Dwarf Galaxies United by Dark Bosons

TL;DR

This paper proposes that dark matter in dwarf galaxies consists of multiple ultra-light bosons, forming soliton cores that explain observed galaxy properties and suggest a connection to string theory's axiverse.

Contribution

It introduces a model where different dwarf galaxy classes are explained by distinct boson masses, supported by observational data and simulations, indicating multiple boson species in dark matter.

Findings

Core radius inversely related to velocity dispersion, consistent with Bose-Einstein condensate behavior.

Different dwarf classes have distinct boson masses, differing by an order of magnitude.

Soliton core profiles match observed stellar distributions and simulation predictions.

Abstract

Low mass galaxies in the Local Group are dominated by dark matter and comprise the well studied ``dwarf Spheroidal" (dSph) class, with typical masses of $10^{9-10}M_\odot$ and also the equally numerous ``ultra faint dwarfs" (UFD), discovered recently, that are distinctly smaller and denser with masses of only $10^{7-8}M_\odot$. This bimodality amongst low mass galaxies contrasts with the scale free continuity expected for galaxies formed under gravity, as in the standard Cold Dark Matter (CDM) model for heavy particles. Within each dwarf class we find the core radius $R_c$ is inversely related to velocity dispersion $\sigma$, quite the opposite of standard expectations, but indicative of dark matter in a Bose-Einstein state, where the Uncertainty Principle requires $R_c \times \sigma$ is fixed by Planks constant, $h$. The corresponding boson mass, $m_b=h/R_c \sigma$, differs by one order of magnitude between the UDF and dSph classes, with $10^{-21.4}$eV and $10^{-20.3}$eV respectively. Two boson species is reinforced by parallel relations seen between the central density and radius of UDF and dSph dwarfs respectively, each matching the steep prediction, $\rho_c \propto R_c^{-4}$, for soliton cores in the ground state. Furthermore, soliton cores accurately fit the stellar profiles of UDF and dSph dwarfs where prominent, dense cores appear surrounded by low density halos, as predicted by our simulations. Multiple bosons may point to a String Theory interpretation for dark matter, where a discrete mass spectrum of axions is generically predicted to span many decades in mass, offering a unifying "Axiverse" interpretation for the observed "diversity" of dark matter dominated dwarf galaxies.