  Cosmological simulations, as well as mounting evidence from observations,
have shown that supermassive black holes play a fundamental role in regulating
the formation of stars throughout cosmic time. This has been clearly
demonstrated in the case of galaxy clusters in which powerful feedback from the
central black hole is preventing the hot intracluster gas from cooling
catastrophically, thus reducing the expected star formation rates by orders of
magnitude. These conclusions have however been almost entirely based on nearby
clusters. Based on new Chandra X-ray observations, we present the first
observational evidence for massive, runaway cooling occurring in the absence of
supermassive black hole feedback in the high-redshift galaxy cluster
SpARCS104922.6+564032.5 ($z=1.709$). The hot intracluster gas appears to be
fueling a massive burst of star formation ($\approx900$~M$_\odot$yr$^{-1}$)
that is offset by dozens of kpc from the central galaxy. The burst is
co-spatial with the coolest intracluster gas but not associated with any galaxy
in the cluster. In less than 100 million years, such runaway cooling can form
the same amount of stars as in the Milky Way. Intracluster stars are therefore
not only produced by tidal stripping and the disruption of cluster galaxies,
but can also be produced by runaway cooling of hot intracluster gas at early
times. Overall, these observations show the dramatic impact when supermassive
black hole feedback fails to operate in clusters. They indicate that in the
highest overdensities such as clusters and proto-clusters, runaway cooling may
be a new and important mechanism for fueling massive bursts of star formation
in the early universe.
