Abstract
Recently, some very tight constraints of annihilating dark matter have been obtained from gamma-ray data of the Milky Way and Milky Way dwarf spheroidal satellite galaxies. In this article, we report that there are two excellent galaxy clusters (A2877 and Fornax) which can provide interesting constraints for annihilating dark matter. The lower limits of the dark matter mass for the thermal relic annihilation cross section are 25 GeV, 6 GeV, 130 GeV and 100 GeV respectively for the e+e−, μ+μ−, τ+τ− and bbar{b} channels. For some configuration of our working assumptions, our results improve the Fermi-LAT upper limits of annihilation cross sections by a factor of 1.3 − 1.8 for wide ranges of dark matter mass for e+e−, μ+μ− and bbar{b} channels, and a factor of 1.2–1.8 for τ+τ− channel with dark matter mass ≤100 GeV. These limits basically rule out most of the existing popular dark matter interpretation of the GeV excess in the Milky Way.
Highlights
In the past few years, gamma-ray observations of the Milky Way initiated a hot research area about dark matter annihilation
By using the latest gamma-ray data of galaxy clusters from Fermi-LAT, we show that the A2877 and Fornax clusters can give very stringent constraints for annihilating dark matter
Standard cosmology indicates that the thermal relic annihilation cross section should be = 2.2 × 10−26 cm[3] s−1 so that it gives the correct amount of dark matter in our universe[36]
Summary
There are two different kinds of gamma-ray emissions for annihilating dark matter in galaxy clusters. Since dark matter annihilation produces electron and positron pairs, these electron and positron pairs can interact with the thermal electrons in galaxy clusters via Bremsstrahlung cooling This process would emit gamma-ray photons if the energy of the electrons or positrons is high enough. Where ρDM is the dark matter density profile, R200 is the radius of a galaxy cluster, and dNγ/dE is the energy spectrum of gamma-ray produced per one annihilation, which can be obtained in[24]. By assuming the dark matter density profiles of the A2877 and Fornax clusters follow the Navarro-Frenk-White (NFW) density profile ρDM = ρsrs3[r(r + rs)2]−1 27 and using the corresponding parameters in[28], we can get the total photons produced with energy greater than E0 for direct emission. The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request
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