Dwarf Galaxies United by Dark Bosons
First author: Alvaro Pozo
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.