A Massive Starburst at the Center of the Most Distant Relaxed Galaxy Cluster: SPT-CL J2215-3537

For years we have grappled with the “cooling flow problem” in galaxy clusters, where the massive reserves of hot (10⁷ K) gas in the intracluster medium (ICM) have been universally observed to form stars with an efficiency of only 1-10%. Feedback from accreting active galactic nuclei (AGN) has been identified as the likely heating source capable of suppressing runaway cooling by up to two orders of magnitude. However, with the recent discovery of the Phoenix cluster exhibiting the only known pure cooling flow, the thermostat of AGN feedback appears to be broken in this and a few other strongly cooling systems. Additionally, little is known about the behavior of this feedback-cooling cycle at higher redshifts, when galaxy clusters were still in the process of forming and the balance between the two processes was just barely being established. To explore these questions, we utilize new space- and ground-based multiwavelength observations from Chandra, Hubble, Magellan, and ASKAP telescopes in the discovery of the most distant, dynamically relaxed cool core cluster, SPT-CL J2215-3537 (SPT2215) and its central brightest cluster galaxy (BCG) at a redshift of z = 1.2. These data reveal massive filaments of warm (10⁴ K) ionized gas likely fed by the exceptionally strong cooling flow (dM_gas/dt ~ 2000 Msun yr^-1) from the hot ICM, as well as bright [O II] line emission implying an extreme and highly-efficient star formation rate (SFR) of ~320 Msun yr^-1. We also detect a weak radio source suggesting that AGN feedback has already started operating in this high-z system, though the implied jet power is less than half of the cooling luminosity of the hot ICM, consistent with cooling overpowering heating. The extreme cooling and SFR of SPT2215 is rare among known cool core clusters, and it is even more remarkable that we observe these at such a high redshift, when most clusters are still dynamically disturbed. The high mass of this cluster, coupled with the fact that it is dynamically relaxed with a highly-isolated BCG, suggests that it is an exceptionally rare system that must have formed very rapidly in the early Universe. Combined with the high SFR, SPT2215 may be a high-z analog of the Phoenix cluster, potentially providing insight into the limits as well as the onset of AGN feedback in the most massive galaxies.