Particle energy cascade in the intergalactic medium

We study the development of high-energy (E-in < 1 TeV) cascades produced by a primary electron of energy E-in injected into the intergalactic medium (IGM). To this aim we have developed the new code medea (Monte Carlo Energy Deposition Analysis) which includes Bremsstrahlung and inverse Compton (IC) processes, along with H/He collisional ionizations and excitations, and electron-electron collisions. The cascade energy partition into heating, excitations and ionizations depends primarily not only on the IGM ionized fraction, x(e), but also on redshift, z, due to IC on cosmic microwave background (CMB) photons. While Bremsstrahlung is unimportant under most conditions, IC becomes largely dominant at energies E-in >= 1 MeV. The main effect of IC at injection energies E-in < 100 MeV is a significant boost of the fraction of energy converted into low-energy photons (h nu < 10.2 eV) which do not further interact with the IGM. For energies E-in >= 1 GeV CMB photons are preferentially upscattered within the X-ray spectrum (h nu > 104 eV) and can free stream to the observer. Complete tables of the fractional energy depositions as a function of redshift, E-in and ionized fraction are given. Our results can be used in many astrophysical contexts, with an obvious application related to the study of decaying/annihilating dark matter (DM) candidates in the high-z Universe.