Reticulum II: Particle Dark Matter and Primordial Black Holes Limits

Abstract

Reticulum II (Ret II) is a satellite galaxy of the Milky Way and presents a prime target to investigate the nature of dark matter (DM) because of its high mass-to-light ratio. We evaluate a dedicated INTEGRAL observation campaign data set to obtain $\gamma$-ray fluxes from Ret II and compare those with expectations from DM. Ret II is not detected in the $\gamma$-ray band 25--8000 keV, and we derive a flux limit of $\lesssim 10^{-8}\mathrm{erg}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$. The previously reported 511 keV line is not seen, and we find a flux limit of $\lesssim 1.7\times 10^{-4}\mathrm{ph}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$. We construct spectral models for primordial black hole (PBH) evaporation and annihilation/decay of particle DM, and subsequent annihilation of positrons produced in these processes. We exclude that the totality of DM in Ret II is made of a monochromatic distribution of PBHs of masses $\lesssim 8\times 10^{15}\mathrm{g}$. Our limits on the velocity-averaged DM annihilation cross section into $e^{+}{e}^{-}$ are $⟨\sigma v⟩\lesssim 5\times 10^{-28}{\left(m_{\mathrm{DM}}/\mathrm{MeV}\right)}^{2.5}\mathrm{c}{\mathrm{m}}^3{\mathrm{s}}^{-1}$. We conclude that analysing isolated targets in the MeV $\gamma$-ray band can set strong bounds on DM properties without multi-year data sets of the entire Milky Way, and encourage follow-up observations of Ret II and other dwarf galaxies.

8 pages, 10 figures, submitted

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