How well is angular momentum accretion modelled in semi-analytic galaxy
  formation models?
  Gas cooling and accretion in haloes delivers mass and angular momentum onto
galaxies. In this work, we investigate the accuracy of the modelling of this
important process in several different semi-analytic (SA) galaxy formation
models (GALFORM, L-GALAXIES and MORGANA) through comparisons with a
hydrodynamical simulation performed with the moving-mesh code AREPO. Both SA
models and the simulation were run without any feedback or metal enrichment, in
order to focus on the cooling and accretion process. All of the SA models
considered here assume that gas cools from a spherical halo. We found that the
assumption that the gas conserves its angular momentum when moving from the
virial radius, $r_{\rm vir}$, to the central region of the halo, $r\sim 0.1
r_{\rm vir}$, is approximately consistent with the results from our simulation,
in which gas typically retains $70-80\%$ of its angular momentum during this
process. We also found that, compared to the simulation, the MORGANA model
tends to overestimate the mean specific angular momentum of cooled-down gas,
the L-GALAXIES model also tends to overestimate this in low-redshift massive
haloes, while the two older GALFORM models tend to underestimate the angular
momentum. In general, the predictions of the new GALFORM cooling model
developed by Hou et al. agree the best with the simulation.
