Search for Dark Matter Annihilation and Decay with H$\alpha$ Line Emission

Abstract

I present a new indirect search for dark matter (DM) using Hydrogen-$\alpha$ (H$\alpha$) recombination emission. DM annihilation or decay products can ionize neutral gas; subsequent recombination cascades generate H$\alpha$ photons through the $3\to 2$ transition. In quiet gas-rich dwarf galaxies, the $n=2$ population is negligible, so H$\alpha$ is effectively unabsorbed and traces the DM-energy injection site. Using the non-detection of extended H$\alpha$ emission in the Leo T dwarf galaxy with Multi Unit Spectroscopic Explorer (MUSE) observations, I derive the first H$\alpha$-based limits on DM annihilation and decay, reaching leading sensitivity for parts of the eV-GeV mass range. Existing and upcoming telescopes can further extend this reach, establishing H$\alpha$ imaging as a powerful DM search strategy.

Comments: 5+10 pages, 2+4 figures, 1 red spectral line at 656 nanometers, version accepted for publication in PRL