A Model For Halo Formation With Axion Mixed Dark Matter (1307.1705v3)

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.

A Model for Halo Formation with Axion Mixed Dark Matter

The research paper authored by D. J. E. Marsh and J. Silk presents a model that explores the potential role of ultra-light axions as a component of dark matter in addressing several prominent issues observed in the small-scale structure of the universe within the standard $\Lambda$CDM cosmology framework. This paper specifically examines how axion mixed dark matter (aMDM), featuring ultra-light axions with mass ranges from $10^{-24}\text{ eV}$ to $10^{-20}\text{ eV}$, could offer solutions to the missing satellites problem (MSP), the cusp-core problem (CCP), and the too big to fail problem (MFP).

Key Findings and Insightful Outcomes

1. Structure Formation and Suppression:
   • The inclusion of ultra-light axions leads to the suppression of structure formation below a characteristic scale. This suppression is analogous to the effects of warm dark matter (WDM) but involves a distinct mass range of axions, thereby not subjecting ultra-light axions to the same constraints as thermal relic WDM particles.
   • A specific axion mass of $m_a = 10^{-22}\text{ eV}$ demonstrates suppression at scales equivalent to a WDM particle mass of approximately $m_W=0.84 \text{ keV}$.
2. Halo Mass Function and Possible Resolutions to the MSP:
   • The halo mass function (HMF) in aMDM models demonstrates a suppression starting at characteristic masses. With large axion fractions, this leads to a natural cut-off—similar to what is seen in numerical simulations of WDM—within the low mass halo range.
   • A partially mixed model (containing CDM) lessens the severity of this suppression, suggesting a means to address the overprediction of satellite galaxy numbers observed in $\Lambda$CDM simulations.
3. Core Formation in Dwarf Galaxies:
   • The paper models the formation of cores within dark matter halos, slightly modifying the NFW profile by introducing cores with sizes dependent on the axion's mass.
   • A significant finding is that ultra-light axions can induce kiloparsec-scale cores for dwarf galaxies with masses insufficiently affected by traditional large-scale structure analyses, implying the potential resolution of the CCP.
4. Resolution of the MFP Through Concentration and Velocity:
   • The presence of axions reduces the concentration parameters of large halos and their associated maximum circular velocities, which may assist in resolving the MFP by altering the internal dynamics of predicted 'failed' massive subhalos.
5. Implications for High-Redshift Galaxies:
   • Axions in the $10^{-22}\text{ eV}$ range could come under tension with high-redshift galaxy observational data, suggesting an exploration of higher axion masses ($m_a\gtrsim 10^{-21}\text{ eV}$) as an alternative within the aMDM framework.

Implications and Future Prospects

The model proposed by Marsh and Silk is significant in that it extends beyond monolithic WDM narratives and explores axion-based mixed dark matter as a viable solution to longstanding cosmological issues. Further investigation using hydrodynamical simulations and observational analysis would be essential to fine-tune constraints on axion properties. Progress in this area could yield novel insights into both cosmological structure formation and particle astrophysics, promising a reevaluation of the components that compose dark matter in the universe.

Overall, this paper introduces a comprehensive approach to understanding complex astronomical problems through the lens of particle physics and cosmology, broadening the potential framework within which dark matter candidates are considered.