Insights into Systematic Uncertainties in DESI's Emission Line Galaxies Data

The recent paper titled "ELG Spectroscopic Systematics Analysis of the DESI Data Release 1" by Jiaxi Yu and a team of 60 authors presents findings from the Dark Energy Spectroscopic Instrument (DESI) regarding Emission Line Galaxies (ELGs). The study focuses on the spectroscopic systematics of over 2.4 million ELGs utilized for three-dimensional large-scale structure analyses in DESI's Data Release 1 (DR1).

Key findings include the introduction of a redshift success rate, denoted as $f_{\rm goodz}$, which measures the fraction of secure redshifts among all measurements. This metric is influenced by observing conditions, leading to non-cosmological variations in large-scale structure analyses. To address these variations, the authors developed a redshift failure weight ($w_{\rm zfail}$) and a per-fibre correction ($\eta_{\rm zfail}$), which have minor impacts on galaxy clustering.

The study identifies two types of systematic uncertainties for ELGs with secure redshifts: catastrophic systematics, which are significant but rare, and redshift uncertainties, which are smaller but more prevalent. Specifically, catastrophic errors account for 0.26% of the total ELGs in DR1, arising from issues such as confusion between spectral lines and double objects.

Additionally, the authors simulated different scenarios of these catastrophic errors and their effects on galaxy clustering, revealing that the biases introduced by these errors are non-negligible but generally smaller than 0.2 sigma in terms of redshift space distortions (RSD) parameters. The redshift uncertainty for the ELGs studied is quantified at 8.5 km/s, characterized by a Lorentzian profile.

The findings from this research contribute to a better understanding of systematic uncertainties in galaxy surveys, which is crucial for accurate cosmological measurements. The full paper can be accessed at arXiv:2405.16657.