The 4MOST-Cosmology Redshift Survey: target selection of bright galaxies and luminous red galaxies

ABSTRACT The Cosmology Redshift Survey of the 4-metre Multi-Object Spectroscopic Telescope (4MOST-CRS) will provide redshift measurements of galaxies and quasars over 5800 $\rm {deg^{2}}$ in the Southern hemisphere. As targets for the 4MOST-CRS, we present a selection of an $r< 19.25$ magnitude-limited sample of bright galaxies (BGs) and a colour selected sample of luminous red galaxies (LRGs) based on DESI Legacy Survey DR10.1 photometric data, in the redshift ranges $0.1< z< 0.5$ and $0.4< z< 1$, respectively. These samples are selected using the g, r, z, and $W1$ (from unWISE) photometric bands. For BGs, the star–galaxy separation is performed based on Gaia and Tycho-2 star catalogues. Following 4MOST requirements, the target densities of BGs and LRGs are 250 and 400 $\rm {deg^{-2}}$, respectively. We quantified the stellar contamination to be $< 2{\!-\!}4~{{\ \rm per\ cent}}$ for both galaxy samples using data from the first DESI data release. Using angular clustering, we show that both samples are robust against imaging systematics, confirming a low stellar contamination. Finally, we provide forecasts of the baryonic acoustic oscillation (BAO) and growth of structure ($\sigma _8$) measurements from the 4MOST-CRS alone and in combination with DESI. From the 4MOST-CRS, we expect 3 per cent and 25 per cent precision on BAO and $\sigma _8$ measurements, respectively. Combining 4MOST-CRS with DESI improves the constraints from DESI alone by 12 per cent in the 0.4 < z < 0.8 redshift range, leading to the most stringent constraints on BAO measurements from spectroscopic galaxy clustering.