On Estimating Lyman-alpha Forest Correlations between Multiple Sightlines

TL;DR

This paper investigates how spectroscopic surveys can effectively measure 3D Lyman-alpha forest correlations, proposing methods to optimize survey design and data weighting to improve cosmological parameter constraints.

Contribution

It introduces a simple weighting method for sightlines, analyzes survey sensitivity scaling, and provides guidelines for survey optimization to enhance 3D correlation measurements.

Findings

Sensitivity depends on a noise-weighted source density metric.

Optimal survey depth achieves S/N ~ 2 per pixel for L_* quasars.

Survey design focusing on certain exposure times maximizes correlation detection.

Abstract

The next frontier of Lyman-alpha forest studies is the reconstruction of 3D correlations from a dense sample of background sources. The measurement of 3D correlations has the potential to improve constraints on fundamental cosmological parameters, ionizing background models, and the reionization history. This study addresses the sensitivity of spectroscopic surveys to 3D correlations in the Lyman-alpha forest. We show that the sensitivity of a survey to this signal can be quantified by just a single number, a noise-weighted number density of sources on the sky. We investigate how the sensitivity of a spectroscopic quasar (or galaxy) survey scales as a function of its depth, area, and redshift. We propose a simple method for weighting sightlines with varying S/N levels to estimate the correlation function, and we show that this estimator generally performs nearly as well as the minimum variance quadratic estimator. In addition, we show that the sensitivity of a quasar survey to the flux correlation function is generally maximized if it observes each field just long enough to achieve S/N ~ 2 in a 1 A pixel on an L_* quasar while acquiring spectra for all quasars with L > L_*: Little is gained by integrating longer on the same targets or by including fainter quasars. We quantify how these considerations relate to constraints on the angular diameter distance, the curvature of space-time, and the reionization history.