A Model For Halo Formation With Axion Mixed Dark Matter

Understand

There are several issues to do with dwarf galaxy predictions in the standard $\Lambda$CDM cosmology that have suscitated much recent debate about the possible modification of the nature of dark matter as providing a solution.

• We explore a novel solution involving ultra-light axions that can potentially resolve the missing satellites problem, the cusp-core problem, and the `too big to fail' problem.
• We discuss approximations to non-linear structure formation in dark matter models containing a component of ultra-light axions across four orders of magnitude in mass, $10^{-24}\text{ eV}\lesssim m_a \lesssim 10^{-20}\text{ eV}$, a range too heavy to be well constrained by linear cosmological probes such as the CMB and matter power spectrum, and too light/non-interacting for other astrophysical or terrestrial axion searches.
• We find that an axion of mass $m_a\approx 10^{-21}\text{ eV}$ contributing approximately $85\%$ of the total dark matter can introduce a significant kpc scale core in a typical Milky Way satellite galaxy in sharp contrast to a thermal relic with a transfer function cut off at the same scale, while still allowing such galaxies to form in significant number.