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.
