  Galaxy merger simulations have explored the behaviour of gas within the
galactic disk, yet the dynamics of hot gas within the galaxy halo has been
neglected. We report on the results of high-resolution hydrodynamic simulations
of colliding galaxies with hot halo gas. We explore a range of mass ratios, gas
fractions and orbital configurations to constrain the shocks and gas dynamics
within the progenitor haloes. We find that : (i) A strong shock is produced in
the galaxy haloes before the first passage, increasing the temperature of the
gas by almost an order of magnitude to $T\sim 10^{6.3}$ K. (ii) The X-ray
luminosity of the shock is strongly dependent on the gas fraction; it is
$\gtrsim 10^{39}$ erg/s for halo gas fractions larger than 10%. (iii) The hot
diffuse gas in the simulation produces X-ray luminosities as large as $10^{42}$
erg/s. This contributes to the total X-ray background in the Universe. (iv) We
find an analytic fit to the maximum X-ray luminosity of the shock as a function
of merger parameters. This fit can be used in semi-analytic recipes of galaxy
formation to estimate the total X-ray emission from shocks in merging galaxies.
(v) $\sim$ 10-20% of the initial gas mass is unbound from the galaxies for
equal-mass mergers, while $3-5%$ of the gas mass is released for the 3:1 and
10:1 mergers. This unbound gas ends up far from the galaxy and can be a
feasible mechanism to enrich the IGM with metals.
