Abstract
Upcoming 21-cm intensity surveys will use the hyperfine transition in
emission to map out neutral hydrogen in large volumes of the universe.
Unfortunately, large spatial scales are completely contaminated with spectrally
smooth astrophysical foregrounds which are orders of magnitude brighter than
the signal. This contamination also leaks into smaller radial and angular modes
to form a foreground wedge, further limiting the usefulness of 21-cm
observations for different science cases, especially cross-correlations with
tracers that have wide kernels in the radial direction. In this paper, we
investigate reconstructing these modes within a forward modeling framework.
Starting with an initial density field, a suitable bias parameterization and
non-linear dynamics to model the observed 21-cm field, our reconstruction
proceeds by maximizing the likelihood of a forward simulation to match the
observations, under given modeling error and a data noise model. For redshifts
$z=2$ and 4, we are able to reconstruct 21cm field with cross correlation, $r_c
> 0.8$ on all scales for both our optimistic and pessimistic assumptions about
foreground contamination and for different levels of thermal noise. The
performance deteriorates slightly at $z=6$. The large-scale line-of-sight modes
are reconstructed almost perfectly. We demonstrate how our method also provides
a technique for density field reconstruction for baryon acoustic oscillations,
outperforming standard methods on all scales. We also describe how our
reconstructed field can provide superb clustering redshift estimation at high
redshifts, where it is otherwise extremely difficult to obtain dense
spectroscopic samples, as well as open up a wealth of cross-correlation
opportunities with projected fields (e.g. lensing) which are restricted to
modes transverse to the line of sight.
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