Search for dark matter annihilation signatures in H.E.S.S. observations of dwarf spheroidal galaxies

Dwarf spheroidal galaxies of the Local Group are close satellites of the Milky Way characterized by a large mass-to-light ratio and are not expected to be the site of nonthermal high-energy gamma-ray emission or intense star formation. Therefore they are among the most promising candidates for indirect dark matter searches. During the last years the High Energy Stereoscopic System (H.E.S.S.) of imaging atmospheric Cherenkov telescopes observed five of these dwarf galaxies for more than 140 hours in total, searching for TeV gamma-ray emission from annihilation of dark matter particles. The new results of the deep exposure of the Sagittarius dwarf spheroidal galaxy, the first observations of the Coma Berenices and Fornax dwarves and the reanalysis of two more dwarf spheroidal galaxies already published by the H.E.S.S. Collaboration, Carina and Sculptor, are presented. In the absence of a significant signal new constraints on the annihilation cross section applicable to weakly interacting massive particles (WIMPs) are derived by combining the observations of the five dwarf galaxies. The combined exclusion limit depends on the WIMP mass and the best constraint is reached at 1–2 TeV masses with a cross-section upper bound of $\sim 3.9\times {10}^{-24}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ at a 95% confidence level.

Figure 1

Significance distributions and corresponding Gaussian fits over the camera field of view for the five analyzed dSphs. No significant excess is seen at the nominal target positions.

Figure 2

Left: significance skymap in equatorial coordinates. Right: ${\theta }^2$ radial distribution of the ON events for gamma-ray-like events from the Sgr target position. The estimated background is also shown by black crosses. No significant excess is seen within an angular region around the source position of $\theta \le 0.1°$.

Figure 3

Upper limits at 95% C.L. on the flux as a function of $m_{\chi }$ and under the hypothesis of DM particle annihilation in the $W^{+}{W}^{-}$ and $ZZ$ final states as parametrized in Eq. (4), obtained with the likelihood approach.

Figure 4

Upper limits at 95% C.L. on the flux for Sagittarius dwarf galaxy as a function of $m_{\chi }$ and under the hypothesis of different DM particle annihilation channels, obtained with the likelihood approach.

Figure 5

Exclusion limit at 95% C.L. on the velocity-weighted WIMP self-annihilation cross section versus the DM particle mass $m_{\chi }$, under the hypothesis of DM particle annihilation in the $W^{+}{W}^{-}$ and $ZZ$ final states as parametrized in Eq. (4) and for the two hypotheses of NFW and Burkert halo profiles.

Figure 6

Combined exclusion limits at 95% C.L. on the velocity-weighted WIMP self-annihilation cross section versus the DM particle mass $m_{\chi }$, under the hypothesis of DM particle annihilation in the $W^{+}{W}^{-}$ and $ZZ$ final states as parametrized in Eq. (4) and for the two hypotheses of NFW and Burkert halo profiles. The results refer to the combination of all five dwarf galaxies examined in this work and the combination of all but Sgr. The Sgr bounds are also shown for comparison.

Figure 7

Exclusion limit from the Sgr on the velocity-weighted annihilation cross section versus the DM particle mass $m_{\chi }$ and under the hypothesis of DM particle annihilation in different channels.

Figure 8

Exclusion limits on the velocity-weighted annihilation cross section versus the DM particle mass. The limits combine the results from the five dwarf galaxies assuming a NFW DM density profile and two WIMP annihilation final states: $W^{+}{W}^{-}$, $ZZ$ and ${\tau }^{+}{\tau }^{-}$ channels. NMSSM models scan is also shown (blue markers).

Figure 9

Exclusion limits on the velocity-weighted annihilation cross section versus the DM particle mass. The limits combine the results from the five dwarf galaxies observed with H.E.S.S. and assume a NFW DM density profile. The results are compared with regions of the $(m_{\chi },⟨{\sigma }_{\text{ann}}v⟩)$ plane favored by AMS and PAMELA measurements of the positron fraction (blue contours) and by Fermi-LAT and H.E.S.S. measurements of the electron and positron fluxes (green contours), both at $3\sigma$ and $5\sigma$, assuming annihilation into a ${\mu }^{+}{\mu }^{-}$ final state. The corresponding results for the $e^{+}{e}^{-}{e}^{+}{e}^{-}$ and ${\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ channels are also shown.