HISTORY OF GALAXY INTERACTIONS AND THEIR IMPACT ON STAR FORMATION OVER THE LAST 7 Gyr FROM GEMS

Accepted by the Astrophysical Journal We perform a comprehensive estimate of the frequency of galaxy mergers and their impact on star formation over z ∼ 0.24–0.80 (lookback time Tb ∼ 3–7 Gyr) using ∼ 3600 (M ≥ 1 × 10 9 M⊙) galaxies with GEMS HST, COMBO-17, and Spitzer data. Our results are: (1) Among ∼ 790 high mass (M ≥ 2.5 × 10 10 M⊙) galaxies, the visually-based merger fraction over z ∼ 0.24–0.80, ranges from 9% ± 5 % to 8 % ± 2%. Lower limits on the major merger and minor merger fraction over this interval range from 1.1 % to 3.5 % , and 3.6 % to 7.5%, respectively. This is the first, albeit approximate, empirical estimate of the frequency of minor mergers over the last 7 Gyr. Assuming a visibility timescale of ∼ 0.5 Gyr, it follows that over Tb ∼ 3–7 Gyr, ∼ 68 % of high mass systems have undergone a merger of mass ratio> 1/10, with ∼ 16%, 45%, and 7 % of these corresponding respectively to major, minor, and ambiguous ‘major or minor ’ mergers. The average merger rate is ∼ a few ×10 −4 galaxies Gyr −1 Mpc −3. Among ∼ 2840 blue cloud galaxies of mass M ≥ 1.0 ×10 9 M⊙, similar results hold. (2) We compare the empirical merger fraction and merger rate for high mass galaxies to three ΛCDM-based models: halo occupation distribution models, semi-analytic models, and hydrodynamic SPH simulations. We find