The distribution of gas in the Local Group from constrained cosmological simulations: the case for Andromeda and the Milky Way galaxies

We study the gas distribution in the Milky Way and Andromeda using a constrained cosmological simulation of the Local Group (LG) within the context of the CLUES (Constrained Local UniversE Simulations) project. We analyse the properties of gas in the simulated galaxies at z = 0 for three different phases: ‘cold’, ‘hot’ and H i, and compare our results with observations. The amount of material in the hot halo (M hot ≈ 4–5 × 1010 M⊙), and the cold (M cold(r ≲ 10 kpc) ≈ 108 M⊙) and H i ( $M_{\rm H\,{\small i}}(r\lesssim 50\,{\rm kpc})\approx 3{-}4\times 10^8\, {M_{\odot}}$ ) components displays reasonable agreement with observations. We also compute the accretion/ejection rates together with the H i (radial and all-sky) covering fractions. The integrated H i accretion rate within r = 50 kpc gives ∼0.2–0.3 M⊙ yr−1, i.e. close to that obtained from high-velocity clouds in the Milky Way. We find that the global accretion rate is dominated by hot material, although ionized gas with T ≲ 105 K can contribute significantly too. The net accretion rates of all material at the virial radii are 6–8 M⊙ yr−1. At z = 0, we find a significant gas excess between the two galaxies, as compared to any other direction, resulting from the overlap of their gaseous haloes. In our simulation, the gas excess first occurs at z ∼ 1, as a result of the kinematical evolution of the LG.