Impact of dark matter models on the EoR 21-cm signal bispectrum
  The nature of dark matter sets the timeline for the formation of first
collapsed halos and thus affects the sources of reionization. Here, we consider
two different models of dark matter: cold dark matter (CDM) and thermal warm
dark matter (WDM), and study how they impact the epoch of reionization (EoR)
and its 21-cm observables. Using a suite of simulations, we find that in the
WDM scenarios, the structure formation on small scales gets suppressed
resulting in a smaller number of low mass dark matter halos compared to the CDM
scenario. Assuming that the efficiency of sources in producing ionizing photons
remain the same, this leads to a lower number of total ionizing photons
produced at any given cosmic time and thus in a delay in the reionization
process. We also find visual differences in the neutral hydrogen (HI) topology
and in 21-cm maps in case of the WDM compared to the CDM. However, differences
in the 21-cm power spectra, at the same neutral fraction, are found to be
small. Thus, we focus on the non-Gaussianity in the EoR 21-cm signal,
quantified through its bispectrum. We find that the 21-cm bispectra (driven by
the HI topology) are significantly different in WDM models when compared with
CDM, even for same mass averaged neutral fractions. This establishes that the
21-cm bispectrum is a unique and promising way to differentiate between
different dark matter models, and can be used to constrain the nature of the
dark matter in the future EoR observations.
