Analytic and numerical realisations of a disk galaxy
  Recent focus on the importance of cold, unshocked gas accretion in galaxy
formation -- not explicitly included in semi-analytic studies -- motivates the
following detailed comparison between two inherently different modelling
techniques: direct hydrodynamical simulation and semi-analytic modelling. By
analysing the physical assumptions built into the Gasoline simulation, formulae
for the emergent behaviour are derived which allow immediate and accurate
translation of these assumptions to the Galform semi-analytic model. The
simulated halo merger history is then extracted and evolved using these
equivalent equations, predicting a strikingly similar galactic system. This
exercise demonstrates that it is the initial conditions and physical
assumptions which are responsible for the predicted evolution, not the choice
of modelling technique. On this level playing field, a previously published
Galform model is applied (including additional physics such as chemical
enrichment and feedback from active galactic nuclei) which leads to starkly
different predictions.
