4MOST Cosmology Redshift Survey: Survey Design, Clustering Tests, and Early Science Validation Results
Date:
The upcoming 4MOST Cosmology Redshift Survey (CRS) is designed to obtain nearly 5.4 million redshifts over ~5700deg^2 of the southern sky. CRS aims to deliver precise measurements of the baryon acoustic oscillation (BAO) scale and redshift-space distortions (RSD), providing an independent test of dark energy in light of recent indications of evolving dark energy. CRS will also complement major southern datasets, including Rubin Observatory LSST, Euclid, and CMB measurements, through joint analyses and cross-correlations. A central requirement for per cent-level cosmology is that the spectroscopic samples are selected in a way that is both efficient and spatially uniform. Variations in imaging depth, extinction, masking, and stellar contamination can imprint angular structure that mimics true clustering, biasing cosmological fits and distorting environmental measures. We therefore use two-point statistics as a validation tool, turning clustering into a set of quantitative null tests for target selection, masking strategy, and residual contamination. In this talk, we will present the CRS target selection and validation tests for its main tracers, focusing on bright galaxies, luminous red galaxies, and quasars. We will first summarise the sample characterisation and cosmological performance forecasts presented by Verdier et al. (2025), based on selections from DESI Legacy Surveys DR10.1 imaging. We will then present the clustering-based validation from Bandi et al. (2026). We measure the angular correlation function w(θ) for magnitude- and colour-selected subsamples, test sensitivity to photometric systematics, and quantify the impact of masking and veto choices, including Legacy Surveys quality flags and WISE-related artefact regions. We compare measurements between Galactic caps as a stringent uniformity test, perform Limber-scaling consistency checks across bright-galaxy magnitude slices, and infer redshift distributions via cross-correlation with DESI spectroscopy. These tests translate into concrete mitigation strategies and a systematics budget for early CRS cosmology. Finally, we will report results from the 4MOST Science Programme Validation (SPV) campaign from February to April 2026. We will assess instrument and pipeline performance, spectral quality, and redshift success rates, and present on-sky tests of CRS target selection using early spectra, including empirical contamination measurements and completeness as a function of masking and observing conditions. We will conclude by showing the first clustering measurements from 4MOST SPV spectra and their implications for CRS analyses ahead of routine survey operations.