Constraints on radiative dark-matter decay from the cosmic microwave background

If dark matter decays to electromagnetically interacting particles, it can inject energy into the baryonic gas and thus affect the processes of recombination and reionization. This leaves an imprint on the cosmic microwave background (CMB): the large-scale polarization is enhanced, and the small-scale temperature fluctuation is damped. We use the Wilkinson Microwave Anisotropy Probe (WMAP) three-year data combined with galaxy surveys to constrain radiatively decaying dark matter. Our new limits to the dark-matter decay width are about 10 times stronger than previous limits. For dark-matter lifetimes that exceed the age of the Universe, a limit of _<1. 710^-25 s^-1 (95% C. L. ) is derived, where is the efficiency of converting decay energy into ionization energy. Limits for lifetimes short compared with the age of the Universe are also derived. We forecast improvements expected from the Planck satellite.