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 3-7Gyr) using 3600 (M≥ 1 × 109 M ⊙) 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 ⊙) 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 7Gyr. Assuming a visibility timescale of 0.5Gyr, it follows that over T b 3-7Gyr, 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 galaxiesGyr-1 Mpc-3. Among 2840 blue-cloud galaxies of mass M≥ 1.0 × 109 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≥ 1.0 × 109 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 3-7Gyr. This complements the results of Wolf etal. over a shorter time interval of T b 6.2-6.8Gyr, and suggests that the behavior of the cosmic SFR density over the last 7Gyr is predominantly shaped by non-interacting galaxies. © 2009. The American Astronomical Society. All rights reserved.