Dark matter model favoured by reionization data: 7 keV sterile neutrino vs cold dark matter.

One of possible explanations of a faint narrow emission line at 3.5 keV reported in our Galaxy, Andromeda galaxy and a number of galaxy clusters is the dark matter made of 7 keV sterile neutrinos. Another signature of such sterile neutrino dark matter could be fewer ionizing sources in the early Universe (compared to the standard "cold dark matter" (CDM) scenario), which should affect the reionization of the Universe. By using a semi-analytical model of reionization, we compare the model predictions for CDM and two different models of 7 keV sterile neutrino dark matter (consistent with the 3.5 keV line interpretation as decaying dark matter line) with available observations of epoch of reionization (including the final measurements of electron scattering optical depth made by Planck observatory). We found that both CDM and 7 keV sterile neutrino dark matter well describe the data. The overall fit quality for sterile neutrino dark matter is slightly (with $\Delta \chi^2 \simeq 2-3$) better than for CDM, although it is not possible to make a robust distinction between these models on the basis of the given observations.