  We present a new approach to study galaxy evolution in a cosmological
context. We combine cosmological merger trees and semi-analytic models of
galaxy formation to provide the initial conditions for multi-merger
hydrodynamic simulations. In this way we exploit the advantages of merger
simulations (high resolution and inclusion of the gas physics) and
semi-analytic models (cosmological background and low computational cost), and
integrate them to create a novel tool. This approach allows us to study the
evolution of various galaxy properties, including the treatment of the hot
gaseous halo from which gas cools and accretes onto the central disc, which has
been neglected in many previous studies. This method shows several advantages
over other methods. As only the particles in the regions of interest are
included, the run time is much shorter than in traditional cosmological
simulations, leading to greater computational efficiency. Using cosmological
simulations, we show that multiple mergers are expected to be more common than
sequences of isolated mergers, and therefore studies of galaxy mergers should
take this into account. In this pilot study, we present our method and
illustrate the results of simulating ten Milky Way-like galaxies since z=1. We
find good agreement with observations for the total stellar masses, star
formation rates, cold gas fractions and disc scale length parameters. We expect
that this novel numerical approach will be very useful for pursuing a number of
questions pertaining to the transformation of galaxy internal structure through
cosmic time.
