Astrophysics & Cosmology from Line Intensity Mapping vs Galaxy Surveys
  Line intensity mapping (LIM) proposes to efficiently observe distant faint
galaxies and map the matter density field at high redshift. Building upon the
formalism in the companion paper, we first highlight the degeneracies between
cosmology and astrophysics in LIM. We discuss what can be constrained from
measurements of the mean intensity and redshift-space power spectra. With a
sufficient spectral resolution, the large-scale redshift-space distortions of
the 2-halo term can be measured, helping to break the degeneracy between bias
and mean intensity. With a higher spectral resolution, measuring the
small-scale redshift-space distortions disentangles the 1-halo and shot noise
terms. Cross-correlations with external galaxy catalogs or lensing surveys
further break degeneracies. We derive requirements for experiments similar to
SPHEREx, HETDEX, CDIM, COMAP and CONCERTO. We then revisit the question of the
optimality of the LIM observables, compared to galaxy detection, for
astrophysics and cosmology. We use a matched filter to compute the luminosity
detection threshold for individual sources. We show that LIM contains
information about galaxies too faint to detect, in the high-noise or
high-confusion regimes. We quantify the sparsity and clustering bias of the
detected sources and compare them to LIM, showing in which cases LIM is a
better tracer of the matter density. We extend previous work by answering these
questions as a function of Fourier scale, including for the first time the
effect of cosmic variance, pixel-to-pixel correlations, luminosity-dependent
clustering bias and redshift-space distortions.
