The parametrization of gas flows in discs in the Auriga simulations
  We study the radial motions of cold, star-forming gas in the secular
evolution phase of a set of 14 magnetohydrodynamical cosmological zoom-in
simulations of Milky Way-mass galaxies. We study the radial transport of
material within the disc plane in a series of concentric rings. For the gas in
each ring at a given time we compute two quantities as a function of time and
radius: 1) the radial bulk flow of the gas; and 2) the radial spread of the gas
relative to the bulk flow. Averaging the data from all the halos, we find that
the radial spread increases with radius in the form of a power law with strong
secondary dependencies on the fraction of accreted material and the local
radial velocity dispersion of the gas. We find that the bulk motion of gas is
well described in the inner disc regions by a radially-independent mean inward
flow speed of $-$2.4 km s$^{-1}$. The spread around this value relates to the
change in angular momentum of the gas and also the amount of accreted material.
These scalings from fully cosmological, MHD simulations of galaxy formation can
then be used in semi-analytic models to better parameterise the radial flow of
gas in discs.
