The Energy Cascade from Warm Dark Matter Decays

TL;DR

This paper uses detailed Monte Carlo simulations to analyze how keV-scale electrons from warm dark matter decays deposit energy into the intergalactic medium, affecting ionization, heating, and photon production relevant for early universe observations.

Contribution

It introduces a comprehensive simulation including previously neglected processes and provides fitting formulas for energy deposition fractions from WDM decay electrons.

Findings

Derived precise energy partitioning into heating, ionization, and photon production.

Separated Ly-alpha photons from non-interacting photons to assess their impact on 21 cm signals.

Provided fitting formulas for energy deposition fractions as a function of ionization state.

Abstract

We use a set of Monte Carlo simulations to follow the cascade produced by a primary electron of energy E_in in the intergalactic medium. We choose E_in=3-10 keV as expected from the decay of one of the most popular Warm Dark Matter (WDM) candidates, sterile neutrinos. Our simulation takes into account processes previously neglected such as free-free interactions with ions and recombinations and uses the best available cross sections for collisional ionizations and excitations with H and He and for electron-electron collisions. We precisely derive the fraction of the primary electron energy that heats the gas, ionizes atoms and produces line and continuum photons as a function of the ionization fraction. Handy fitting formulae for all the above energy depositions are provided. By keeping track of the individual photons we can distinguish between photons in the Ly-alpha resonance and those with energy E < 10.2 eV that do not interact further with gas. This separation is important because a Ly-alpha background can heat or cool the gas depending on the nature of the photons, and can have effects on the 21 cm radiation emitted by neutral H, which will probably become detectable at z > 6 in the near future by the next generation radio interferometers.