Warm dark matter signatures on the 21cm power spectrum: Intensity
  mapping forecasts for SKA
  We investigate the impact that warm dark matter (WDM) has in terms of 21cm
intensity mapping in the post-reionization Universe at z = 3 - 5. We perform
hydrodynamic simulations for 5 different models: cold dark matter and WDM with
1,2,3,4 keV (thermal relic) mass and assign the neutral hydrogen a-posteriori
using two different methods that both reproduce observations in terms of column
density distribution function of neutral hydrogen systems. Contrary to naive
expectations, the suppression of power present in the linear and non-linear
matter power spectra, results in an increase of power in terms of neutral
hydrogen and 21cm power spectra. This is due to the fact that there is a lack
of small mass halos in WDM models with respect to cold dark matter: in order to
distribute a total amount of neutral hydrogen within the two cosmological
models, a larger quantity has to be placed in the most massive halos, that are
more biased compared to the cold dark matter cosmology. We quantify this effect
and address significance for the telescope SKA1-LOW, including a realistic
noise modeling. The results indicate that we will be able to rule out a 4 keV
WDM model with 5000 hours of observations at z > 3, with a statistical
significance of > 3 sigma, while a smaller mass of 3 keV, comparable to present
day constraints, can be ruled out at more than 2 sigma confidence level with
1000 hours of observations at z > 5.
