Comparing galaxy formation in semi-analytic models and hydrodynamical
  simulations
  It is now possible for hydrodynamical simulations to reproduce a
representative galaxy population. Accordingly, it is timely to assess
critically some of the assumptions of traditional semi-analytic galaxy
formation models. We use the Eagle simulations to assess assumptions built into
the Galform semi-analytic model, focussing on those relating to baryon cycling,
angular momentum and feedback. We show that the assumption in Galform that
newly formed stars have the same specific angular momentum as the total disc
leads to a significant overestimate of the total stellar specific angular
momentum of disc galaxies. In Eagle, stars form preferentially out of low
specific angular momentum gas in the interstellar medium (ISM) due to the
assumed gas density threshold for stars to form, leading to more realistic
galaxy sizes. We find that stellar mass assembly is similar between Galform and
Eagle but that the evolution of gas properties is different, with various
indications that the rate of baryon cycling in Eagle is slower than is assumed
in Galform. Finally, by matching individual galaxies between Eagle and Galform,
we find that an artificial dependence of AGN feedback and gas infall rates on
halo mass doubling events in Galform drives most of the scatter in stellar mass
between individual objects. Put together our results suggest that the Galform
semi-analytic model can be significantly improved in light of recent advances.
