Heavyweight Galaxies in the Early Universe Puzzle Astronomers

An international team of astronomers has discovered large galaxies existing when the Universe was young - some two thirds of the way back to the Big Bang - casting doubt on current theories of how the biggest galaxies form. The conventional view is that the heaviest galaxies in the universe started out very small and light and have gained most of their weight relatively recently by cannibalising other galaxies that came too close. However, these new findings, published in Nature on April 2nd 2009, suggest that rather than being svelte, some galaxies in the distant past weighed just as much as the heaviest galaxies we see today.

The discovery was made using one of the largest optical telescopes in the World, called Subaru (named after the Japanese word for the Pleiades star cluster), located on the Island of Hawaii and owned by the National Observatory of Japan. By analysing the light from these remote galaxies - called Brightest Cluster Galaxies, because they are the brightest members of clusters of galaxies - the team has effectively weighed them and found that, despite feeding on a constant diet of small galaxies, the heaviest galaxies have not increased their weight over the last 9 billion years. In a Universe whose age is 13.7 billion years old, these results spark a debate as to how these galaxies put on so much weight in the first few billion years after the big bang.

Professor Chris Collins from Liverpool John Moores University and leader of the team who made the breakthrough said "Current predictions using simulations run on super computers suggest that at such a young age these galaxies should be only 20% of their final weight, so to find galaxies so large suggests that galaxy formation is a much more rapid process than we previously thought and perhaps the theories are missing some important physics."

Dr John Stott who carried out the analysis said:

"We were surprised to find that the largest and brightest galaxies in the Universe have remained essentially unchanged for the last 9 billion years, having grown rapidly soon after the Big Bang"

One possibility being considered is that the galaxies formed by the collapse of an already massive cloud at the dawn of the universe.

Dr Bob Mann, a team member from the University of Edinburgh's Institute for Astronomy at the Royal Observatory Edinburgh commented

"This result is the latest from a fruitful long-term study of Brightest Cluster Galaxies and the properties of the galaxy clusters which host them. These are the largest of galaxies, and our results show how they can be used to test theories of galaxy formation and evolution. It remains a challenge for theoretical astrophysicists to explain how these galaxies can have grown so much so quickly in the early Universe."

This study was led by Chris Collins and John Stott from Liverpool John Moores University, as part of the XMM Cluster Survey (XCS) project. Other XCS team members are M. Hilton (KwaZulu-Natal, SA), S. Kay (The University of Manchester), A. Stanford (University of California, Davis), M Davidson (University of Edinburgh), M. Hosmer (university of Sussex), B. Hoyle (University of Portsmouth), A Liddle (University of Sussex), E. Lloyd-Davies (University od Sussex), R. Mann (University of Edinburgh), N. Mehrtens (University of Sussex), C. Miller (CTIO, USA), R. Nichol (University of Portsmouth), A. Romer (University of Sussex), Martin Sahlen (University of Sussex), P. Viana (Universiadade do Porto, Portugal), M West (ESO, Chile).


Cluster image

An infrared image of the cluster XMMU J2235.3-2557 taken with Subaru, seen at a distance corresponding to 65% of the way back to the Big Bang. The image shows the central 1.5 x 1.5 arc min of the cluster corresponding to 0.75 Mpc at this distance. The clusters X-ray emission is used to pinpoint the location of the brightest galaxy in the cluster as shown by the green contours which represent the X-ray intensity as measured by the XMM-Newton X-ray satellite.



This press release refers to a paper published in Nature: “Early assemblage of the most massive galaxies” (2 April 2009)

Media contacts

Catriona Kelly
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Eleanor Gilchrist
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Science Contact

Bob Mann
School of Physics & Astronomy, University of Edinburgh
Tel: 0131-668-8338
Email: rgm@roe.ac.uk

Notes for Editors

This work was carried with funding from the Science and Technology Facilities Council (STFC) , which also funds the UK subscription to ESA.

Subaru is operated by the National Astronomical Observatory of Japan and XMM-Newton is an ESA science mission funded by contributions to ESA members and from NASA.

Bob Mann is at the Institute for Astronomy which is part of the University of Edinburgh and which is located at the Royal Observatory Edinburgh.