A Model For Halo Formation With Axion Mixed Dark Matter

TL;DR

This paper proposes that ultra-light axions, making up about 85% of dark matter, can resolve small-scale structure issues in dwarf galaxies while remaining consistent with cosmological observations.

Contribution

It introduces a novel dark matter model with ultra-light axions that addresses multiple small-scale galaxy formation problems simultaneously.

Findings

An axion mass around 10^{-21} eV creates kpc scale cores in dwarf galaxies.

The model allows sufficient high-redshift galaxy formation.

It reduces dwarf galaxy maximum velocities, aiding feedback-based solutions.

Abstract

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. Therefore ultra-light axions do not suffer from the \emph{Catch 22} that applies to using a warm dark matter as a solution to the small scale problems of cold dark matter. 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 MWG dwarf satellites.