Gas flows in galaxy mergers: supersonic turbulence in bridges, accretion
  from the circumgalactic medium, and metallicity dilution
  In major galaxy mergers, the orbits of stars are violently perturbed, and gas
is torqued to the centre, diluting the gas metallicity and igniting a
starburst. In this paper, we study the gas dynamics in and around merging
galaxies using a series of cosmological magneto-hydrodynamical (MHD) zoom-in
simulations. We find that the gas bridge connecting the merging galaxies
pre-coalescence is dominated by turbulent pressure, with turbulent Mach numbers
peaking at values of 1.6-3.3. This implies that bridges are dominated by
supersonic turbulence, and are thus ideal candidates for studying the impact of
extreme environments on star formation. We also find that gas accreted from the
circumgalactic medium (CGM) during the merger significantly contributes (27-51
per cent) to the star formation rate (SFR) at the time of coalescence and
drives the subsequent reignition of star formation in the merger remnant.
Indeed, 19-53 per cent of the SFR at z=0 originates from gas belonging to the
CGM prior the merger. Finally, we investigate the origin of the
metallicity-diluted gas at the centre of merging galaxies. We show that this
gas is rapidly accreted onto the galactic centre with a time-scale much shorter
than that of normal star-forming galaxies. This explains why coalescing
galaxies are not well-captured by the fundamental metallicity relation.
