Nihao - VIII. Circum-galactic medium and outflows - The puzzles of H I and O VI gas distributions

We study the hot and cold circum-galactic medium (CGM) of 86 galaxies of the cosmological, hydrodynamical simulation suite, Numerical Investigation of a Hundred Astrophysical Objects (NIHAO). NIHAO allows a study of how the z = 0 CGM varies across five orders of magnitude of stellar mass using O VI and H I as proxies for hot and cold gas. The cool H I covering fraction and column density profiles match observations well, particularly in the inner CGM. O VI shows increasing column densities with mass, a trend seemingly echoed in the observations. As in multiple previous simulations, the O VI column densities in simulations are lower than observed and optically thick H I does not extend as far out as in observations. We take a look at the collisional ionization fraction of O VI 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 haloes (M_Halo < 10¹¹ M☉), but outside that radius, the CGM is too well mixed to detect an elongated shape. Larger haloes 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.