Monday

Galaxy clusters lie at the peaks of the cosmic matter distribution and host a bright X-ray atmosphere, making them useful tools to study both cosmology and galaxy formation and evolution. Estimates of cluster mass are vital to their use as cosmological probes, and can be inferred from X-ray or SZ observations under the assumption of hydrostatic equilibrium (HE).

However, galaxy clusters undergo many mergers during their growth history, and are still accreting matter at z=0. Therefore the assumption of HE does not always hold, leading to an under or over-estimate of the cluster mass – the hydrostatic mass bias. Major mergers drive the largest changes in this bias, as they significantly disturb the gaseous haloes of clusters, rendering the HE assumption, at least temporarily, incorrect. A number of simulations have been used to characterise and correct for this bias, with common estimates being of order 20%.

We use the FABLE suite of cluster simulations (Henden+2018), that implement an improved version of the Illustris feedback model, to investigate the co-evolution of the bias, X-ray morphology and ICM properties, as a function of cosmic time. By repeating similar analyses to other groups and averaging the bias across our sample, we find a bias of order 5-10% at R500 at z=0, albeit with significant scatter. We also find a mild decrease in the bias with redshift, and with increasing halo mass. Interestingly, we find that the bias varies significantly with time as the cluster evolves and undergoes mergers, and is rarely at the static value found by averaging a sample of clusters. We show that significant changes in the bias correlate with shocks caused by both recent major mergers, and sometimes AGN feedback , which has important implications about future estimates of galaxy cluster mass from observations by Athena and ACT.