Spherical Harmonic Analyses of Intensity Mapping Power Spectra

TL;DR

This paper introduces a spherical Fourier-Bessel framework for intensity mapping data analysis, enabling accurate large-scale structure measurements across the full sky without flat-sky limitations, improving systematics diagnosis and power spectrum estimation.

Contribution

It develops a novel spherical analysis framework for intensity mapping, extending flat-sky techniques to curved sky, and generalizes key data analysis methods.

Findings

Framework effectively incorporates curved sky effects.

Generalizes power spectrum estimators to full-sky surveys.

Addresses foreground contamination and interloper line treatments.

Abstract

Intensity mapping is a promising technique for surveying the large scale structure of our Universe from $z=0$ to $z \sim 150$, using the brightness temperature field of spectral lines to directly observe previously unexplored portions of out cosmic timeline. Examples of targeted lines include the $21\,\textrm{cm}$ hyperfine transition of neutral hydrogen, rotational lines of carbon monoxide, and fine structure lines of singly ionized carbon. Recent efforts have focused on detections of the power spectrum of spatial fluctuations, but have been hindered by systematics such as foreground contamination. This has motivated the decomposition of data into Fourier modes perpendicular and parallel to the line-of-sight, which has been shown to be a particularly powerful way to diagnose systematics. However, such a method is well-defined only in the limit of a narrow-field, flat-sky approximation. This limits the sensitivity of intensity mapping experiments, as it means that wide surveys must be separately analyzed as a patchwork of smaller fields. In this paper, we develop a framework for analyzing intensity mapping data in a spherical Fourier-Bessel basis, which incorporates curved sky effects without difficulty. We use our framework to generalize a number of techniques in intensity mapping data analysis from the flat sky to the curved sky. These include visibility-based estimators for the power spectrum, treatments of interloper lines, and the "foreground wedge" signature of spectrally smooth foregrounds.