Upper bounds on matter-antimatter admixture from gamma-ray observations of colliding clusters of galaxies with the Fermi Large Area Telescope

We examined whether significant constraints on matter-antimatter admixture can be derived from gamma-ray observations of colliding clusters of galaxies with Fermi-LAT. We selected ten known systems of colliding clusters of galaxies for the analysis and computed the upper bounds on matter-antimatter admixture in these systems, which range from $7\times 10^{-9}$ to $2\times 10^{-6}$. This allowed us to exclude a symmetric universe on scales of order $\sim 20\mathrm{Mpc}$ at the confidence level of 99.9%. Adopting the number of systems of colliding galaxy clusters from the Marenostrum Universe cosmological simulation, we checked if the Fermi-LAT second source catalog contains a sufficient number of gamma-ray sources to provide us with the required number of sources possibly associated with $p\overline{p}$ annihilation from cluster-anticluster collisions. We found that a matter-antimatter-symmetric universe is strongly ruled out on scales of order $\sim 20\mathrm{Mpc}$ if a matter-antimatter admixture in these bullet-like systems is of $f\gtrsim 10^{-5}$, and on scales of order $\sim 400\mathrm{Mpc}$ if $f\gtrsim 10^{-4}$.

Figure 1

Illustration of the behavior of the likelihood with respect to changing the hypothesis, i.e. the increase of the $p\overline{p}$ annihilation flux, for one of the selected colliding systems, Abell 520. $L0$ is the maximized likelihood for the “null” hypothesis.