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It’s not easy being green – what colours tell us about galaxy evolution

Last Updated on Friday, 01 July 2016 07:46
Published on Thursday, 30 June 2016 08:28

Scientists may have answered why green galaxies are rare in our universe and why their colour could reveal a troubled past. Their research is presented today (Thursday 30 June) at the National Astronomy Meeting at the University of Nottingham.

The international team, led from Durham University's Institute for Computational Cosmology (ICC), used new computer modelling of the universe to investigate the colours that galaxies have and what those colours might tell us about how galaxies evolve. Using the state of the art EAGLE simulations, the researchers modelled how both the ages of stars in galaxies and what those stars are made from translate into the colour of light that they produce.

composite 300dpiComposite image of blue, green and red galaxies: L-R Virtual images of blue, green and red galaxies produced by the EAGLE simulations. The green galaxy is caught in the act of transforming from blue to red as its gas supply runs out. Credit: James Trayford/EAGLE/Durham University. Click for a full size image

The team said their simulations showed that colours of galaxies can also help diagnose how they evolve.

While red and blue galaxies are relatively common, rare green galaxies are likely to be at an important stage in their evolution, when they are rapidly turning from blue – when new stars and planets are being born – to red as stars begin to burn themselves out.

Lead researcher James Trayford, PhD student in the ICC, said: “Galaxies emit a healthy blue glow while new stars and planets are being born. However, if the formation of stars is halted galaxies turn red as stars begin to age and die.

“In the real universe we see many blue and red galaxies, but these intermediate ‘green’ galaxies are more rare.

“This suggests that the few green galaxies we catch are likely to be at a critical stage in their evolution; rapidly turning from blue to red.”

Because stars form from dense gas, a powerful process is needed to rapidly destroy their gas supply and cause such dramatic changes in colour, the research found.

James added: “In a recent study we followed simulated galaxies as they changed colour, and investigated what processes caused them to change.

“We typically find that smaller green galaxies are being violently tossed around by the gravitational pull of a massive neighbour, causing their gas supply to be stripped away.

“Meanwhile, bigger green galaxies may self-destruct as immense explosions triggered by supermassive black holes at their centres can blow dense gas away.”

However, the research found that there was some hope for green galaxies as a lucky few might absorb a fresh supply of gas from their surroundings. This can revive the formation of stars and planets, and restore galaxies to a healthy blue state.

James said: “By using simulations to study how galaxy colours change, we can speed up the process of galaxy evolution from the billions of years it takes in the real Universe to just a matter of days in a computer.

“This means we don’t just see galaxy colours frozen in time, we can watch them evolve. Another advantage is that we can remove unwanted factors that may change the colours we see, such as pesky dust clouds that can prevent light escaping from galaxies.

“As the EAGLE simulations we use represent a new level of realism, we can have greater confidence in applying these results to the real universe.”


Media contacts

Dr Robert Massey
Royal Astronomical Society
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Ms Anita Heward
Royal Astronomical Society
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NAM 2016 press office (from Monday 27 June to Friday 1 July)
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Science contact

James Trayford
Institute for Computational Cosmology
Durham University
(Available for interview on Wednesday, June 29 and Thursday, June 30, 2016)
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Images and captions


Composite image of blue, green and red galaxies: L-R Virtual images of blue, green and red galaxies produced by the EAGLE simulations. The green galaxy is caught in the act of transforming from blue to red as its gas supply runs out. Credit: James Trayford/EAGLE/Durham University

Image of blue galaxy from EAGLE simulation. Credit: James Trayford/EAGLE/Durham University

Image of green galaxy from EAGLE simulation. Credit: James Trayford/EAGLE/Durham University

Image of red galaxy from EAGLE simulation: Credit: James Trayford/EAGLE/Durham University


Further information

The research is funded by the Science and Technology Facilities Council (STFC) and the European Research Council (ERC).

It's not easy being green: The evolution of galaxy colour in the EAGLE simulation, Trayford James, W, et al is being presented at the Royal Astronomical Society’s National Astronomy Meeting, at the University of Nottingham, Thursday, June 30, 2016.

The EAGLE simulation project is a flagship of the Virgo consortium, and is led by scientists in Durham, Leiden and Liverpool John Moores Universities. The simulations created by the project were carried out on the DiRAC computing facility in Durham and at the Curie computing facility based in France

Institute for Computational Cosmology


Notes for editors

The RAS National Astronomy Meeting 2016 (NAM 2016) takes place this year at the University of Nottingham from 27 June to 1 July. NAM 2016 brings together more than 550 space scientists and astronomers to discuss the latest research in their respective fields. The conference is principally sponsored by the Royal Astronomical Society and the Science and Technology Facilities Council. Follow the conference on Twitter

About Durham University
- A world top 100 university with a global reputation and performance in research and education
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- Durham was named as The Times and Sunday Times 'Sports University of the Year 2015' in recognition of outstanding performance in both the research and teaching of sport, and student and community participation in sport at all levels.

The University of Nottingham has 43,000 students and is ‘the nearest Britain has to a truly global university, with a “distinct” approach to internationalisation, which rests on those full-scale campuses in China and Malaysia, as well as a large presence in its home city.’ (Times Good University Guide 2016). It is also one of the most popular universities in the UK among graduate employers and the winner of ‘Outstanding Support for Early Career Researchers’ at the Times Higher Education Awards 2015. It is ranked in the world’s top 75 by the QS World University Rankings 2015/16, and 8th in the UK by research power according to the Research Excellence Framework 2014. It has been voted the world’s greenest campus for four years running, according to Greenmetrics Ranking of World Universities.

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