Cosmological hydrodynamical simulations as well as observations indicate that spiral galaxies are comprised of five different components : dark matter halo , stellar disc , stellar bulge , gaseous disc and gaseous halo .
While the first four components have been extensively considered in numerical simulations of binary galaxy mergers , the effect of a hot gaseous halo has usually been neglected even though it can contain up to 80 % of the total gas within the galaxy virial radius .
We present a series of hydrodynamic simulations of major mergers of disc galaxies , that for the first time include a diffuse , rotating , hot gaseous halo .
Through cooling and accretion , the hot halo can dissipate and refuel the cold gas disc before and after a merger .
This cold gas can subsequently form stars , thus impacting the morphology and kinematics of the remnant .
Simulations of isolated systems with total mass M \sim 10 ^ { 12 } M _ { \odot } show a nearly constant star formation rate of \sim 5 M _ { \odot } { ~ { } yr } ^ { -1 } if the hot gaseous halo is included , while the star formation rate declines exponentially if it is neglected .
We conduct a detailed study of the star formation efficiency during mergers and find that the presence of a hot gaseous halo reduces the starburst efficiency ( e = 0.5 ) compared to simulations without a hot halo ( e = 0.68 ) .
The ratio of the peak star formation rate in mergers compared to isolated galaxies is reduced by almost an order of magnitude ( from 30 to 5 ) .
Moreover we find cases where the stellar mass of the merger remnant is lower than the sum of the stellar mass of the two progenitor galaxies when evolved in isolation .
This suggests a revision to semi-analytic galaxy formation models which assume that a merger always leads to enhanced star formation .
In addition , the bulge-to-total ratio after a major merger is decreased if hot gas is included in the halo , due to the formation of a more massive stellar disc in the remnant .
We show that adding the hot gas component has a significant effect on the kinematics and internal structure of the merger remnants , like an increased abundance of fast rotators and an r ^ { 1 / 4 } surface brightness profile at small scales .
The consequences on the population of elliptical galaxies formed by disc mergers are discussed .