[Paper Review] The Dark Energy Spectroscopic Instrument: One-dimensional power spectrum from first Lyman-$α$ forest samples with Fast Fourier Transform

C. Ravoux, Marie Lynn Abdul Karim+64 more|arXiv (Cornell University)|Jun 9, 2023

Galaxies: Formation, Evolution, Phenomena69 references4 citations

TL;DR

This paper presents the first one-dimensional Lyman-α forest power spectrum measurement from the Dark Energy Spectroscopic Instrument (DESI) using Fast Fourier Transform (FFT) techniques on 26,330 quasar spectra at z > 2.1. The measurement achieves 1% precision agreement with the QMLE method up to half the Nyquist frequency and provides high-resolution constraints on the intergalactic medium, neutrino masses, and warm dark matter models, with future data expected to reach sub-percent precision on a sample of up to 3 million sub-forests.

ABSTRACT

We present the one-dimensional Lyman-$α$ forest power spectrum measurement using the first data provided by the Dark Energy Spectroscopic Instrument (DESI). The data sample comprises $26,330$ quasar spectra, at redshift $z > 2.1$, contained in the DESI Early Data Release and the first two months of the main survey. We employ a Fast Fourier Transform (FFT) estimator and compare the resulting power spectrum to an alternative likelihood-based method in a companion paper. We investigate methodological and instrumental contaminants associated to the new DESI instrument, applying techniques similar to previous Sloan Digital Sky Survey (SDSS) measurements. We use synthetic data based on log-normal approximation to validate and correct our measurement. We compare our resulting power spectrum with previous SDSS and high-resolution measurements. With relatively small number statistics, we successfully perform the FFT measurement, which is already competitive in terms of the scale range. At the end of the DESI survey, we expect a five times larger Lyman-$α$ forest sample than SDSS, providing an unprecedented precise one-dimensional power spectrum measurement.

Motivation & Objective

• To measure the one-dimensional Lyman-α forest power spectrum from the first DESI data release using Fast Fourier Transform (FFT) techniques.
• To assess systematic uncertainties from pixel masking, continuum fitting, and spectral resolution in the FFT-based pipeline.
• To compare the DESI measurement with prior moderate- and high-resolution surveys, particularly eBOSS, to validate consistency and identify discrepancies.
• To lay the foundation for future high-precision cosmological constraints using upcoming DESI data with up to 1 million Lyα forest systems.

Proposed method

• The FFT estimator is applied to 26,330 quasar spectra from DESI’s Early Data Release and the first two months of the main survey, covering redshifts z > 2.1.
• Synthetic data are used to correct for systematic effects introduced by pixel masking, continuum fitting, and spectral resolution modeling.
• Side-band power spectrum techniques are improved to minimize contamination from atmospheric emission lines and residual systematics.
• A comprehensive review of systematic uncertainties is performed, including noise and resolution estimation on pixel-level simulations of the DESI CCD camera.
• The FFT-based power spectrum is validated against the QMLE method in a companion study, showing 1% level agreement up to half the Nyquist frequency.
• High-resolution hydrodynamical simulations and Gaussian process emulators are planned for future use to refine cosmological parameter constraints.

Experimental results

Research questions

• RQ1 How accurately can the FFT method measure the one-dimensional Lyman-α forest power spectrum in the first DESI data release?
• RQ2 What are the dominant systematic uncertainties in the FFT pipeline, and how do they affect the power spectrum measurement?
• RQ3 How does the DESI measurement compare with previous high-resolution surveys such as eBOSS, particularly at large scales?
• RQ4 To what extent does the improved spectral resolution of DESI (approximately twice that of SDSS) enhance small-scale sensitivity to the intergalactic medium?
• RQ5 Can the FFT method achieve sub-percent precision in future DESI data, and how will systematic errors be controlled as data volume increases?

Key findings

• The FFT-based power spectrum measurement from DESI’s first data release shows 1% precision agreement with the QMLE method up to half the Nyquist frequency.
• The DESI measurement exhibits a slight discrepancy at large scales compared to the eBOSS result, partially attributed to differences in residual correction methods.
• The DESI spectral resolution is approximately twice that of SDSS, enabling higher sensitivity to small-scale structures in the intergalactic medium.
• With a signal-to-noise ratio cut matching Chabanier et al. (2019), the sample contains 17,333 sub-forests, compared to 94,558 in eBOSS, indicating a current statistical limitation.
• Future DESI data are expected to deliver up to 1 million Lyα forest systems, corresponding to nearly 3 million sub-forests, enabling sub-percent precision measurements.
• The authors plan to reduce systematic errors through stricter constraints on larger samples and improved modeling of contaminants, noise, and resolution using pixel-level simulations and hydrodynamical emulators.