Combining galaxy and 21cm surveys

TL;DR

This paper investigates how combining 21cm intensity mapping surveys with galaxy redshift surveys can enhance the measurement of baryon acoustic oscillations (BAO) by using reconstruction techniques based on Lagrangian perturbation theory.

Contribution

It demonstrates the potential of low-order Lagrangian perturbation theory to assess the benefits of survey combination and identifies the critical galaxy density needed for effective BAO reconstruction.

Findings

Reconstruction effectiveness depends on the linear power spectrum peak.

Upcoming surveys like eBOSS and DESI can achieve the necessary galaxy densities.

Combining surveys improves BAO measurement precision.

Abstract

Acoustic waves traveling through the early Universe imprint a characteristic scale in the clustering of galaxies, QSOs and inter-galactic gas. This scale can be used as a standard ruler to map the expansion history of the Universe, a technique known as Baryon Acoustic Oscillations (BAO). BAO offer a high-precision, low-systematics means of constraining our cosmological model. The statistical power of BAO measurements can be improved if the `smearing' of the acoustic feature by non-linear structure formation is undone in a process known as reconstruction. In this paper we use low-order Lagrangian perturbation theory to study the ability of $21\,$cm experiments to perform reconstruction and how augmenting these surveys with galaxy redshift surveys at relatively low number densities can improve performance. We find that the critical number density which must be achieved in order to benefit $21\,$cm surveys is set by the linear theory power spectrum near its peak, and corresponds to densities achievable by upcoming surveys of emission line galaxies such as eBOSS and DESI. As part of this work we analyze reconstruction within the framework of Lagrangian perturbation theory with local Lagrangian bias, redshift-space distortions, ${\bf k}$-dependent noise and anisotropic filtering schemes.