Observational challenges in Ly-alpha intensity mapping

Intensity mapping (IM) is sensitive to the cumulative line emission of galaxies. As such it represents a promising technique for statistical studies of galaxies fainter than the limiting magnitude of traditional galaxy surveys. The strong hydrogen Ly-alpha line is the primary target for such an experiment, as its intensity is linked to star formation activity and the physical state of the interstellar (ISM) and intergalactic (IGM) medium. However, to extract the meaningful information one has to solve the confusion problems caused by interloping lines from foreground galaxies. We discuss here the challenges for a Ly-alpha IM experiment targeting z > 4 sources. We find that the Ly-alpha power spectrum can be in principle easily (marginally) obtained with a 40 cm space telescope in a few days of observing time up to z < 8 (z = 10) assuming that the interloping lines (e.g. H-alpha, [O II], [O III] lines) can be efficiently removed. We show that interlopers can be removed by using an ancillary photometric galaxy survey with limiting AB mag 26 in the NIR bands (Y, J, H, or K). This would enable detection of the Ly-alpha signal from 5 < z < 9 faint sources. However, if a [C II] IM experiment is feasible, by cross-correlating the Ly-alpha with the [C II] signal the required depth of the galaxy survey can be decreased to AB mag 24. This would bring the detection at reach of future facilities working in close synergy.