Galaxies are thought to grow gradually from the early Universe by star formation and mergers, according to huge recent galaxy formation simulations. But here Collins et al. report that Brightest Cluster Galaxies, the most luminous stellar objects, have not increased much in mass over the past 9 billion years or so: they were already almost fully grown 4 or 5 billion years after the Big Bang.
- Chris A. Collins
- John P. Stott
- Michael J. West
