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HI Intensity Mapping (IM) will be used to do precision cosmology using manyexisting and upcoming radio observatories. The signal will be contaminated dueto absorption, the largest component of which will be the flux absorbed by theHI emitting sources themselves from the flux incident on them from backgroundradio continuum sources. We, for the first time, provide a quantitativeestimate of the magnitude of the absorbed flux compared to the emitted HI fluxfor various voxels placed at redshifts between 0.1 and 2.5. We use acosmological sky simulation of the atomic HI emission line, and sum over theemitted and absorbed fluxes for all sources within voxels at differentredshifts. For estimating the absorbed flux we use various relations based onexisting observations as well as simulations. We find that for the sameco-moving volume of sky, the HI emission falls off quickly with increasingredshift, while the absorption varies much less with redshift and follows theredshift distribution of faint sources that dominate the number counts of radiocontinuum sources. This results in the fraction of absorption compared toemission 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 about30% up to a redshift of 2.5. These numbers can vary significantly due to theuncertainties on the exact forms of the various relations used, the largestvariation being driven by the uncertainty on the number counts of radiocontinuum sources at sub-mJy flux densities. Absorption of flux incident frombackground radio continuum sources might become an important contaminant to HIIM signals beyond redshifts of 0.5, and needs to be quantified more accuratelyusing inputs from upcoming deep high resolution surveys of radio continuumsources, HI absorption, and HI emission with the SKA and its precursors.

A first quantification of the effects of absorption for HI Intensity Mapping experiments