The effects of a hot gaseous halo on disc thickening in galaxy minor
  mergers
  We employ hydrodynamical simulations to study the effects of dissipational
gas physics on the vertical heating and thickening of disc galaxies during
minor mergers. For the first time we present a suite of simulations that
includes a diffuse, rotating, cooling, hot gaseous halo, as predicted by
cosmological hydrodynamical simulations as well as models of galaxy formation.
We study the effect of this new gaseous component on the vertical structure of
a Milky Way-like stellar disc during 1:10 and 1:5 mergers. For 1:10 mergers we
find no increased final thin disc scale height compared to the isolated
simulation, leading to the conclusion that thin discs can be present even after
a 1:10 merger if a reasonable amount of hot gas is present. The reason for this
is the accretion of new cold gas, leading to the formation of a massive new
thin stellar disc that dominates the surface brightness profile. In a previous
study, in which we included only cold gas in the disk, we showed that the
presence of cold gas decreased the thickening by a minor merger relative to the
no-gas case. Here, we show that the evolution of the scale height in the
presences of a cooling hot halo is dominated by the formation of the new
stellar disc. In this scenario, the thick disc is the old stellar disc that has
been thickened in a minor merger at z>1, while the thin disc is the new stellar
disc that reforms after this merger. In addition, we study the evolution of the
scale height during a 1:5 merger and find that a thin disc can be present even
after this merger, provided enough hot gas is available. The final scale height
in our simulations depends on the mass of the hot gaseous halo, the efficiency
of the winds and the merger mass ratio. We find post-merger values in the range
0.5<z0<1.0 kpc in good agreement with observational constraints by local
galaxies.
