Studying baryon acoustic oscillations using photometric redshifts from the DESI Legacy Imaging survey DR9

TL;DR

This study uses photometric redshifts from the DESI Legacy Imaging Survey DR9 to measure baryon acoustic oscillations, revealing variance across regions and systematic biases that impact BAO scale measurements, supporting the sample variance hypothesis for existing tensions.

Contribution

It introduces a method to measure BAO scales using photometric redshifts from DR9 and identifies systematic biases affecting these measurements.

Findings

BAO scale measurements are consistent with Planck's observations.

Variance between different survey footprints aligns with sample variance.

Systematic biases in photometric BAO studies need correction for accurate results.

Abstract

Context.The Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Survey DR9 (DR9 hereafter), with its extensive dataset of galaxy locations and photometric redshifts, presents an opportunity to study baryon acoustic oscillations (BAOs) in the region covered by the ongoing spectroscopic survey with DESI. Aims. We aim to investigate differences between different parts of the DR9 footprint. Furthermore, we want to measure the BAO scale for luminous red galaxies within them. Our selected redshift range of 0.6 to 0.8 corresponds to the bin in which a tension between DESI Y1 and eBOSS was found. Methods. We calculated the anisotropic two-point correlation function in a modified binning scheme to detect the BAOs in DR9 data. We then used template fits based on simulations to measure the BAO scale in the imaging data. Results. Our analysis reveals the expected correlation function shape in most of the footprint areas, showing a BAO scale consistent with Planck's observations. Aside from identified mask-related data issues in the southern region of the South Galactic Cap, we find a notable variance between the different footprints. Conclusions. We find that this variance is consistent with the difference between the DESI Y1 and eBOSS data, and it supports the argument that that tension is caused by sample variance. Additionally, we also uncovered systematic biases not previously accounted for in photometric BAO studies. We emphasize the necessity of adjusting for the systematic shift in the BAO scale associated with typical photometric redshift uncertainties to ensure accurate measurements.