How to get cool in the heat: comparing analytic models of hot, cold, and
  cooling gas in haloes and galaxies with EAGLE
  We use the hydrodynamic, cosmological EAGLE simulations to investigate how
hot gas in haloes condenses to form and grow galaxies. We select haloes from
the simulations that are actively cooling and study the temperature,
distribution, and metallicity of their hot, cold, and transitioning `cooling'
gas, placing these in context of semi-analytic models. Our selection criteria
lead us to focus on Milky Way-like haloes. We find the hot-gas density profiles
of the haloes form a progressively stronger core over time, the nature of which
can be captured by a beta profile that has a simple dependence on redshift. In
contrast, the hot gas that will cool over a time-step is broadly consistent
with a singular isothermal sphere. We find that cooling gas carries a few times
the specific angular momentum of the halo and is offset in spin direction from
the rest of the hot gas. The gas loses ~60% of its specific angular momentum
during the cooling process, generally remaining greater than that of the halo,
and it precesses to become aligned with the cold gas already in the disc. We
find tentative evidence that angular-momentum losses are slightly larger when
gas cools onto dispersion-supported galaxies. We show that an exponential
surface density profile for gas arriving on a disc remains a reasonable
approximation, but a cusp containing ~20% of the mass is always present, and
disc scale radii are larger than predicted by a vanilla Fall & Efstathiou
model. These scale radii are still closely correlated with the halo spin
parameter, for which we suggest an updated prescription for galaxy formation
models.
