A disturbing FABLE of mergers, feedback and mass biases

The use of galaxy clusters as cosmological probes often relies on
understanding the properties and evolution of the intracluster medium (ICM).
However, the ICM is a complex plasma, regularly stirred by mergers and
feedback, with non-negligible bulk and turbulent motions and a non-thermal
pressure component, making it difficult to construct a coherent and
comprehensive picture. To this end, we use the FABLE simulations to investigate
how the hydrostatic mass bias is affected by cluster dynamical state, mergers,
AGN feedback and turbulence. Following in detail a single, massive cluster we
find the bias varies significantly over cosmic time, rarely staying at the
average value found at a particular epoch. Variations of the bias at a given
radius are contemporaneous with periods where outflows dominate the mass flux,
either due to mergers or AGN feedback. The $z=0$ ensemble median mass bias in
FABLE is $sim13$ per cent at $R_mathrm{500}$ and $sim15$ per cent at
$R_mathrm{200}$, but with a large scatter in individual values. In halo
centres, we see an increase in temperature and a decrease in non-thermal
pressure support with cosmic time as turbulence thermalises, leading to a
reduction in the mass bias within $sim0.2 R_mathrm{200}$. When using a fitted
pressure profile, instead of the `raw’ simulation data, to estimate the bias,
we find there can be significant differences, particularly at larger radii and
higher redshift. We therefore caution over the use of such fits in future work
with the next generation of X-ray and SZ observations.
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