Galaxy formation with L-GALAXIES: Model description

In this supplementary material, we give a full description of the treatment of astrophysical processes in our model of galaxy formation, a variant of the Henriques et al. 2020 public release. The most significant modification of our model with respect to Henriques et al. 2020 is the implementation of a novel gas stripping method. Like its predecessors, our model is built on subhalo merger trees constructed from the Millennium andMillennium-II simulations after scaling to represent the first-year Planck cosmology. A set of coupled differential equations allow us to model the formation and evolution of baryonic matter. In L-Galaxies, baryonic matter bound to each galaxy/subhalo is divided into seven main components: hot gas, cold gas (partitioned into HI and H2), stellar disc, bulge stars, halo stars, the supermassive black hole, and ejected material. There is also diffuse primordial gas associated with dark matter, which is not yet part of any halo. Primordial gas falls with the dark matter onto sufficiently massive haloes, where it is shock-heated. The efficiency of radiative cooling then determines whether it is added directly to the cold gas of the central galaxy, or resides for a while in a hot gas atmosphere. The properties of cold interstellar gas are followed in concentric rings where cold gas is partitioned into HI and H2 and the latter is converted into stars, both quiescently and in merger-induced starbursts which also drive the growth of central supermassive black holes. Stellar evolution is tracked independently in each ring and not only determines the photometric appearance of the final galaxy, but also heats and enriches its gas components, in many cases driving material into the wind reservoir, from which it may later fall back into the galaxy. Accretion of hot gas onto central black holes gives rise to radio mode feedback, regulating condensation of hot gas onto the galaxy. Galaxy mergers affect the gas components of galaxies, as well as the partition of stars between discs, bulges, and the intracluster light (the halo stars), a diffuse component built from tidally disrupted systems. The radial structure of discs and bulge sizes are estimated from simple energy and angular momentum-based arguments. Environmental processes such as tidal and ram-pressure stripping influence the gas content of galaxies and play a key role in quenching star formation of galaxies in dense environments. These processes are implemented based on local background environmentmeasurementsmade directly on the particle data of the underlying dark matter only simulations.