Validation of the DESI 2024 Lyman Alpha Forest BAL Masking Strategy

TL;DR

This study evaluates how different strategies for masking broad absorption line features in quasars affect the accuracy of large-scale structure measurements from the Lyman-alpha forest in DESI data, finding minimal impact across methods.

Contribution

It provides a comprehensive analysis of BAL masking strategies and demonstrates their negligible effect on BAO measurements in DESI's Lyman-alpha forest data.

Findings

Masking all BAL pixels has minimal impact on BAO results.

Alternate masking strategies do not significantly alter BAO measurements.

Synthetic data analysis confirms robustness of BAO results against BAL masking variations.

Abstract

Broad absorption line quasars (BALs) exhibit blueshifted absorption relative to a number of their prominent broad emission features. These absorption features can contribute to quasar redshift errors and add absorption to the Lyman-alpha (LyA) forest that is unrelated to large-scale structure. We present a detailed analysis of the impact of BALs on the Baryon Acoustic Oscillation (BAO) results with the LyA forest from the first year of data from the Dark Energy Spectroscopic Instrument (DESI). The baseline strategy for the first year analysis is to mask all pixels associated with all BAL absorption features that fall within the wavelength region used to measure the forest. We explore a range of alternate masking strategies and demonstrate that these changes have minimal impact on the BAO measurements with both DESI data and synthetic data. This includes when we mask the BAL features associated with emission lines outside of the forest region to minimize their contribution to redshift errors. We identify differences in the properties of BALs in the synthetic datasets relative to the observational data, as well as use the synthetic observations to characterize the completeness of the BAL identification algorithm, and demonstrate that incompleteness and differences in the BALs between real and synthetic data also do not impact the BAO results for the LyA forest.