Green With Envy: Why Green Galaxies Are So Rare

June 30, 2016 | Joanne Kennell

Composite image of blue, green, and red galaxies.
Photo credit: James Trayford/EAGLE/Durham University

Colors provide information about galaxy evolution.

An international team of scientists, led by researchers from Durham University’s Institute for Computational Cosmology (ICC), may have figured out why green galaxies are rarely spotted in the known universe. Turns out they have likely had a rough life.

The team used computer models of the universe to study how the ages and compositions of stars influence the color of light produced by a galaxy. Simulations can speed up galaxy evolution from billions of years in the real world to just days in a computer, allowing researchers to diagnose a galaxy’s evolution based on its color.

Red and blue galaxies are fairly common.

Lead researcher James Trayford, a doctoral student at ICC, explained in a Royal Astronomical Society press release, “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.”

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However, green galaxies are the result of an important stage in their evolution: when they rapidly turn from blue to red.

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

As you know, stars are made of very hot and dense gas — mostly hydrogen and helium — so there must be a process strong enough to quickly strip them of their gas supply, causing this rapid color change.

The results of the study suggest three likely scenarios.

For smaller green galaxies, they can be “violently tossed around by the gravitational pull of a massive neighbour, causing their gas supply to be stripped away,” said Trayford. “Meanwhile, bigger green galaxies may self-destruct as immense explosions triggered by supermassive black holes at their centres can blow dense gas away.”

However, it’s not only unpleasantness that causes green galaxies to lose their lovely shade. A few of them may absorb a fresh supply of gas from their surroundings. This accumulation can restore the formation of stars and planets, returning the galaxy to a healthy blue hue.

The research was presented today (June 30) at the National Astronomy Meeting in Nottingham, UK.

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