Implications of multi-axion dark matter on structure formation

Axions are candidates for dark matter in the universe.We develop an accurate Boltzmann code to calculate the linear growth of the plasma. As an interesting example, we investigate a mixed dark matter model consisting of cold dark matter (CDM) and two-axion dark matter. We analyze the growth of the structure numerically and analytically. We find that an effective single axion with an effective mass and an effective abundance is useful to characterize the two-axion cosmology. Moreover, we generalize the effective single axion description to multi-axion dark matter cosmology. We also compare the results with those of warm dark matter (WDM) model. Moreover, we calculate halo mass functions for the mixed model by using the Press-Schechter model and linear perturbations and then determine the mass function as a function of masses and axion abundance.