Properties of gas in and around galaxy haloes
  We study the properties of gas inside and around galaxy haloes as a function
of radius and halo mass, focussing mostly on z=2, but also showing some results
for z=0. For this purpose, we use a suite of large cosmological, hydrodynamical
simulations from the OverWhelmingly Large Simulations project. The properties
of cold- and hot-mode gas, which we separate depending on whether the
temperature has been higher than 10^5.5 K while it was extragalactic, are
clearly distinguishable in the outer parts of massive haloes (virial
temperatures >> 10^5 K. The differences between cold- and hot-mode gas resemble
those between inflowing and outflowing gas. The cold-mode gas is mostly
confined to clumpy filaments that are approximately in pressure equilibrium
with the diffuse, hot-mode gas. Besides being colder and denser, cold-mode gas
typically has a much lower metallicity and is much more likely to be infalling.
However, the spread in the properties of the gas is large, even for a given
mode and a fixed radius and halo mass, which makes it impossible to make strong
statements about individual gas clouds. Metal-line cooling causes a strong
cooling flow near the central galaxy, which makes it hard to distinguish gas
accreted through the cold and hot modes in the inner halo. Stronger feedback
results in larger outflow velocities and pushes hot-mode gas to larger radii.
The gas properties evolve as expected from virial arguments, which can also
account for the dependence of many gas properties on halo mass. We argue that
cold streams penetrating hot haloes are observable as high-column density HI
Lyman-alpha absorption systems in sightlines near massive foreground galaxies.
