Accretion, radial flows and abundance gradients in spiral galaxies
  The metal-poor gas continuously accreting onto the discs of spiral galaxies
is unlikely to arrive from the intergalactic medium (IGM) with exactly the same
rotation velocity as the galaxy itself and even a small angular momentum
mismatch inevitably drives radial gas flows within the disc, with significant
consequences to galaxy evolution. Here we provide some general analytic tools
to compute accretion profiles, radial gas flows and abundance gradients in
spiral galaxies as a function of the angular momentum of accreting material. We
generalize existing solutions for the decomposition of the gas flows, required
to reproduce the structural properties of galaxy discs, into direct accretion
from the IGM and a radial mass flux within the disc. We then solve the equation
of metallicity evolution in the presence of radial gas flows with a novel
method, based on characteristic lines, which greatly reduces the numerical
demand on the computation and sheds light on the crucial role of boundary
conditions on the abundance profiles predicted by theoretical models. We also
discuss how structural and chemical constraints can be combined to disentangle
the contributions of inside-out growth and radial flows in the development of
abundance gradients in spiral galaxies. Illustrative examples are provided
throughout with parameters plausible for the Milky Way. We find that the
material accreting on the Milky Way should rotate at 70-80 per cent of the
rotational velocity of the disc, in agreement with previous estimates.
