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

We present a search for $\gamma$-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 $\sim 2–10\mathrm{GeV}$ and is consistent with annihilation of dark matter for particle masses less than a few $\times 10^2\mathrm{GeV}$. Modeling the background as a Poisson process based on Fermi-LAT diffuse models, and taking into account trial factors, we detect emission with $p$ value less than $9.8\times 10^{-5}$ ($>3.7\sigma$). An alternative, model-independent treatment of the background reduces the significance, raising the $p$ value to $9.7\times 10^{-3}$ ($2.3\sigma$). Even in this case, however, Reticulum II has the most significant $\gamma$-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.

Figure 1

Energy spectrum of events detected within 0.5° of RetII (red points) with 68% Poisson error bars. Two background estimates are shown: (1) the sum (solid black) of the Fermi Collaboration’s models for isotropic (dashed line) and galactic diffuse (dot-dashed line) emission at the location of RetII, and (2) the average intensity (gray triangles) within 3306 ROIs that lie within 10° of RetII and overlap neither known sources nor the ROI centered on RetII. The number of events detected from RetII in each energy bin is shown above the error bar. The number expected from the Fermi background model is shown below the solid black curve.

Figure 2

Significance of the $\gamma$-ray excess in the direction of Reticulum II as a function of dark matter particle mass. Left: curves correspond to the result of the search in various channels (i.e., using different ways of weighting events) using background model Diffuse 1. The curve for $e^{+}{e}^{-}$ is similar to ${\mu }^{+}{\mu }^{-}$, $ZZ$ is similar to $W^{+}{W}^{-}$, and $q$ represents $u,d,c,s$ quarks and gluons. Right: significance in the ${\tau }^{+}{\tau }^{-}$ channel for four different background models (see text).

Figure 3

Left: significance of $\gamma$-ray detection for annihilation into ${\tau }^{+}{\tau }^{-}$ for various masses, calculated using the model-independent procedure of Ref. [41]. Solid and dashed blue lines correspond to RetII and Seg1 (another attractive nearby target). Gray curves correspond to the collection of dwarfs used in Ref. [41] as well as the eight other newly discovered DES dwarfs. Right: the $\text{Fermi}\text{isotropic}+\text{diffuse}\text{model}$ intensity near RetII. The color corresponds to intensity normalized to the value in the direction of RetII (at an energy of 8 GeV; other energies are similar). A 0.5° ROI is shown at the center and the small dots show the centers of the ROIs used for the empirical background estimation. White $\mathsf{x}$’s mark the locations of known $\gamma$-ray sources. Green circles are the ROIs which have a test statistic larger than that in the central ROI (when searching for a 25 GeV particle annihilating to ${\tau }^{+}{\tau }^{-}$).

Figure 4

An exploration of a dark matter interpretation of the observed $\gamma$-ray excess for four representative annihilation channels. $J=J_{19}10^{19}{\mathrm{GeV}}^2{\mathrm{cm}}^{-5}$ and $⟨\sigma v⟩=⟨\sigma v{⟩}_{-26}10^{-26}{\mathrm{cm}}^3{\sec }^{-1}$. The data constrain only the product of $J⟨\sigma v⟩$ since the dark matter content of Reticulum II is currently unknown. Contours represent 68%, 95%, and 99.7% confidence regions. Note that this figure does not quantify which annihilation channel is preferred by the data, i.e., which channel provides the best fit to the $\gamma$-ray spectrum.