History of galaxy interactions and their impact on star formation over the last 7 Gyr from GEMS

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 T b~ 3-7 Gyr) using ~3600 (M>= 1 × 109 M sun) galaxies with GEMS Hubble Space Telescope, COMBO-17, and Spitzer data. Our results are as follows. (1) Among ~790 high-mass (M>= 2.5 × 1010 M sun) 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 T b~ 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 × 109 M sun, similar results hold. (2) We compare the empirical merger fraction and merger rate for high-mass galaxies to three Lambda cold dark matter-based models: halo occupation distribution models, semi-analytic models, and hydrodynamic SPH simulations. We find qualitative agreement between observations and models such that the (major+minor) merger fraction or rate from different models bracket the observations, and show a factor of 5 dispersion. Near-future improvements can now start to rule out certain merger scenarios. (3) Among ~3698 M>= 1.0 × 109 M sun galaxies, we find that the mean star formation rate (SFR) of visibly merging systems is only modestly enhanced compared to non-interacting galaxies over z~ 0.24-0.80. Visibly merging systems only account for a small fraction (