2026ApJ..1000...22F

We present imaging and spectroscopic analyses of Chandra and XMM-Newton observations of ACT-CL J0123.5-0428, one of the most massive, highest-redshift galaxy clusters detected within the survey fields of the Atacama Cosmology Telescope. The Chandra data are sufficient to characterize the morphology of this cluster and constrain the geometrically deprojected temperature in two spatial bins out to r₂₅₀₀, revealing a dynamically relaxed system whose temperature drops to kT = 1.8 ± 0.6 keV in the inner ∼40 kpc. Within this same inner radius, the surface brightness and density of the intracluster medium are sharply peaked, and the cooling time falls to ${t}_{{\rm{cool}}}=28{0}_{-120}^{+150}$ Myr. A novel forward-modeling analysis of the XMM data extends imaging and spectroscopic measurements of this system out to r₅₀₀, constraining the redshift to z = 1.50 ± 0.03, with a mean temperature of kT = 7.3 ± 1.1 keV and an emission-weighted mean metallicity of $Z/{Z}_{\odot }=0.4{3}_{-0.25}^{+0.46}$. We also utilize the limited optical-IR photometric coverage of the cluster to characterize the properties of the brightest cluster galaxy (BCG), which is coincident with the X-ray peak. Despite the high redshift and strong cool core, the BCG exhibits no signs of recent or ongoing star formation, suggesting active galactic nucleus feedback has been acting persistently to stem star formation since z ∼ 2.5. These measurements identify ACT-CL J0123.5-0428 as the highest-redshift, dynamically relaxed, cool core galaxy cluster discovered to date, making it a premier target for future astrophysical and cosmological studies.