Tomographic Intensity Mapping versus Galaxy Surveys: Observing the
  Universe in H-alpha emission with new generation instruments
  The H-alpha line emission is an important probe for a number of fundamental
quantities in galaxies, including their number density, star formation rate
(SFR) and overall gas content. A new generation of low-resolution intensity
mapping probes, e.g. SPHEREx and CDIM, will observe galaxies in H-alpha
emission over a large fraction of the sky from the local Universe till a
redshift of z ~ 6 to 10, respectively. This will also be the target line for
observations by the high-resolution Euclid and WFIRST instruments in the z ~
0.7 - 2 redshift range. In this paper, we estimate the intensity and power
spectra of the H-alpha line in the z ~ 0 - 5 redshift range using observed line
luminosity functions (LFs), when possible, and simulations, otherwise. We
estimate the significance of our predictions by accounting for the modelling
uncertainties (e.g. SFR, extinction, etc.) and observational contamination. We
find that Intensity Mapping (IM) surveys can make a statistical detection of
the full H-alpha emission between z ~ 0.8 - 5. Moreover, we find that the
high-frequency resolution and the sensitivity of the planned CDIM surveys allow
for the separation of H-alpha emission from several interloping lines. We
explore ways to use the combination of these line intensities to probe galaxy
properties. As expected, our study indicates that galaxy surveys will only
detect bright galaxies that contribute up to a few percent of the overall
H-alpha intensity. However, these surveys will provide important constraints on
the high end of the H-alpha LF and put strong constraints on the AGN LF.
