Possible Evidence for MeV Dark Matter In Dwarf Spheroidals

TL;DR

The paper investigates whether the Galactic bulge 511 keV line could originate from annihilating MeV-scale dark matter in the mass range $1$–$100$ MeV. It proposes testing this hypothesis with observations of nearby dwarf spheroidal galaxies, notably Sagittarius and Draco, modeling DM halos with cusped and cored profiles, and computing the expected 511 keV flux via the line-of-sight integral (the J-factor) while accounting for positron propagation. The authors derive constraints from the bulge signal that fix a relation $ (\sigma v / \mathrm{pb}) (1\,\mathrm{MeV}/m_{\rm dm})^2 \approx 4.8^{+4.4}_{-2.4} \times 10^{-4}$ and predict Sagittarius fluxes of order $\Phi \sim 10^{-4}\ \mathrm{cm^{-2}\,s^{-1}}$ (Draco ~10× smaller), potentially detectable by INTEGRAL/SPI; a non-detection would challenge the DM interpretation, while a detection would be a smoking gun for MeV DM. The work also notes that a nearby Canis Major dwarf could yield even larger fluxes, strengthening the testability of the scenario with current data.

Abstract

It has been recently proposed that the observed 511 keV emission from the Galactic bulge could be the product of very light (1-100 MeV) annihilating dark matter particles. Other possible explanations for this signal are associated with stellar objects, such as hypernovae. In order to distinguish between annihilating light dark matter scenario and more conventional astrophysical sources for the bulge emission, we here propose the study of dwarf spheroidals such as Sagittarius. These galaxies have typical luminosities of $10^5 L_\odot$ but mass-to-light ratios of $\sim 100$. As there are comparatively few stars, the prospects for 511 keV emission from standard astrophysical scenarios are minimal. The dwarf spheroidals do, however, contain copious amounts of dark matter. INTEGRAL/SPI has observed the Sagittarius region. Analysis of this data for 511 keV emission will provide a test of MeV dark matter which can distinguish between annihilating dark matter and more standard astrophysics. The observation of such a signal from Sagittarius should be a ``smoking gun'' for MeV dark matter.