Galaxy clusters are the most massive gravitationally self-bound objects in the Universe. These clusters form at the rare high peaks of the primordial density fluctuations and subsequently trace the growth of structure in the Universe as they evolve in mass and abundance. As such, clusters serve as powerful cosmological probes for constraining the properties of primordial fluctuations as well as cosmological parameters, including the nature of dark energy. However, clusters are also susceptible to various systematics, such as selection biases and projection effects. Furthermore, recent cosmological results from DES using photometrically identified clusters—which favored lower values of Ωₘ and higher values of σ₈ compared to other constraints from the CMB and large-scale structure—have raised questions about the reliability of optical clusters as cosmological probes.
We have developed a novel analysis method that fully forward-models the abundances, weak lensing, and clustering of galaxy clusters, including accurate treatments of systematics such as projection effects. Projection effects—i.e., the misidentification of interloper galaxies as cluster members—are considered the most significant source of systematic uncertainty. We find that projection effects not only alter the mass–observable relation but also boost the amplitudes of clustering and lensing signals due to the anisotropic distribution of optical clusters. Modeling these effects is therefore crucial for accurate cosmological analyses using optical cluster samples.
In this talk, I will present the results of a joint analysis using Sloan Digital Sky Survey (SDSS) redMaPPer clusters and the Hyper Suprime-Cam (HSC) Year 3 shape catalog.
