Do hungry galaxies eat their neighbours?

Exactly how massive galaxies attain their size is poorly understood, not least because it happens over billions of years.

A combination of observation and modelling led by Australia’s ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) may have provided a clue, however.

The scientists combined data from Australia’s Multi-Object Spectroscopic Emission Line (MOSEL) survey with an international cosmological modelling program to catch a glimpse of the forces that create ancient galactic monsters.

By analysing how gases within galaxies move, says Anshu Gupta, lead author of a paper in The Astrophysical Journal, it is possible to discover the proportion of stars made internally – and the proportion effectively cannibalised from elsewhere.

“We found that in old massive galaxies – those around 10 billion light years away from us – things move around in lots of different directions,” she says. “That strongly suggests that many of the stars within them have been acquired from outside. In other words, the big galaxies have been eating the smaller ones.”

Because light takes time to travel through the Universe, galaxies further away from the Milky Way are seen at an earlier point in their existence.

Gupta’s team, which included scientists from Australia, the US, Canada, Mexico, Belgium and the Netherlands, found that observation and modelling of these very distant galaxies revealed much less variation in their internal movements.

“We then had to work out why ‘older’, closer big galaxies were so much more disordered than the ‘younger’, more distant ones,” says second author Kim-Vy Tran.

“The most likely explanation is that in the intervening billions of years the surviving galaxies have grown fat and disorderly through incorporating smaller ones.”

The researchers ran their modelling on a specially designed set of simulations known as IllustrisTNG – a program so big that it has to run simultaneously on several of world’s most powerful supercomputers.