that incorporates the full photometric redshift posteriors and corrections for stellar mass completeness, we measure galaxy pair-counts for projected separations between 5 and 30 kpc in stellar mass selected samples at 9.7 < log₁₀(M _∗/M _o) < 10.3 and log₁₀(M _∗/M _o) > 10.3. We find that the major merger pair fraction rises with redshift to z ≈ 6 proportional to (1 + z)^m, with m = 0.8 ±; 0.2 (m = 1.8 ±; 0.2) for log₁₀(M _∗/M _o) > 10.3 (9.7 < log₁₀(M _∗/M _o) < 10.3). Investigating the pair fraction as a function of mass ratio between 1:20 and 1:1, we find no evidence for a strong evolution in the relative numbers of minor to major mergers out to z < 3. Using evolving merger timescales, we find that the merger rate per galaxy () rises rapidly from 0.07 ±; 0.01 Gyr^-1 at z < 1 to 7.6 ±; 2.7 Gyr^-1 at z = 6 for galaxies at log₁₀(M _∗/M _o) > 10.3. The corresponding comoving major merger rate density remains roughly constant during this time, with rates of Γ ≈ 10^-4 Gyr^-1 Mpc^-3. Based on the observed merger rates per galaxy, we infer specific mass accretion rates from major mergers that are comparable to the specific star formation rates for the same mass galaxies at z > 3 - observational evidence that mergers are as important a mechanism for building up mass at high redshift as in situ star formation.