Intergalactic medium heating by dark matter

We derive the evolution of the energy deposition in the intergalactic medium (IGM) by dark matter (DM) decays/annihilations for both sterile neutrinos and light dark matter (LDM) particles. At z > 200 sterile neutrinos transfer a fraction fabs ~ 0.5 of their rest mass energy into the IGM; at lower redshifts this fraction becomes <~0.3 depending on the particle mass. The LDM particles can decay or annihilate. In both the cases fabs ~ 0.4-0.9 at high (>300) redshift, dropping to ~0.1 below z = 100. These results indicate that the impact of DM decays/annihilations on the IGM thermal and ionization history is less important than previously thought. We find that sterile neutrinos (LDM) decays are able to increase the IGM temperature by z = 5 at most up to 4K (100K), about 50-200 times less than predicted by estimates based on the assumption of complete energy transfer to the gas.