Green Galaxies Are Going Through Violent Aging, Study Finds

An international research program, led by Durham University has resorted to computer modeling to better understand what lies behind the differently colored galaxies and what — if anything — those colors — as picked up by telescopes, since they may not be perceptible to the naked eye — tell us about each one of them.

Using a simulation system called EAGLE, the researchers modeled how galaxies age over time and how the change in stars' chemical composition could affect the color of the light they emit.

What the team's scientists already knew was that galaxies have a "healthy" blue glow to them when new stars and planets are being born, whereas they turn to a reddish hue when ageing stars burn themselves out and eventually die.

What the researchers discovered, though, is that green galaxies represent a sort of "middle-ground" stage between full blue splendor and red demise.

"From looking at real galaxies, there seems to be two common colors… blue and red. This is thought to reflect the generally 'on' or 'off' nature of star formation in galaxies (galaxies that are forming stars have a relatively blue colour because of their hot, young stars)," lead researcher James Trayford told Sputnik.

"The galaxies that do appear green are thought to be ephemeral: rapidly turning from blue to red after the formation of stars has been more or less shut down. The green galaxies we see are then interesting to study, because they may be caught in the act of changing color after some monumental event in their history that caused them to stop forming stars."

Usually, the ageing process from blue to red is slow and gradual, but researchers found green galaxies to be going through a much more dramatic 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," Trayford said in a statement. "Meanwhile, bigger green galaxies may self-destruct as immense explosions triggered by super-massive black holes at their centers can blow dense gas away."

But the researchers added that sometimes, in an overwhelming reversal of fate, green galaxies can manage to absorb some fresh gas from their surroundings, which restores them to their blue health.  

"In our simulations we find instances where, for example, a collision of two galaxies can trigger a huge explosion from the central supermassive black hole and destroy all the dense gas in the galaxy, so that stars stop forming," Trayford told Sputnik. "However it seems that we can get gas falling back onto the galaxy in dense clouds later on in the history of the universe, which may rekindle star formation inside the galaxy."