Relativistic SZ temperatures and hydrostatic mass bias for massive clusters in the FLAMINGO simulations

The relativistic Sunyaev-Zel'dovich (SZ) effect can be used to measure intracluster gas temperatures independently of X-ray spectroscopy. Here, we use the large-volume FLAMINGO simulation suite to determine whether SZ y-weighted temperatures lead to more accurate hydrostatic mass estimates in massive (M_ 500c > 7.5× 10^14 M_⊙) clusters than when using X-ray spectroscopic-like temperatures. We find this to be the case, on average. The median bias in the SZ mass at redshift zero is < b > ≡ 1-< M_ 500c,hse/M_ 500c,true> = -0.05 ± 0.01, over 4 times smaller in magnitude than the X-ray spectroscopic-like case, < b > = 0.22 ± 0.01. However, the scatter in the SZ bias, σ_b≈ 0.2, is around 40 per cent larger than for the X-ray case. We show that this difference is strongly affected by clusters with large pressure fluctuations, as expected from shocks in ongoing mergers. Selecting the clusters with the best-fitting generalized NFW pressure profiles, the median SZ bias almost vanishes, < b > = -0.009 ± 0.005, and the scatter is halved to σ_b≈ 0.1. We study the origin of the SZ/X-ray difference and find that, at R_ 500c and in the outskirts, SZ weighted gas better reflects the hot, hydrostatic atmosphere than the X-ray weighted gas. The SZ/X-ray temperature ratio increases with radius, a result we find to be insensitive to variations in baryonic physics, cosmology and numerical resolution.