A model for halo formation with axion mixed dark matter

There are several issues to do with dwarf galaxy predictions in the standard Lambda cold dark matter (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 ultralight 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 ultralight axions across four orders of magnitude in mass, 10(-24) less than or similar to m(a) less than or similar to 10(-20) eV, a range too heavy to be well constrained by linear cosmological probes such as the cosmic microwave background 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) approximate to 10(-21) eV contributing approximately 85 per cent 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. Therefore, ultralight axions do not suffer from the Catch 22 that applies to using a warm dark matter as a solution to the small-scale problems of CDM. Our model simultaneously allows formation of enough high-redshift galaxies to allow reconciliation with observational constraints, and also reduces the maximum circular velocities of massive dwarfs so that baryonic feedback may more plausibly resolve the predicted overproduction of massive Milky Way Galaxy dwarf satellites.