Reconstructing large-scale structure with neutral hydrogen surveys

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.

Large scale structure reconstruction with short-wavelength modes

“…d (1) m,rs ( k) Z 1 ( k; a = 1)δ (1) m,ini ( k) = (1+f (a = 1)µ 2 )δ (1) m,ini ( k). (B1) where again we use the following approximation: V d 3 r e −i q• r f (a(r)) f (a = 1) (2π) 3 δ D ( q) (B2)…”

“…d (1) m,rs ( k)d (1) m,rs ( k ) (2π) 3 is the leading order redshift space matter power spectrum first derived by N. Kaiser [18]. We can decompose its direction-dependence by Legendre polynomials expansion as:…”

“…d (1) m,rs ( k s )d (2) m,rs ( k s ) = Z 1 ( k s ; a = 1) δ (1) m,ini ( k s )d (2) m,rs ( k s ) ×2C Z 2 (− k s , k s + k s ; a = 1)P m,ini (k s )d (1) m,rs ( k s + k s )…”

“…When evaluating the two-point function of the full density contrast, cross-terms appear. For example, δ (1) m ( k; a)δ (2) m ( k ; a) is proportional to δ (1) m ( k + k ; a) if both k and k correspond to short wavelengths but their sum is small (long wavelength). Explicitly, keeping terms up to second order, (1) m ( k l ; a).…”

“…Directly measuring the distribution of matter on large scales is extremely difficult right now as pointed out, e.g., by [1]. The attempt to use small scale perturbations to infer large scale information has been frequently discussed in recent years [2][3] [4][5] [6].…”