Observational Evidence for Large-scale Gas Heating in a Galaxy Protocluster at z = 2.30

Abstract We report a z = 2.30 galaxy protocluster (COSTCO-I) in the COSMOS field, where the Ly α forest as seen in the CLAMATO IGM tomography survey does not show significant absorption. This departs from the transmission–density relationship (often dubbed the fluctuating Gunn–Peterson approximation; FGPA) usually expected to hold at this epoch, which would lead one to predict strong Ly α absorption at the overdensity. For comparison, we generate mock Ly α forest maps by applying the FGPA to constrained simulations of the COSMOS density field and create mocks that incorporate the effects of finite sight-line sampling, pixel noise, and Wiener filtering. Averaged over r = 15 h −1 Mpc around the protocluster, the observed Ly α forest is consistently more transparent in the real data than in the mocks, indicating a rejection of the null hypothesis that the gas in COSTCO-I follows the FGPA ( p = 0.0026, or 2.79 σ significance). It suggests that the large-scale gas associated with COSTCO-I is being heated above the expectations of the FGPA, which might be due to either large-scale AGN jet feedback or early gravitational shock heating. COSTCO-I is the first known large-scale region of the IGM that is observed to be transitioning from the optically thin photoionized regime at cosmic noon to eventually coalesce into an intracluster medium (ICM) by z = 0. Future observations of similar structures will shed light on the growth of the ICM and allow constraints on AGN feedback mechanisms.