2009ApJ...697.1971J

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≥ 1x10⁹ M_☉) galaxies with GEMS Hubble Space Telescope, COMBO-17, and Spitzer data. Our results are as follows. (1) Among ∼790 high-mass (M≥ 2.5x10¹⁰ 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 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 x10^–4 galaxies/Gyr/Mpc3. Among ∼2840 blue-cloud galaxies of mass M≥ 1.0x10⁹ M_☉, similar results hold. (2) We compare the empirical merger fraction and merger rate for high-mass galaxies to three Λ 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≥ 1.0x10⁹ M_☉ 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 (<30%) of the cosmic SFR density over T_b∼ 3-7 Gyr. This complements the results of Wolf et al. over a shorter time interval of T_b∼ 6.2-6.8 Gyr, and suggests that the behavior of the cosmic SFR density over the last 7 Gyr is predominantly shaped by non-interacting galaxies.