First Ly$\alpha$ 1D Bispectrum Measurement in eBOSS

TL;DR

This paper reports the first robust measurement of the one-dimensional Lyα forest bispectrum from eBOSS quasar data, demonstrating its potential to complement power spectrum analyses in cosmology.

Contribution

It introduces a novel bispectrum measurement method for Lyα forest data, validated with mocks, and extends modeling to include SiIII correlations, paving the way for future large-scale surveys.

Findings

First robust Lyα bispectrum measurement from eBOSS data

Detection of SiIII-Lyα correlations in the bispectrum

A perturbation theory model explains the bispectrum reasonably well

Abstract

We present the first robust measurement of the one-dimensional Lyman alpha (Ly$\alpha$) forest bispectrum using the complete extended Baryon Oscillation Spectroscopic Survey (eBOSS) quasar sample, corresponding to the sixteenth data release (DR16) of the Sloan Digital Sky Survey (SDSS). The measurement employs an FFT estimator over 12 redshift bins, ranging from $z=2.2$ to $z=4.4$, and extends to scales of $0.02 ~ (\mathrm{km/s})^{-1}$. The sample consists of 122,066 quasar spectra, although only the first six redshift bins contain sufficient data to extract a physical bispectrum. To validate and correct the bispectrum measurement, we use synthetic datasets generated from lognormal and 2LPT mocks. Additionally, we detect clear evidence of correlations between Si$_\mathrm{III}$ absorption lines and the Ly$\alpha$ forest within the bispectrum signal, which we describe with an extension of the model used for the analogue of 1d power spectrum signal. In this context, the pipeline developed for this study addresses the impact of instrumental and methodological systematics and is ready for application to larger spectroscopic datasets, such as those from the first year of DESI observations. Finally, A simple perturbation theory model provides a reasonable explanation of the eBOSS bispectrum, suggesting that higher-order one-dimensional statistics in the Ly$\alpha$ forest can complement cosmological model inference based on the power spectrum in future analyses.