Inflow of low-metallicity cool gas in the halo of the Andromeda galaxy

As the closest $L^{\ast}$ galaxy to our own Milky Way, the Andromeda galaxy (M31) is an ideal laboratory for studies of galaxy evolution. The AMIGA project has recently provided observations of the cool ($T\sim10^4$ K) phase of the circumgalactic medium (CGM) of M31, using HST/COS absorption spectra along $\sim40$ background QSO sightlines, located up to and beyond the galaxy virial radius. Based on these data, and by the means of semi-analytic models and Bayesian inference, we provide here a physical description of the origin and dynamics of the cool CGM of M31. We investigate two competing scenarios, in which (i) the cool gas is mostly produced by supernova(SN)-driven galactic outflows or (ii) it mostly originates from infall of gas from the intergalactic medium. In both cases, we take into account the effect of gravity and hydrodynamical interactions with a hot corona, which has a cosmologically motivated angular momentum. We compare the outputs of our models to the observed covering factor, silicon column density and velocity distribution of the AMIGA absorbers. We find that, to explain the observations, the outflow scenario requires an unphysically large (> 100\%) efficiency for SN feedback. Our infall models, on the other hand, can consistently account for the AMIGA observations and the predicted accretion rate, angular momentum and metallicity are consistent with a cosmological infall from the intergalactic medium.
Accepted for publication in MNRAS