The SRG/eROSITA All-Sky Survey

We present the weak-lensing mass calibration and constrain the relation between the stellar mass of the brightest cluster galaxy (BCG), halo mass, and redshift ( M ⋆, BCG – M – z ) for a sample of 124 galaxy clusters and groups at redshift 0.1 < z < 0.8 from the first Data Release of the eROSITA All-Sky Survey (eRASS1), using data from the Hyper Suprime-Cam (HSC) Subaru Strategic Program. The cluster survey is conducted by the eROSITA X-ray telescope aboard the Spectrum-Roentgen-Gamma (SRG) space observatory. The cluster sample is X-ray-selected and optically confirmed with a negligibly low contamination rate (≈5%). On the basis of individual clusters, the shear profiles g + of 96 clusters are derived using the HSC Three-Year (HSC-Y3) weak-lensing data, while the BCG stellar masses M ⋆, BCG of 101 clusters are estimated using the SED template fitting to the HSC five-band ( grizY ) photometry. The observed X-ray photon count rate C R is used as the mass proxy, based on which individual halo masses M are obtained at the given C R in a population modelling, while accounting for systematic uncertainties in the weak-lensing modelling through a simulation-calibrated weak-lensing mass-to-halo-mass ( M WL – M – z ) relation. The count rate ( C R – M – z ) and BCG stellar mass ( M ⋆, BCG – M – z ) relations are simultaneously constrained in forward modelling and population modelling. In agreement with the results based on the weak-lensing data from the DES and KiDS surveys, we obtain a C R – M – z relation with a self-similar redshift scaling and a mass trend that is steeper than the self-similar prediction. We cannot simultaneously place stringent constraints on the power-law indices of the mass ( B BCG ) and redshift ( γ BCG ) trends, due to the parameter degeneracy arising from the sample selection and the limited sample size. By adopting an informative prior on γ BCG to break the B BCG – γ BCG degeneracy, we obtain a M ⋆, BCG – M – z relation with the mass slope increasing to B BCG = 0.38 ± 0.11. Informed by the prior, our results suggest that the BCG stellar mass at a fixed halo mass has remained stable with a moderate increase at a level of (20±8)% since redshift z ≈ 0.8. This finding supports the picture of the rapid-then-slow BCG formation, where the majority of the stellar mass must have been assembled at a much earlier cosmic time.