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

Abstract

We perform a comprehensive estimate of the frequency of galaxy mergers and\ntheir impact on star formation over z~0.24--0.80 (lookback time T_b~3--7 Gyr)\nusing 3698 (M*>=1e9 Msun) galaxies with GEMS HST, COMBO-17, and Spitzer data.\nOur results are: (1) Among 790 high mass (M*>=2.5e10 Msun) galaxies, the\nvisually-based merger fraction over z~0.24--0.80, ranges from 9%+-5% to 8%+-2%.\nLower limits on the major and minor merger fractions over this interval range\nfrom 1.1% to 3.5%, and 3.6% to 7.5%, respectively. This is the first\napproximate empirical estimate of the frequency of minor mergers at z<1. For a\nvisibility timescale of ~0.5 Gyr, it follows that over T_b~3--7 Gyr, ~68% of\nhigh mass systems have undergone a merger of mass ratio >1/10, with ~16%, 45%,\nand 7% of these corresponding respectively to major, minor, and ambiguous\n`major or minor' mergers. The mean merger rate is a few x 1e-4 Gyr-1 Mpc-3. (2)\nWe compare the empirical merger fraction and rate for high mass galaxies to a\nsuite of Lambda CDM-based models: halo occupation distribution models,\nsemi-analytic models, and hydrodynamic SPH simulations. We find qualitative\nagreement between observations and models such that the (major+minor) merger\nfraction or rate from different models bracket the observations, and show a\nfactor of five dispersion. Near-future improvements can now start to rule out\ncertain merger scenarios. (3) Among ~3698 M*>=1e9 Msun galaxies, we find that\nthe mean SFR of visibly merging systems is only modestly enhanced compared to\nnon-interacting galaxies over z~0.24--0.80. Visibly merging systems only\naccount for less than 30% of the cosmic SFR density over T_b~3--7 Gyr. This\nsuggests that the behavior of the cosmic SFR density over the last 7 Gyr is\npredominantly shaped by non-interacting galaxies.\n