Simulations and modelling of the ISM in galaxies
  The latest observations of molecular gas and the atomic hydrogen content of
local and high-redshift galaxies, coupled with how these correlate with star
formation activity, have revolutionized our ideas about how to model star
formation in a galactic context. A successful theory of galaxy formation has to
explain some key facts: (i) high-redshift galaxies have higher molecular gas
fractions and star formation rates than local galaxies, (ii) scaling relations
show that the atomic-to-stellar mass ratio decreases with stellar mass in the
local Universe, and (iii) the global abundance of atomic hydrogen evolves very
weakly with time. We review how modern cosmological simulations of galaxy
formation attempt to put these pieces together and highlight how approaches
simultaneously solving dark matter and gas physics, and approaches first
solving the dark matter N-body problem and then dealing with gas physics using
semi-analytic models, differ and complement each other. We review the
observable predictions, what we think we have learned so far and what still
needs to be done in the simulations to allow robust testing by the new
observations expected from telescopes such as ALMA, PdBI, LMT, JVLA, ASKAP,
MeerKAT, SKA.
