The Glow of Axion Quark Nugget Dark Matter: (III) The Mysteries of the Milky Way UV Background

Abstract

Axion quark nuggets (AQNs) are hypothetical objects with nuclear density that would have formed during the quark-hadron transition and could make up most of the dark matter today. These objects have a mass greater than a few grams and are sub-micrometer in size. They would also help explain the matter-antimatter asymmetry and the similarity between visible and dark components of the universe, i.e. $Ω_{\text{DM}} \sim Ω_{\text{visible}}$. These composite objects behave as cold dark matter, interacting with ordinary matter and producing pervasive electromagnetic radiation. This work aims to calculate the FUV electromagnetic signature in a 1 kpc region surrounding the solar system, resulting from the interaction between antimatter AQNs and baryons. To this end, we use the high-resolution hydrodynamic simulation of the Milky Way, FIRE-2 Latter suite, to select solar system-like regions. From the simulated gas and dark matter distributions in these regions, we calculate the FUV background radiation generated by the AQN model. We find that the results are consistent with the FUV excess recently confirmed by the Alice spectrograph aboard New Horizons, which corroborated the FUV excess initially discovered by GALEX a decade ago. We also discuss the potential cosmological implications of our work, which suggest the existence of a new source of FUV radiation in galaxies, linked to the interaction between dark matter and baryons.

Figures (8)

• Figure 1: Gas distribution in a 20x20x4 kpc$^3$ cube around the centre of the m12i galaxy simulation. $xy$-plane projections summed over $z\in[-1,1]\;\text{kpc}$. Left: gas mass densities, obtained by binning particles over an overlaid grid. Right: gas mass density field, generated by applying the Voronoi tessellation and nearest-voxel search process discussed in section \ref{['subsec:voronoi']} to the data in the left plot.
• Figure 2: Probability density of the electron fraction $X_e$ and gas temperature $T_{\rm gas}$ for a 20x20x20 kpc cube centered on the Galactic centre of the m12i simulation.
• Figure 3: The grey histogram shows the distribution of the relative speed $\Delta \rm{v}$ between dark and visible matter from FIRE's m12i simulation. $\Delta \rm{v}$ values are taken at the solar system neighbourhood locations as described in Section \ref{['subsec:solar-neighbourhood']}. The solid line shows the shifted Maxwell Boltzmann $\Delta \rm{v}$ model described by Eq. \ref{['eqn:max-boltz-dv']}.
• Figure 4: Pre-convolution and post-convolution ionized gas density distributions, using a spherical averaging kernel with $R=0.6\;\text{kpc}$. The $xy$-planes are projections along the $z$-axis, averaged over $z\in[-1,1]\;\text{kpc}$.
• Figure 5: Grey histogram: distribution of the average ionized gas density $\overline{n_{\rm ion}}$ of the central voxel of the solar system regions selected on dark matter density $\rho_{\rm DM}$, neutral gas $n_{\rm neut}$ and distance from the galactic center $R_\odot$ (see Table \ref{['tab:solar-sys-params']}). The post-convolution of the ionized gas distribution was used to select the central voxel (see right panel of Figure \ref{['fig:convolution']}). Blue histogram: random sub-sample of the grey histogram such that it follows the gaussian distribution (dashed line) for $n_{\rm ion}$ as indicated by Table \ref{['tab:solar-sys-params']}. A total of 281 solar system-like regions fullfil all selection criteria of Table \ref{['tab:solar-sys-params']}.