Characterization of contaminants in the Lyman-alpha forest auto-correlation with DESI

Abstract Baryon Acoustic Oscillations can be measured with sub-percent precision above redshift two with the Lyman- α (Ly α ) forest auto-correlation and its cross-correlation with quasar positions. This is one of the key goals of the Dark Energy Spectroscopic Instrument (DESI) which started its main survey in May 2021. We present in this paper a study of the contaminants to the Ly α forest which are mainly caused by correlated signals introduced by the spectroscopic data processing pipeline as well as astrophysical contaminants due to foreground absorption in the intergalactic medium. Notably, an excess signal caused by the sky background subtraction noise is present in the Ly α auto-correlation in the first line-of-sight separation bin. We use synthetic data to isolate this contribution, we also characterize the effect of spectro-photometric calibration noise, and propose a simple model to account for both effects in the analysis of the Ly α forest. We then measure the auto-correlation of the quasar flux transmission fraction of low redshift quasars, where there is no Ly α forest absorption but only its contaminants. We demonstrate that we can interpret the data with a two-component model: data processing noise and triply ionized Silicon and Carbon auto-correlations. This result can be used to improve the modeling of the Ly α auto-correlation function measured with DESI.

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