In the present universe galaxy clusters cores are typically populated by massive "red and dead" galaxies. At some point in cosmic history, however, these galaxies must have formed the bulk of their stars. Now Herschel has observed a massive cluster labelled XDCP0044 at redshift z=1.58 (lookback time ~9.5 Gyr) where galaxies in the cluster core exhibit strikingly high amounts of star formation, the first time this has been seen in a massive cluster (Santos et al. 2015).
For more information on this discovery and figure captions see the ESA Herschel SciTech web release.