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.
Scientists may have answered why green galaxies are rare in our universe and why their color could reveal a troubled past. Their research is presented 30 June at the National Astronomy Meeting at the University of Nottingham.
The international team, led from Durham University's Institute for Computational Cosmology, used new computer modeling of the universe to investigate the colors that galaxies have and what those colors might tell us about how galaxies evolve. Using the state of the art EAGLE simulations, the researchers modeled how both the ages of stars in galaxies and what those stars are made from translate into the color of light that they produce.
The team said their simulations showed that colors 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 color, the research found.
James added: "In a recent study we followed simulated galaxies as they changed color, 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 neighbor, causing their gas supply to be stripped away.
"Meanwhile, bigger green galaxies may self-destruct as immense explosions triggered by super-massive black holes at their centers 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 colors 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 colors frozen in time, we can watch them evolve. Another advantage is that we can remove unwanted factors that may change the colors 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."