Comparing galaxy formation in semi-analytic models and hydrodynamical simulations

TL;DR

This study compares semi-analytic galaxy formation models with hydrodynamical simulations, revealing key differences in assumptions about angular momentum, baryon cycling, and feedback, and suggesting improvements for semi-analytic models.

Contribution

It critically assesses assumptions in the Galform semi-analytic model using Eagle simulations, highlighting areas for refinement based on hydrodynamical results.

Findings

Galform overestimates stellar specific angular momentum.

Stars form from low angular momentum gas in Eagle, leading to realistic galaxy sizes.

Baryon cycling in Eagle appears slower than in Galform.

Abstract

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.