When galaxies rotate, size matters


Sky survey provides clues to how things change over time.


A simulation showing galaxies gradually accreting along a cosmic filament.
CREDIT: Greg PoolE

The direction in which a galaxy spins depends on its mass, astrophysicists have found.

Their analysis of 1418 galaxies showed that small ones are likely to spin on a different axis to large ones. The rotation was measured in relation to each galaxy’s closest “cosmic filament” – the largest structures in the Universe.

A simulation showing a section of the Universe at its broadest scale – a web of filaments forms a lattice of matter, enclosing vast voids.

Tiamat simulation, Greg Poole.

The international study was driven by the ARC Centre of Excellence in All Sky Astrophysics (ASTRO 3D), based in Australia. The paper, to be published in the Monthly Notices of the Royal Astronomical Society, is available on the preprint site arXiv.

Filaments are massive thread-like formations comprising huge amounts of matter, including galaxies, gas and, modelling implies, dark matter. They can be 500 million light-years long but just 20 wide – and they are why the universe looks a bit like honeycomb.

At their largest scale, the filaments divide the universe into a vast gravitationally linked lattice interspersed with enormous dark matter voids.

“It's worth noticing that the spine of cosmic filaments is pretty much the highway of galactic migration, with many galaxies encountering and merging along the way,” says lead author Charlotte Welker, now at McMaster University in Canada.

Using data gathered by an instrument called the Sydney-AAO Multi-object Integral-field spectrograph (SAMI) at Australia’s Anglo-Australian Telescope (AAT), Welker and colleagues from Australia, the US, France and Korea studied each of the target galaxies and measured its spin in relation to its nearest filament.

They found that smaller ones tended to rotate in direct alignment to the filaments, while larger ones turned at right angles. The alignment changes from the first to the second as galaxies, drawn by gravity towards the spine of a filament, collide and merge with others, thus gaining mass.

“The flip can be sudden,” Welker says. “Merging with another galaxy can be all it takes.

“Imagine you are skating after a friend and catching up. If you grab your friend’s hand while you are still moving faster, you will both start rotating on a vertical axis – a spin perpendicular to your horizontal path.

“However, if a small cat – a much lighter bit of matter – runs after your friend and jumps on her she probably won’t start spinning. It would take a lot of cats leaping on her at once to change her rotation.”

Co-author Scott Croom, from Australia’s University of Sydney, says the result offers insight into the deep structure of the Universe.

“Virtually all galaxies rotate, and this rotation is fundamental to how galaxies form,” he says.

“For example, most galaxies are in flat rotating disks, like our Milky Way. Our result is helping us to understand how that galactic rotation builds up across cosmic time.”

  1. https://arxiv.org/abs/1909.12371
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