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 3698 (M*>=1e9 Msun) galaxies with GEMS HST, COMBO-17, and Spitzer data.
Our results are: (1) Among 790 high mass (M*>=2.5e10 Msun) galaxies, the
visually-based merger fraction over z~0.24--0.80, ranges from 9%+-5% to 8%+-2%.
Lower limits on the major and minor merger fractions over this interval range
from 1.1% to 3.5%, and 3.6% to 7.5%, respectively. This is the first
approximate empirical estimate of the frequency of minor mergers at z<1. For 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 mean merger rate is a few x 1e-4 Gyr-1 Mpc-3. (2)
We compare the empirical merger fraction and rate for high mass galaxies to a
suite of Lambda CDM-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 five dispersion. Near-future improvements can now start to rule out
certain merger scenarios. (3) Among ~3698 M*>=1e9 Msun galaxies, we find that
the mean 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 less than 30% of the cosmic SFR density over T_b~3--7 Gyr. This
suggests that the behavior of the cosmic SFR density over the last 7 Gyr is
predominantly shaped by non-interacting galaxies.
