The impact of dark matter decays and annihilations on the formation of the first structures

We derive the effects of dark matter (DM) decays and annihilations on structure formation. We consider moderately massive DM particles (sterile neutrinos and light DM), as they are expected to give the maximum contribution to heating and reionization. The energy injection from DM decays and annihilations produces both an enhancement in the abundance of coolants (H2 and HD) and an increase of gas temperature. We find that for all the considered DM models the critical halo mass for collapse, mcrit, is generally higher than in the unperturbed case. However, the variation of mcrit is small. In the most extreme cases, that is, considering light DM annihilations (decays) and haloes virializing at redshift zvir > 30 (zvir ~ 10), mcrit increases by a factor of ~4 (~2). In the case of annihilations the variations of mcrit are also sensitive to the assumed profile of the DM halo. Furthermore, we note that the fraction of gas which is retained inside the halo can be substantially reduced (to ~40 per cent of the cosmic value), especially in the smallest haloes, as a consequence of the energy injection by DM decays and annihilations.