NIHAO VIII: Circum-galactic medium and outflows - The puzzles of HI and
  OVI gas distributions
  We study the hot and cold circum-galactic medium (CGM) of 86 galaxies of the
cosmological, hydrodynamical simulation suite NIHAO. NIHAO allows a study of
how the $z=0$ CGM varies across 5 orders of magnitude of stellar mass using OVI
and HI as proxies for hot and cold gas. The cool HI covering fraction and
column density profiles match observations well, particularly in the inner CGM.
OVI shows increasing column densities with mass, a trend seemingly echoed in
the observations. As in multiple previous simulations, the OVI column densities
in simulations are lower than observed and optically thick HI does not extend
as far out as in observations. We take a look at the collisional ionisation
fraction of OVI as a function of halo mass. We make observable predictions of
the bipolarity of outflows and their effect on the general shape of the CGM.
Bipolar outflows can be seen out to around 40 kpc in intermediate and low mass
halos ($M_{\mathrm{halo}}<10^{11}M_{\mathrm{sun}}$), but outside that radius,
the CGM is too well mixed to detect an elongated shape. Larger halos have
extended gas discs beyond the stellar disc that dominate the shape of the inner
CGM. The simulated CGM is remarkably spherical even in low mass simulations.
The chemical enrichment of both halo and disc gas follow expected increasing
trends as a function of halo mass that are well fit with power laws. These
relations can be used in non-hydrodynamic models, such as semi-analytic models.
