Galaxy mergers moulding the circum-galactic medium - I. The impact of a
  major merger
  Galaxies are surrounded by sizeable gas reservoirs which host a significant
amount of metals: the circum-galactic medium (CGM). The CGM acts as a mediator
between the galaxy and the extra-galactic medium. However, our understanding of
how galaxy mergers, a major evolutionary transformation, impact the CGM remains
deficient. We present a theoretical study of the effect of galaxy mergers on
the CGM. We use hydrodynamical cosmological zoom-in simulations of a major
merger selected from the Illustris project such that the z=0 descendant has a
halo mass and stellar mass comparable to the Milky Way. To study the CGM we
then re-simulated this system at a 40 times better mass resolution, and
included detailed post-processing ionization modelling. Our work demonstrates
the effect the merger has on the characteristic size of the CGM, its
metallicity, and the predicted covering fraction of various commonly observed
gas-phase species, such as H I, C IV, and O VI. We show that merger-induced
outflows can increase the CGM metallicity by 0.2-0.3 dex within 0.5 Gyr
post-merger. These effects last up to 6 Gyr post-merger. While the merger
increases the total metal covering fractions by factors of 2-3, the covering
fractions of commonly observed UV ions decrease due to the hard ionizing
radiation from the active galactic nucleus, which we model explicitly. Our
study of the single simulated major merger presented in this work demonstrates
the significant impact that a galaxy interaction can have on the size,
metallicity, and observed column densities of the CGM.
