The changing circumgalactic medium over the last 10 Gyr - I. Physical and dynamical properties

We present an analysis of the physical and dynamical states of two sets of EAGLE zoom simulations of galaxy haloes, one at high redshift (z = 2-3) and the other at low redshift (z = 0), with masses of ≈10¹² M_⊙. Our focus is how the circumgalactic medium (CGM) of these L* star-forming galaxies change over the last 10 Gyr. We find that the high-z CGM is almost equally divided between the 'cool' (T < 10⁵ K) and 'hot' (T ≥ 10⁵ K) phases, while at low-z the hot CGM phase contains 5 × more mass than the cool phase. The high-z hot CGM contains 60 per cent more metals than the cool CGM, while the low-z cool CGM contains 35 per cent more metals than the hot CGM. The metals are evenly distributed radially between the hot and cool phases throughout the high-z CGM. At high z, the CGM volume is dominated by hot outflows, but also contains cool gas mainly inflowing and cool metals mainly outflowing. At low z, the cool metals dominate the interior and the hot metals are more prevalent at larger radii. The low-z cool CGM has tangential motions consistent with rotational support out to 0.2R₂₀₀, often exhibiting r ≈ 40 kpc disc-like structures. The low-z hot CGM has several times greater angular momentum than the cool CGM, and a more flattened radial density profile than the high-z hot CGM. This study verifies that, just as galaxies demonstrate significant transformations over cosmic time, the gaseous haloes surrounding them also undergo considerable changes of their own both in physical characteristics of density, temperature, and metallicity, and dynamic properties of velocity and angular momentum.