Infalling ultra-faint dwarfs as emissaries of the Axiverse

TL;DR

This paper proposes that ultra-faint dwarf galaxies are linked to specific axion masses in the Axiverse, with their properties explained by light bosons, and suggests observational tests using pulsar timing with SKA.

Contribution

It introduces a cosmological simulation model connecting UFDs to axion masses in the Axiverse, explaining their distinct properties and predicting observable pulsar timing signatures.

Findings

UFDs correspond to a heavy axion of 3×10^{-21} eV

Larger galaxies are associated with a lighter axion of 10^{-22} eV

Predicted pulsar timing residuals could test the model

Abstract

Recent discoveries of ultra-faint dwarf galaxies (UFDs) infalling onto the Milky Way, namely Leo K \& M at $r \simeq 450$kpc, considerably strengthens the case that UFDs constitute a distinct galaxy class that is inherently smaller and fainter, and metal-poorer than the classical dwarf spheroidals (dSph). This distinction is at odds with the inherent continuity of galaxy halo masses formed under scale-free gravity for any standard dark-matter (DM) model. Here, we show that distinct galaxy classes do evolve in cosmological simulations of multiple light bosons representing the ``Axiverse'' proposal of string theory, where a discrete mass spectrum of axions is generically predicted to span many decades in mass. In this context, the observed UFD class we show corresponds to a relatively heavy boson of $3\times 10^{-21}$ eV, including Leo K \& M, whereas a lighter axion of $10^{-22}$ eV comprises the bulk of DM in all larger galaxies including the dSphs. Although Leo M is larger in size than Leo K, we predict its velocity dispersion to be smaller $(\simeq 1.7$km/s) than that of Leo K $(\simeq 4.5$km/s) because of the inverse de Broglie scale dependence on momentum. This scenario can be definitively tested using millisecond pulsars close to the Galactic center, where the Compton frequencies of the heavy and light bosons imprint monotone timing residuals that may be detected by the Square Kilometre Array (SKA) on timescales of approximately one week and four months, respectively.