Targeting the future of spectroscopic galaxy surveys -- A cosmological study case with the Wide-field Spectroscopic Telescope

Abstract

The next generation of Stage-V spectroscopic surveys are designed to measure hundreds of millions of galaxy and quasar spectra, extending precision large-scale-structure cosmology to redshifts and volumes well beyond the reach of current Stage-IV facilities. Realising this potential requires revisiting the target selection strategies and designing them explicitly around the depth, multiplexing, and survey time of the new instruments, rather than a direct extrapolation of existing recipes. This work develops and characterises new strategies for the Wide-field Spectroscopic Telescope (WST), a proposed 12-metre, highly multiplexed facility expected to operate in the 2040s. Using deep multi-band photometry, including COSMOS-field data from CLAUDS and HSC, we revisit low-redshift $z<1.6$ tracer selection of SDSS, BOSS/eBOSS, DESI and 4MOST-CRS -- Bright Galaxies, Luminous Red Galaxies, Emission Line Galaxies and quasars -- to photometric depth achievable in the WST era, and introduce a purely magnitude-limited sample (MagMax) exploiting three-dimensional colour-colour star-galaxy separation, and a set of Lyman-Break Galaxy dropout samples extending target selection to $z>2$. Using Fisher-matrix forecasts, we showcase the constraining power of a WST-like cosmological survey on BAO distance parameters $α_\parallel$ and $α_\perp$, the growth rate $f$, the evolving dark energy equation $w_0w_a$, the local-type primordial non-Gaussianity $f_{\rm NL}^{\rm loc}$, and the sum of the neutrino mass $Σm_ν$. The main outcome of this work is an optimisation framework that directly links instrumental capabilities and observing time to cosmological constraining power, providing a science-driven template applicable to the design of future generations of Stage-V spectroscopic surveys.