Press Release

Massive Galaxies Born Earlier than Expected

April 3, 2009

Astronomers have discovered large galaxies that have not changed much in size for the last 9 billion years—a finding that conflicts with widely held views of when and how galaxies form.

The Conventional Model

The most popular model for galaxy formation is a bottom-up or hierarchical model in which small galaxies gradually develop into larger ones over a long period of time. The model predicts that smaller galaxies would merge to form larger ones, growing to 90% of their size about 11 billion years after the Big Bang. This means that smaller galaxies would be older and larger ones, younger.

A team of astrophysicists, led by Professor Chris Collins from Liverpool John Moores University (LJMU), used observations from the Subaru Telescope (one of the world’s largest optical-infrared telescopes) as well as the XMM-Newton (an x-ray observatory in space) to collect data about galaxies that existed two-thirds of the way back in time since the universe began. By being able to extend the baseline of their data so far back in time, they have a foundation for concluding that large galaxies were almost fully formed 4-5 billion years after the Big Bang, 6-7 billion years earlier than the hierarchical model would forecast. Dr. John Scott, who carried out the analysis, stressed the significance of the research: “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.”

Candles for Studying the Universe

The galaxies examined in the observations were brightest cluster galaxies (BCGs), located at the centers of galaxy clusters. Because they are so bright and uniform, they are often used as beacons for studying the universe.

At the beginning of their research, the scientists examined the ages of the stars in the galaxies. Their findings indicated a consistent epoch of formation for the majority of the stars-- about 2-3 billion years after the Big Bang. Since the galaxies they studied were almost fully formed 4-5 billion years after the Big Bang, they concluded that they probably grew rapidly and formed much sooner than the hierarchical model of galaxy evolution suggests.

Questions May Enrich or Change Models for Galaxy Formation

These findings add fuel to the longstanding debate about when galaxies form. Although there is evidence that some merging of galaxies is occurring, as the hierarchical model predicts, it does not seem to be the explanation for the earlier birth of large galaxies that this research discovered.

The breakthrough findings of the LJMU astronomers’ research may alter the way that simulation models are constructed in the future. Simulation models provide a useful framework to compare theory with data. In light of these findings, future simulation models may need to take into account the rapid growth of some galaxies. Professor Chris Collins commented, “Current predictions using simulations run on supercomputers 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.”

The surprising findings of this research also give scientists new questions to think about: How do these large, Bright Cluster Galaxies form sooner than expected after the Big Bang? And, how have the BCGs remained almost unchanged in size and weight for 9 billion years?

This research paper by Collins et al. was published in Nature, Volume 458, page 603.

This work was carried out 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.

Chris Collins and John Stott (LJMU), on behalf of the Xmm X-ray Cluster Survey team: 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).

Figure1: 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.

Figure2: The mass evolution of brightest cluster galaxies with look-back time. The red points represent the masses of the galaxies we have observed (the average and standard error are also shown) and the gray points the prediction of mass build up from hierarchical models.



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