In the cores of some clusters of galaxies the hot intracluster plasma is dense enough that it should cool radiatively in the clusters lifetime, leading to continuous cooling flows of gas sinking towards the cluster centre, yet no such cooling flow has been observed. The low observed star-formation rates and cool gas masses for these cool-core clusters suggest that much of the cooling must be offset by feedback to prevent the formation of a runaway cooling flow. Here we report X-ray, optical and infrared observations of the galaxy cluster SPT-CLJ2344-4243 (ref. 11) at redshift z = 0.596. These observations reveal an exceptionally luminous (8.2Ã-10 45 ergs-1) galaxy cluster that hosts an extremely strong cooling flow (around 3,820 solar masses a year). Further, the central galaxy in this cluster appears to be experiencing a massive starburst (formation of around 740 solar masses a year), which suggests that the feedback source responsible for preventing runaway cooling in nearby cool-core clusters may not yet be fully established in SPT-CLJ2344-4243. This large star-formation rate implies that a significant fraction of the stars in the central galaxy of this cluster may form through accretion of the intracluster medium, rather than (as is currently thought) assembling entirely via mergers.
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Acknowledgements M.McD. was supported at MIT by NASA through the Chandra X-ray Observatory. The South Pole Telescope is supported by the National Science Foundation, with partial support provided by the Kavli Foundation, and the Moore Foundation. Support for X-ray analysis was provided by NASA. Work at McGill University is supported by NSERC, the CRC programme, and CIfAR, and at Harvard University by the NSF. S.V. acknowledges a Senior NPP Award held at the NASA Goddard Space Flight Center. R.K. acknowledges a NASA Hubble Fellowship, B.A.B. acknowledges a KICP Fellowship, M.A.D. acknowledges an Alfred P. Sloan Research Fellowship, and O.Z. acknowledges a BCCP fellowship.