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

We present the discovery of the most distant, dynamically relaxed cool core cluster, SPT-CL J2215-3537 (SPT2215), and its central brightest cluster galaxy (BCG) at z = 1.16. Using new X-ray observations, we demonstrate that SPT2215 harbors a strong cool core with a central cooling time of 200 Myr (at 10 kpc) and a maximal intracluster medium cooling rate of 1900 +/- 400 Me yr-1. This prodigious cooling may be responsible for fueling the extended, star-forming filaments observed in Hubble Space Telescope imaging. Based on new spectrophotometric data, we detect bright [O II] emission in the BCG, implying an unobscured star formation rate (SFR) of 320 140230 +/- Me yr-1. The detection of a weak radio source (2.0 +/- 0.8 mJy at 0.8 GHz) suggests ongoing feedback from an active galactic nucleus (AGN), though the implied jet power is less than half the cooling luminosity of the hot gas, consistent with cooling overpowering heating. The extreme cooling and SFR of SPT2215 are rare among known cool core clusters, and it is even more remarkable that we observe these at such 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 of AGN feedback and star formation in the most massive galaxies.

Automated summary

We have discovered the most distant cool core cluster, SPT-CL J2215-3537 (SPT2215), and its central brightest cluster galaxy (BCG) at z = 1.16. The cluster exhibits strong cooling and a high star formation rate (SFR), which are rare at such high redshift. The presence of a weak radio source suggests ongoing feedback from an active galactic nucleus (AGN), but the power is insufficient to counter the cooling. This cluster provides valuable insights into AGN feedback and star formation in massive galaxies.