Morphological evolution and galactic sizes in the L-Galaxies SA model

TL;DR

This paper updates the L-Galaxies semi-analytic model to better simulate galaxy morphological evolution, including bulge growth mechanisms, angular momentum loss, and energy dissipation, resulting in improved agreement with observed galaxy properties.

Contribution

The paper introduces new recipes for bulge growth, angular momentum loss, and energy dissipation in the L-Galaxies model, enhancing its ability to reproduce observed galaxy morphologies and scaling relations.

Findings

Reproduces observed stellar mass and angular momentum relations.

Matches the observed galaxy size-mass relation for bulge-dominated systems.

Provides a diverse galaxy morphology distribution consistent with observations.

Abstract

In this work we update the L-Galaxies semi-analytic model (SAM) to better follow the physical processes responsible for the growth of bulges via disc instabilities (leading to pseudo-bulges) and mergers (leading to classical bulges). We address the former by considering the contribution of both stellar and gaseous discs in the stability of the galaxy, and we update the latter by including dissipation of energy in gas-rich mergers. Furthermore, we introduce angular momentum losses during cooling and find that an accurate match to the observed correlation between stellar disc scale length and mass at z ~ 0.0 requires that the gas loses 20% of its initial specific angular momentum to the corresponding dark matter halo during the formation of the cold gas disc. We reproduce the observed trends between the stellar mass and specific angular momentum for both disc- and bulge-dominated galaxies, with the former rotating faster than the latter of the same mass. We conclude that a two-component instability recipe provides a morphologically diverse galaxy sample which matches the observed fractional breakdown of galaxies into different morphological types. This recipe also enables us to obtain an excellent fit to the morphology-mass relation and stellar mass function of different galactic types. Finally, we find that energy dissipation during mergers reduces the merger remnant sizes and allows us to match the observed mass-size relation for bulge-dominated systems.