Galaxy Mergers Collectively Illuminate the $\gamma$-Ray Sky

The origin and acceleration mechanism of cosmic rays (CRs) remain fundamental open questions. Galaxy mergers are proposed as very high-energy CR accelerators, which are expected to produce high-energy (HE) $\gamma$ rays and neutrinos through interactions with the ambient gas and low-energy background radiation fields. For the first time, we systematically study the HE $\gamma$-ray emission from galaxy mergers utilising 16.7 years of Fermi Large Area Telescope (Fermi-LAT) data with the sample list compiled from eight survey catalogs. Our analysis finds 8 galaxy mergers that exhibit $\gamma$-ray emission with significance $\gtrsim5\sigma$ in the 1-500 GeV energy range. A stacking analysis of the remaining faint galaxy mergers yields a combined $\gamma$-ray emission detected at $\sim 35\sigma$ significance, a best-fit spectral index of $\Gamma \approx 2.07$, and an energy flux of $\sim \rm 2\times10^{-14}~erg~cm^{-2}~s^{-1}$. We compare the stacked spectral energy distributions of the galaxy mergers with the projected sensitivity of the upcoming $\gamma$-ray telescope Cherenkov Telescope Array (CTA). Furthermore, we find that 18 previously unassociated Fermi-LAT sources are spatially coincident with galaxy mergers. Our findings establish galaxy mergers as a new class of HE $\gamma$-ray sources. Future neutrino and $\gamma$-ray observatories will be crucial to discover the particle acceleration mechanism in these newly identified CR sources.