Measurement of the small-scale 3D Lyman-$\alpha$ forest power spectrum

TL;DR

This paper introduces a simple, low-cost method to measure the small-scale 3D power spectrum of the Lyman-alpha forest, validated with simulations and applied to observational data, aiding cosmological studies.

Contribution

A novel, computationally efficient approach to estimate the 3D Ly$ ext{-}\alpha$ forest power spectrum from sparse data samples, demonstrated on simulations and real survey data.

Findings

Validated method with simulations

First $P_{3D}$ measurement from eBOSS data

Potential to improve cosmological constraints

Abstract

Small-scale correlations measured in the Lyman-$\alpha$ (Ly$\alpha$) forest encode information about the intergalactic medium and the primordial matter power spectrum. In this article, we present and implement a simple method to measure the 3-dimensional power spectrum, $P_{\rm 3D}$, of the Ly$\alpha$ forest at wavenumbers $k$ corresponding to small, $\sim$ Mpc scales. In order to estimate $P_{\rm 3D}$ from sparsely and unevenly distributed data samples, we rely on averaging 1-dimensional Fourier Transforms, as previously carried out to estimate the 1-dimensional power spectrum of the Ly$\alpha$ forest, $P_{\rm 1D}$. This methodology exhibits a very low computational cost. We confirm the validity of this approach through its application to Nyx cosmological hydrodynamical simulations. Subsequently, we apply our method to the eBOSS DR16 Ly$\alpha$ forest sample, providing as a proof of principle, a first $P_{\rm 3D}$ measurement averaged over two redshift bins $z=2.2$ and $z=2.4$. This work highlights the potential for forthcoming $P_{\rm 3D}$ measurements, from upcoming large spectroscopic surveys, to untangle degeneracies in the cosmological interpretation of $P_{\rm 1D}$.