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

In this work we update theL-Galaxiessemi-analytic model (SAM) to better follow thephysical 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 thecontribution of both stellar and gaseous discs in the stability of the galaxy, and we update thelatter by including dissipation of energy in gas-rich mergers. Furthermore, we introduce angularmomentum losses during cooling and find that an accurate match to the observed correlationbetween stellar disc scale length and mass atz∼0.0requires that the gas loses 20%of its initialspecific angular momentum to the corresponding dark matter halo during the formation of thecold gas disc. We reproduce the observed trends between the stellar mass and specific angularmomentum for both disc- and bulge-dominated galaxies, with the former rotating faster thanthe latter of the same mass. We conclude that a two-component instability recipe provides amorphologically diverse galaxy sample which matches the observed fractional breakdown ofgalaxies into different morphological types. This recipe also enables us to obtain an excellent fitto 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 allowsus to match the observed mass-size relation for bulge-dominated systems