Universiteit Leiden

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Non-equilibrium chemistry and cooling in simulations of galaxy formation

Promotor: J. Schaye

Alexander James Richings
08 december 2015
Thesis in Leiden Repository

In this thesis we used numerical simulations to explore the role that chemistry plays in galaxy formation. Simulations of galaxies often assume chemical equilibrium, where the chemical reactions between ions and molecules have reached a steady state. However, this assumption may not be valid if the physical conditions of the gas are evolving rapidly. Therefore, we developed a chemical model to follow the non-equilibrium evolution of ions and molecules. We then incorporated this model into hydrodynamic simulations of galaxies. We ran simulations with different metallicities (i.e. different proportions of heavy elements) and UV radiation fields, first using our full non-equilibrium chemical model and then assuming chemical equilibrium. We found that the total star formation rate is higher at higher metallicity and for weaker radiation fields. In contrast, non-equilibrium chemistry does not strongly influence the total star formation rate or outflow properties of the galaxy. However, it does affect the abundances of individual chemical species, for example in molecular outflows. Finally, we explored the properties of molecular clouds in our simulations. At low metallicity, the molecular fraction of young clouds tends to be below equilibrium, as the molecules are still forming. This also affects the observable CO emission from young clouds.

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