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.
