Secondary ionization and heating by fast electrons
  We examine the fate of fast electrons (with energies E>10 eV) in a thermal
gas of primordial composition. To follow their interactions with the background
gas, we construct a Monte Carlo model that includes: (1) electron-electron
scattering (which transforms the electron kinetic energy into heat), (2)
collisional ionization of hydrogen and helium (which produces secondary
electrons that themselves scatter through the medium), and (3) collisional
excitation (which produces secondary photons, whose fates we also follow
approximately). For the last process, we explicitly include all transitions to
upper levels n<=4, together with a well-motivated extrapolation to higher
levels. In all cases, we use recent calculated cross-sections at E<1 keV and
the Bethe approximation to extrapolate to higher energies. We compute the
fractions of energy deposited as heat, ionization (tracking HI and the helium
species separately), and excitation (tracking HI Lyman-alpha separately) under
a broad range of conditions appropriate to the intergalactic medium. The energy
deposition fractions depend on both the background ionized fraction and the
electron energy but are nearly independent of the background density. We find
good agreement with some, but not all, previous calculations at high energies.
Electronic tables of our results are available on request.
