1. Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD, UK
2. Astrophysics and Cosmology Research Unit, School of Mathematical Sciences, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban 4000, South Africa
3. South African Astronomical Observatory, PO Box 9, Observatory, Cape Town 7935, South Africa
4. Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester, M13 9PL, UK
5. Department of Physics, University of California, Davis, California 95616, USA
6. Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, Livermore, California 94551, USA
7. SUPA, Institute of Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
8. Astronomy Centre, University of Sussex, Falmer, Brighton, BN1 9QH, UK
9. ICG, University of Portsmouth, Portsmouth, PO1 2EG, UK
10. Cerro-Tololo Inter-American Observatory, National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, Arizona 85719, USA
11. Departmento de Matemática Aplicada da Faculdade de Ciências da Universiadade do Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal
12. Centro de Astrofisica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
13. European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile

Correspondence to: Chris A. Collins¹ Correspondence and requests for materials should be addressed to C.A.C. (Email: cac@astro.livjm.ac.uk).

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 debate^{2, 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 models^{4, 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.

MORE ARTICLES LIKE THIS

These links to content published by NPG are automatically generated.