CSST strong lensing preparation: forecasting the galaxy–galaxy strong lensing population for the China space station telescope

ABSTRACT Galaxy–galaxy strong gravitational lensing (GGSL) is a powerful probe for the formation and evolution of galaxies and cosmology, while the sample size of GGSLs leads to considerable uncertainties and potential bias. The China Space Station Telescope (CSST, to be launched in late 2026) will conduct observations across 17 500 square degrees of the sky, capturing images in the $ugriz$ bands with a spatial resolution comparable to that of the Hubble Space Telescope. We ran a set of Monte Carlo simulations to predict that the CSST’s wide-field survey will observe $\sim$160 000 galaxy–galaxy strong lenses over its lifespan, increasing the number of existing galaxy–galaxy strong lens samples by three orders of magnitude. This is comparable to the capabilities of the $\it Euclid$ telescope but with the added benefit of additional colour information. Specifically, the CSST can detect strong lenses with Einstein radii about $0.64\pm 0.42 ^{\prime \prime }$, corresponding to the velocity dispersions of $217.19 \pm 50.55 \, \text{km/s}$. These lenses exhibit a median magnification of $\sim$5. The apparent magnitude of the unlensed sources in the g band is $25.87 \pm 1.19$. The signal-to-noise ratio of the lensed images covers a range of ${\sim} 20$ to ${\sim} 1000$, allowing us to determine the Einstein radius with an accuracy ranging from ${\sim} 1~{{\ \rm per\ cent}}$ to ${\sim} 0.1~{{\ \rm per\ cent}}$, ignoring various modelling systematics. Our estimates indicate that CSST can observe rare systems like double source-plane and spiral galaxy lenses. The above selection functions of the CSST strong lensing observation help optimize the strategy of finding and modelling GGSLs.