Constraining cosmological parameters using the splashback radius of galaxy clusters

Abstract

Cosmological parameters such as $\hat{I}{\text{c◯}}_{\mathrm{M}}$ and $\ddot{I}{ƒ}_8$ can be measured indirectly using various methods, including galaxy cluster abundance and cosmic shear. These measurements constrain the composite parameter $S_8$, leading to degeneracy between $\hat{I}{\text{c◯}}_{\mathrm{M}}$ and $\ddot{I}{ƒ}_8$. However, some structural properties of galaxy clusters also correlate with cosmological parameters, due to their dependence on a cluster's accretion history. In this work, we focus on the splashback radius, an observable cluster feature that represents a boundary between a cluster and the surrounding Universe. Using a suite of cosmological simulations with a range of values for $\hat{I}{\text{c◯}}_{\mathrm{M}}$ and $\ddot{I}{ƒ}_8$, we show that the position of the splashback radius around cluster-mass halos is greater in cosmologies with smaller values of $\hat{I}{\text{c◯}}_{\mathrm{M}}$ or larger values of $\ddot{I}{ƒ}_8$. This variation breaks the degeneracy between $\hat{I}{\text{c◯}}_{\mathrm{M}}$ and $\ddot{I}{ƒ}_8$ that comes from measurements of the $S_8$ parameter. We also show that this variation is, in principle, measurable in observations. As the splashback radius can be determined from the same weak lensing analysis already used to estimate $S_8$, this new approach can tighten low-redshift constraints on cosmological parameters, either using existing data, or using upcoming data such as that from Euclid and LSST.

12 pages, 7 figures, 1 table, accepted for publication in ApJ