Abstract
The intracluster medium (ICM) today is comprised largely of hot gas with clouds of cooler gas of unknown origin and lifespan. We analyse the evolution of cool gas (temperatures $\lesssim 10^{4.5}$ K) in the ICM of 352 galaxy clusters from the TNG-Cluster simulations, with present-day mass $\sim 10^{14.3-15.4}\, {\rm M}_\odot$. We follow the main progenitors of these clusters over the past $\sim 13$ billion years (since $z\lesssim 7$) and find that, according to TNG-Cluster, the cool ICM mass increases with redshift at fixed cluster mass, implying that this cooler past of the ICM is due to more than just halo growth. The cool cluster gas at $z\lesssim 0.5$ is mostly located in and around satellite galaxies, while at $z\gtrsim 2$ cool gas can also accrete via filaments from the intergalactic medium. Lower-mass and higher-redshift clusters are more susceptible to cooling. The cool ICM mass correlates with the number of gaseous satellites and inversely with the central supermassive black hole (SMBH) mass. The average number of gaseous satellites decreases since $z=2$, correlating with the decline in the cool ICM mass over cosmic time, suggesting a link between the two. Concurrently, kinetic SMBH feedback shifts the ICM temperature distribution, decreasing the cool ICM mass inside-out. At $z\approx 0.5$, the predicted Mg II column densities are in the ballpark of recent observations, where satellites and other haloes contribute significantly to the total Mg II column density. Suggestively, a non-negligible amount of the ICM cool gas forms stars in situ at early times, reaching $\sim 10^{2}\, {\rm M}_\odot \, {\rm ~yr^{-1}}$ and an H $\alpha$ surface brightness of $\sim 10^{-17}\, {\rm ~erg\, s^{-1}\, cm^{-2}\, arcsec^{-2}}$ at $z\approx 2$, detectable with Euclid and JWST.
