Recent studies of neutral atomic hydrogen (H i) in nearby galaxies found that all field disk galaxies are H i saturated, in that they carry roughly as much H i as permitted before this gas becomes gravitationally unstable. By taking this H i saturation for granted, the atomic gas fraction f atm of galactic disks can be predicted as a function of the stability parameter q = jσ/(GM), where M and j are the baryonic mass and specific angular momentum of the disk and σ is the H i velocity dispersion (Obreschkow et al. 2016). The log-ratio ∆ f q between this predictor and the observed atomic fraction can be seen as a physically motivated 'H i deficiency'. While field disk galaxies have ∆ f q ≈ 0, objects subject to environmental removal of H i are expected to have ∆ f q > 0. Within this framework, we revisit the H i deficiencies of satellite galaxies in the Virgo cluster and in clusters of the EAGLE simulation. We find that observed and simulated cluster galaxies are H i deficient and that ∆ f q slightly increases when getting closer to the cluster centres. The ∆ f q values are similar to traditional H i deficiency estimators, but ∆ f q is more directly comparable between observations and simulations than morphology-based deficiency estimators. By tracking the simulated H i deficient cluster galaxies back in time, we confirm that ∆ f q ≈ 0 until the galaxies first enter a halo with M halo > 10 13 M , at which moment they quickly lose H i by environmental effects. Finally, we use the simulation to investigate the links between ∆ f q and quenching of star formation.

In the protogalactic density field, diffuse gas and collision-less cold dark matter (DM) are often assumed sufficiently mixed that both components experience identical tidal torques. However, haloes in cosmological simulations consistently end up with a higher specific angular momentum (sAM) in gas, even in simulations without radiative cooling and galaxy formation physics. We refine this result by analysing the spin distributions of gas and DM in ∼50,000 well-resolved haloes in a non-radiative cosmological simulation from the SURFS suite. The sAM of the halo gas on average ends up ∼40 per cent above that of the DM. This can be pinned down to an excess AM in the inner halo (<50 per cent virial radius), paralleled by a more coherent rotation pattern in the gas. We uncover the leading driver for this AM difference through a series of control simulations of a collapsing ellipsoidal top-hat, where gas and DM are initially well mixed. These runs reveal that the pressurised inner gas shells collapse more slowly, causing the DM ellipsoid to spin ahead of the gas ellipsoid. The arising torque generally transfers AM from the DM to the gas. The amount of AM transferred via this mode depends on the initial spin, the initial axes ratios and the collapse factor. These quantities can be combined in a single dimensionless parameter, which robustly predicts the AM transfer of the ellipsoidal collapse. This simplistic model can quantitatively explain the average AM excess of the gas found in the more complex non-radiative cosmological simulation.