Spectroscopic search for optical emission lines from dark matter decay

Abstract

We search for narrow-line optical emission from dark matter decay by stacking dark-sky spectra from the Dark Energy Spectroscopic Instrument (DESI) at the redshift of nearby galaxies from DESI's Bright Galaxy and Luminous Red Galaxy samples. Our search uses regions separated by 5 to 20 arcsec from the centers of the galaxies, corresponding to an impact parameter of approximately 50 kpc. No unidentified spectral line shows up in the search, and we place a line flux limit of ${10}^{\ensuremath{-}19}\text{ }\text{ }\mathrm{ergs}/\mathrm{s}/{\mathrm{cm}}^{2}/{\mathrm{arcsec}}^{2}$ on emissions in the wavelength range of $2000--9000\stackrel{\ensuremath{\circ}}{\mathrm{A}}$. This places the tightest constraints yet on the two-photon decay of dark matter in the mass range of 5 to 12 eV, with a particle lifetime exceeding $3\ifmmode\times\else\texttimes\fi{}{10}^{25}\text{ }\text{ }\mathrm{s}$. This detection limit also implies that the line surface brightness contributed from all dark matter along the line of sight is at least 2 orders of magnitude lower than the measured extragalactic background light (EBL), ruling out the possibility that narrow optical-line emission from dark matter decay is a major source of the EBL.