The current consensus is that galaxies begin as small density fluctuations in the early Universe and grow by in situ star formation and hierarchical merging1. Stars begin to form relatively quickly in sub-galactic-sized building blocks called haloes which are subsequently assembled into galaxies. However, exactly when this assembly takes place is a matter of some debate2,3. Here we report that the stellar masses of brightest cluster galaxies, which are the most luminous objects emitting stellar light, some 9 billion years ago are not significantly different from their stellar masses today. Brightest cluster galaxies are almost fully assembled 4-5 billion years after the Big Bang, having grown to more than 90 per cent of their final stellar mass by this time. Our data conflict with the most recent galaxy formation models4,5 based on the largest simulations of dark-matter halo development1. These models predict protracted formation of brightest cluster galaxies over a Hubble time, with only 22 per cent of the stellar mass assembled at the epoch probed by our sample. Our findings suggest a new picture in which brightest cluster galaxies experience an early period of rapid growth rather than prolonged hierarchical assembly.
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This work is based in part on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan and the XMM-Newton, an ESA science mission funded by contributions from ESA member states and from NASA. We acknowledge financial support from Liverpool John Moores University and the STFC. M.H. acknowledges support from the South African National Research Foundation. IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. We thank G. De Lucia for making simulation results available to us in tabular form, I. Tanaka for developing the MCSRED package used to reduce the MOIRCS data, M. Salaris for discussions on stellar population synthesis models and B. Maughan for discussions on cluster masses.
Author Contributions C.A.C. provided the scientific leadership, helped design the experiment, wrote the paper and led the interpretation. J.P.S. performed the photometry and data analysis and made major contributions to the interpretation. M.H. wrote the Subaru proposal, carried out the data reduction and photometric calibration, contributed to the analysis and interpretation and provided detailed comments on the manuscript. S.T.K. independently checked the cluster mass calculations. S.A.S. provided useful discussions on the data and comments on the manuscript. The remaining authors make up the team of the wider XCS project which led to the discovery of J2215. R.G.M., R.C.N., and A.K.R. made useful comments on the text.
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Collins, C., Stott, J., Hilton, M. et al. Early assembly of the most massive galaxies. Nature 458, 603–606 (2009). https://doi.org/10.1038/nature07865
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