The cross-correlation between 21cm intensity mapping maps and the
  Lyman-alpha forest in the post-reionization era
  We investigate the cross-correlation signal between 21cm intensity mapping
maps and the Lyman-alpha forest in the fully non-linear regime using
state-of-the-art hydrodynamic simulations. The cross-correlation signal between
these fields can provide a coherent and comprehensive picture of the neutral
hydrogen (HI) content of our Universe in the post-reionization era, probing
both its mass content and volume distribution. We compute the auto-power
spectra of both fields together with their cross-power spectrum at z = 2.4 and
find that on large scales the fields are completely anti-correlated. This
anti-correlation arises because regions with high (low) 21cm emission, such as
those with a large (low) concentration of damped Lyman-alpha systems, will show
up as regions with low (high) transmitted flux. We find that on scales smaller
than k = 0.2 h/Mpc the cross-correlation coefficient departs from -1, at a
scale where non-linearities show up. We use the anisotropy of the power spectra
in redshift-space to determine the values of the bias and of the redshift-space
distortion parameters of both fields; we find that the errors on the value of
these parameters could decrease by 30% when adding data from the cross-power
spectrum in a conservative analysis. Our results point out that linear theory
is capable of reproducing the shape and amplitude of the cross-power up to
rather non-linear scales. Finally, we find that the 21cm-Lya cross-power
spectrum can be detected by combining data from a BOSS-like survey together
with 21cm intensity mapping observations by SKA1-MID with a S/N ratio higher
than 3 in the range 0.06< k <1 h/Mpc. We emphasize that while the shape and
amplitude of the 21cm auto-power spectrum can be severely affected by residual
foreground contamination, cross-power spectra will be less sensitive to that
and therefore can be used to identify systematics in the 21cm maps.
