4MOST Cosmology Redshift Survey: Quasar Target Selection, and Science Programme Validation Results
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The 4MOST Cosmology Redshift Survey (CRS) will provide the redshift measurement of 1.5 million quasars and Lyα-QSO at redshifts 0.9 2. We present the status of the CRS quasar target selection and its validation using CRS Science Programme Validation (SPV) observations. The baseline selection follows the optical–infrared colour approach introduced by Eltvedt et al. (2023). Within SPV, we test whether modest, physically motivated adjustments can increase the QSO surface density while maintaining tight control over contamination. This includes exploring expanded colour–magnitude boundaries and auxiliary features, and quantifying the resulting completeness–contamination trade-off. The SPV spectra provide a direct assessment of redshift success rate and the dominant contamination pathways, enabling purity measurements for both the baseline and expanded selections and an evaluation of their suitability for cosmological applications. In overlapping regions, we also benchmark performance against the DESI QSO target selection based on a random-forest classifier, using matched samples to compare redshift yield and contamination against an established machine-learning approach. We additionally assess how masking affects contamination and usable area for the CRS QSO selection.