Cosmo study: Middle-aged galaxies are rarer than you'd think

Gorgeous MIlky Way not even close to turning red through stress of ageing

Photo credit: James Trayford/EAGLE/Durham University

Physicists have created a novel simulation which allows users to watch how the colour of a galaxy changes over time as it evolves.

The results will be presented later today at the Royal Astronomical Society National Astronomy Meeting 2016, and are based on a preliminary paper led by researchers at Durham University.

The relationship between colour and age of a galaxy has been established. Galaxies that appear blue are home to young stars burning brightly, and are still active with new stars forming within them.

Since larger stars reach higher temperatures, they deplete their hydrogen reserves through nuclear reactions at a faster rate, leaving older, more red-coloured stars to hobble along the galaxy.

As galaxies age and turn from blue to red, there is a green intermediate stage. The middle-aged galaxies are rare. James Trayford, lead-author of the study and PhD student working at Durham University’s Institute for Computational Cosmology, told The Register that he thought that green galaxies probably made up less than ten per cent of all galaxies.

The rarity suggests that green galaxies are in a critical stage and fizzle out quickly.

Galaxy evolution can be hard to study as observations only allow researchers to see the galaxy at one point in time. It takes billions of years for them to evolve. But with the EAGLE simulations, the lifespan of a galaxy can unfold before the viewer’s eyes.

Green galaxies were quickly torn apart, Trayford told The Register: “Gravity is the most important force controlling galaxies. Small galaxies are at the mercy of larger galaxies which can strip their gas supply away if they get too close.”

The gas is mostly made up of hydrogen - the most abundant element in the universe - which is the vital ingredient for forming stars. Without gas, new stars cannot form and as the existing stars fade away, the galaxy dims. Another way for gas to be dispelled is if a powerful supernova blast blows gas away, Trayford explained.

The researchers hope to test their models with real data. The simulations are a valuable tool for understanding galaxies. By modelling the evolution of galaxies they can match up similar processes happening in the simulations and in space.

“These all help us develop our understanding of how galaxies like our very own Milky Way form,” Trayford said.

Watch the simulation here. ®

Universal note

Each galaxy ages at different rate. If a younger galaxy had a massive supernova explosion and blew out the gas then it would age more quickly despite being around for a shorter time.

According to this research, our 13.2 billion-year-old home galaxy the Milky Way is close to transitioning to green, but is white at the moment.

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