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Abstract

Reticulum II (Ret II) is a satellite galaxy of the Milky Way (MW) 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 γ-ray fluxes from Ret II and compare those with expectations from DM. Ret II is not detected in the γ-ray band 25–8000 keV, and we derive a flux limit of ≲10⁻⁸ergcm⁻²s⁻¹⁠. The previously reported 511 keV line is not seen, and we find a flux limit of ≲1.7×10⁻⁴phcm⁻²s⁻¹⁠. We construct spectral models for primordial black hole (PBH) evaporation and annihilation/decay of particle DM, and subsequent annihilation of e+s produced in these processes. We exclude that the totality of DM in Ret II is made of a monochromatic distribution of PBHs of masses ≲8×10¹⁵g⁠. Our limits on the velocity-averaged DM annihilation cross section into e⁺e⁻ are ⟨σv⟩≲5×10⁻²⁸(mDM/MeV)^2.5cm³s⁻¹⁠. We conclude that analysing isolated targets in the MeV γ-ray band can set strong bounds on DM properties without multi-year data sets of the entire MW, and encourage follow-up observations of Ret II and other dwarf galaxies.