Indication of Gamma-ray Emission from the Newly Discovered Dwarf Galaxy Reticulum II

We present a search for γ-ray emission from the direction of the newly discovered dwarf galaxy Reticulum II. Using Fermi-LAT Collaboration data, we detect a signal that exceeds expected backgrounds between ∼2–10  GeV and is consistent with annihilation of dark matter for particle masses less than a few ×102  GeV. Modeling the background as a Poisson process based on Fermi-LAT diffuse models, and taking into account trial factors, we detect emission withp value less than 9.8×10−5 (>3.7σ). An alternative, model-independent treatment of the background reduces the significance, raising the p value to 9.7×10−3 (2.3σ). Even in this case, however, Reticulum II has the most significant γ-ray signal of any known dwarf galaxy. If Reticulum II has a dark-matter halo that is similar to those inferred for other nearby dwarfs, the signal is consistent with the s-wave relic abundance cross section for annihilation.