Sub-resolution modelling of astrophysical processes
The astrophysical processes involved in the formation and evolution of galaxies in cosmological hydrodynamical simulations span a huge dynamical range of scales. They indeed vary from the $\sim$Mpc scales where gravitational instabilities drive the evolution of the DM component and the hierarchical assembly of haloes, down to the $\sim$parsec and $\sim$sub-parsec scales, where processes like BH accretion and star formation take place, going through the $\sim$kpc scales, where e.g. galactic winds distribute the stellar feedback energy to the ambient medium. The call for sub-grid physics is thus essential: sub-resolution models account for processes that occur below the resolution limit of cosmological hydrodynamical simulations, but that affect the evolution of the simulated structure on scales that are explicitly resolved. Sub-resolution prescriptions usually resort to rather simple analytical or theoretical models, and/or to the phenomenological description of rather complex processes, through a suitable choice of parameters that are calibrated to reproduce observations (see also sub-section §3.16).
A common criticism that has been often raised to numerical astrophysics deals with the number of parameters that are employed when modelling physical processes, in particular when referring to the parameters that enter sub-resolution models. However, observed or expected results cannot be reproduced by just fine-tuning parameters. Results are rather determined by proper parametrizations of processes. For instance: when modelling star formation (see sub-section §3.12), results are mainly driven by the way in which the star formation rate (SFR) relates to the actual fuel (e.g. the mass of molecular or cold gas available), rather than to the exact value of the proportionality constant between the two aforementioned quantities. Parameter space exploration is thus not meant to provide the best fit to observations that simulations want to compare with. It rather aims at enabling a better understanding of the dependence of a model on different unknowns and at making predictions for loosely constrained physical quantities.
In the next sub-sections, we will review the most important astrophysical processes that significantly affect galaxy formation and evolution. They are all self-consistently included in state-of-the-art cosmological hydrodynamical simulations.
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