Galaxies grow by 'eating' their smaller neighbours

Exactly how massive galaxies attain their size is poorly understood, not least because they swell over billions of years. But now a combination of observation and modelling from an international research team has provided a vital clue — one that suggests galaxies grow large by effectively eating their smaller neighbours.

In a paper published in The Astrophysical Journal, scientists combined data from an Australian project called the Multi-Object Spectroscopic Emission Line (MOSEL) survey with a cosmological modelling program running simultaneously on some of the world’s largest supercomputers in order to glimpse the forces that create these ancient galactic monsters. By analysing how gases within galaxies move, 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,” said study leader Dr Anshu Gupta, from the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D).

“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.”

Simulation showing distribution of dark matter density overlayed with the gas density. This image shows the gas channels connecting the central galaxy with its neighbours. Image credit: Gupta et al/ASTRO 3D/IllustrisTNG.

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. Dr Gupta’s team 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,” said ASTRO 3D’s Dr Kim-Vy Tran, second author on the study.

“The most likely explanation is that in the intervening billions of years the surviving galaxies have grown fat and disorderly through incorporating smaller ones. I think of it as big galaxies having a constant case of the cosmic munchies.”

The team of scientists from Australia as well as the US, Canada, Mexico, Belgium and the Netherlands ran their modelling on a specially designed set of simulations known as IllustrisTNG — a multiyear, international project that aims to build a series of large cosmological models of how galaxies form. Dr Gupta said, “The modelling showed that younger galaxies have had less time to merge with other ones.

“This gives a strong clue to what happens during an important stage of their evolution.”

Top image: Simulation showing distribution of dark matter particles around the galaxy. Image credit: Gupta et al/ASTRO 3D/IllustrisTNG.

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