2026ApJ..1000...42S

We present an in-depth Chandra X-ray analysis of the galaxy cluster SPT-CL J0417-4748 (hereafter SPT J0417) at z = 0.58 with a focus on its thermodynamic properties and the apparent absence of central star formation. Utilizing a total Chandra exposure of 103 ks, we find that the large-scale X-ray morphology is consistent with a dynamically relaxed cool-core system. The intracluster medium shows a central density of 0.08 ± 0.01 cm^–3, a central pseudoentropy of $2{6}_{-5}^{+6}\,{\rm{keV}}\,{{\rm{cm}}}^{2}$, and a central cooling time of $51{5}_{-75}^{+96}$ Myr, values typical of massive cool-core clusters. Despite these conditions, no evidence of recent or ongoing star formation is detected in the brightest cluster galaxy (BCG). Spectral energy distribution fitting of Dark Energy Survey photometry indicates that the bulk of the stellar population formed at z ∼ 1.25, with no significant star formation over the past ∼3 Gyr, while optical spectra from Magellan show no [O II] emission. Complementary ASKAP radio and Spitzer infrared data indicate a lack of strong current active galactic nucleus (AGN) activity in the BCG. SPT J0417 exemplifies massive, relaxed cool-core clusters in which cooling and star formation appear almost completely quenched, providing valuable insights into how AGN feedback regulates the long-term thermal balance of the intracluster medium.