Structure Formation with Ultralight Axion Dark Matter

Ultralight axion is a scalar field with an extremely small mass $\sim10^{-22} eV$. It is proposed as an alternative dark matter candidate to the standard cold dark matter (CDM). If self-interactions between axions can be ignored, it is also called fuzzy dark matter (FDM). On large scales, FDM behaves like CDM producing large scale structure of the Universe consistent with current observations. But on scales below the Jeans length, quantum pressure arising from coherent oscillations of the scalar filed counters gravity, leading to a large suppression in the structure formation and cored dark matter halo profiles. In this dissertation, I mainly discuss the cosmic structure formation in the scenario of FDM. First, I show how we implement FDM into the publicly available semi-analytic code for galaxy formation, GALACTICUS. With the modified code, we compute the (sub)halo mass function for FDM with different particle masses and density fractions. Comparing to the standard CDM, the (sub)halo mass function is found to be largely suppressed at lower masses. The suppression scale is inversely proportional to the square root of FDM mass. Then, based on the core mass growth in each binary merger of FDM halos, a simple model for the core-halo mass relation observed in simulations is proposed. The model is verified using the modified GALACTICUS code. Finally, to study tidal disruption of FDM subhalo cores, I perform idealized simulations using a fourth-order pseudo-spectral method. The core mass loss rate due to tidal stripping obtained numerically is compared with the previous results from semi-analytic treatments. We find after some reinterpretations, two results are reasonably consistent. Implementing the numerical results into GALACTICUS, I compute the subhalo mass function with and without considering the core stripping. Fitting formula for the subhalo mass function is given at the end, which will be useful in future studies on constraining FDM from observations related to halo substructure