A first quantification of the effects of absorption for HI Intensity
  Mapping experiments
  HI Intensity Mapping (IM) will be used to do precision cosmology using many
existing and upcoming radio observatories. The signal will be contaminated due
to absorption, the largest component of which will be the flux absorbed by the
HI emitting sources themselves from the flux incident on them from background
radio continuum sources. We, for the first time, provide a quantitative
estimate of the magnitude of the absorbed flux compared to the emitted HI flux
for various voxels placed at redshifts between 0.1 and 2.5. We use a
cosmological sky simulation of the atomic HI emission line, and sum over the
emitted and absorbed fluxes for all sources within voxels at different
redshifts. For estimating the absorbed flux we use various relations based on
existing observations as well as simulations. We find that for the same
co-moving volume of sky, the HI emission falls off quickly with increasing
redshift, while the absorption varies much less with redshift and follows the
redshift distribution of faint sources that dominate the number counts of radio
continuum sources. This results in the fraction of absorption compared to
emission to be negligible in the nearby Universe (up to a redshift of ~0.5),
increases to about 10% at a redshift of 1, and continues to increase to about
30% up to a redshift of 2.5. These numbers can vary significantly due to the
uncertainties on the exact forms of the various relations used, the largest
variation being driven by the uncertainty on the number counts of radio
continuum sources at sub-mJy flux densities. Absorption of flux incident from
background radio continuum sources might become an important contaminant to HI
IM signals beyond redshifts of 0.5, and needs to be quantified more accurately
using inputs from upcoming deep high resolution surveys of radio continuum
sources, HI absorption, and HI emission with the SKA and its precursors.
