A model for halo formation with axion mixed dark matter

Abstract

There are several issues to do with dwarf galaxy predictions in the standard\n$\\Lambda$CDM cosmology that have suscitated much recent debate about the\npossible modification of the nature of dark matter as providing a solution. We\nexplore a novel solution involving ultra-light axions that can potentially\nresolve the missing satellites problem, the cusp-core problem, and the `too big\nto fail' problem. We discuss approximations to non-linear structure formation\nin dark matter models containing a component of ultra-light axions across four\norders of magnitude in mass, $10^{-24}\\text{ eV}\\lesssim m_a \\lesssim\n10^{-20}\\text{ eV}$, a range too heavy to be well constrained by linear\ncosmological probes such as the CMB and matter power spectrum, and too\nlight/non-interacting for other astrophysical or terrestrial axion searches. We\nfind that an axion of mass $m_a\\approx 10^{-21}\\text{ eV}$ contributing\napproximately $85\\%$ of the total dark matter can introduce a significant kpc\nscale core in a typical Milky Way satellite galaxy in sharp contrast to a\nthermal relic with a transfer function cut off at the same scale, while still\nallowing such galaxies to form in significant number. Therefore ultra-light\naxions do not suffer from the \\emph{Catch 22} that applies to using a warm dark\nmatter as a solution to the small scale problems of cold dark matter. Our model\nsimultaneously allows formation of enough high redshift galaxies to allow\nreconciliation with observational constraints, and also reduces the maximum\ncircular velocities of massive dwarfs so that baryonic feedback may more\nplausibly resolve the predicted overproduction of massive MWG dwarf satellites.\n