Star formation as part of galaxy evolution is a highly non-linear process, involving interactions between gas heating and cooling, galactic dynamics, smooth and clumpy accretion, and stellar feedback. Modeling it even on parsecs scales requires the use of hydrodynamic simulations and the modeling of complex physical processes. One area that has been neglected in most previous simulations is the cold, molecular phase of the interstellar medium from which stars form. I addressed that lack in this thesis through the implementation of molecular hydrogen (H2) physics in the galaxy formation code, GASOLINE. In this implementation, the non-equilibrium H2 abundances are calculated based on the local formation and destruction rates of H2, including the effects of shielding. Both self-shielding and shielding from dust are included and protect against the cosmic background radiation and local Lyman-Werner radiation. With the implementation of H2, I was able to implement additional low temperature H2 cooling and H2-based star formation. Using this code, I researched the effects of this cold gas phase and supernova feedback on star formation in galaxy formation simulations.
I first analyzed the effect of supernova feedback on star formation and morphology in galaxies of different masses and resolution, simulated without H2. From this sample, I determined the appropriate spatial and mass resolutions for star formation and feedback to converge. Notably, the 1010 [special characters omitted] galaxy simulation was on the cusp of forming a disk, making its morphology highly sensitivity to resolution. Following the implementation of H2 , I tested the code by calculating the transition from atomic to molecular gas on similar isolated galaxy simulations. I then researched the effect of including H2-physics on star formation in a cosmological dwarf galaxy simulation. The combination of extra cooling from H2 and shielding resulted in the formation of a cold, molecular gas phase within a clumpier interstellar media (ISM). These changes to the ISM resulted in slightly more star formation over a greater area. When H 2 was included in simulations of spiral galaxies, those same clumps resulted in the increased efficiency of supernova feedback and the reduction of stellar bulges. I finally compared star formation in a dwarf and L* galaxy simulated with H2 across their evolution. The dwarf galaxy showed systematically lower fractions of H2 and star formation efficiencies throughout their evolution, even when the two galaxies had similar metallicities and mass fractions of disk gas. This effect was especially clear when comparing the dwarf at z = 0 to a high redshift progenitor of the L* galaxy with similar virial mass and metallicity. Despite their similarity in mass, the L* progenitor had an order of magnitude higher star formation rate resulting from its higher concentration. This study highlighted that even beyond disk averaged quantities, the gas density and temperature distributions are instrumental for simulating star formation.
|School:||University of Washington|
|School Location:||United States -- Washington|
|Source:||DAI-B 73/07(E), Dissertation Abstracts International|
|Keywords:||Galaxy evolution, Interstellar media, Molecular hydrogen, Star formation|
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