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Andromeda galaxy’s ‘cannibalistic’ nature points to new galactic growth theory

New research has found that the neighbouring Andromeda galaxy "cannibalises" smaller galaxies to grow bigger. This is at odds with a calmer picture of how galaxies grow.

Andromeda GalaxyStudying the Andromeda galaxy, pictured here, also helps scientists better understand our own Milky Way galaxy. (Image credit: Robert Gendler / NASA)

The Andromeda galaxy, which will collide with our Milky Way galaxy, in about 4 billion years from now, might be a “cosmic cannibal” that gets bigger by devouring smaller galaxies.

The research, which is currently available on the pre-print server arXiv and will be published in the journal Monthly Notices of the Royal Astronomical Society, bases this theory on the discovery of a structure of stars, called a globular cluster, in the Andromeda Galaxy. These globular clusters have their origin outside the galaxy and have been named the Dulais Structure, after the Welsh word for black stream.

“A few years ago, we discovered that in the far outskirts of Andromeda, there was a sign in the objects orbiting it that the galaxy hadn’t been grazing, but it had eaten large quantities in two distinct epochs. What this new result does is provide a clearer picture of how our local universe has come together – it is telling us that at least in one of the large galaxies, there has been this sporadic feeding of small galaxies,” said lead author Geraint Lewis in a University of Sydney press statement.

Dulais structure and how galaxies grow

The Dulais Structure is formed from the leftovers of a massive feeding event and is a dark stream lit up by star clusters that are different from others in the Andromeda galaxy. According to the researchers, this presents evidence that galaxies grow by consuming smaller systems. As per the University of Sydney, this is at odds with a calmer picture of how galaxies grow.

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The galaxy contains signatures of two major feeding events. The more recent of the two events took place sometime in the last 5 billion years while the other one happened between eight and ten billion years ago.

Studying the Andromeda galaxy, pictured here, also helps scientists better understand our own Milky Way galaxy. Since we are sitting inside the Milky Way, making observations about the galaxy is difficult as they tend to be obscured. The distant Andromeda galaxy, on the other hand, offers a “panoramic view” for scientists. It is still not clear how the Milky Way fed itself, but evidence from Andromeda points to such large feeding events and growth.

For the next phase of the research, the astrophysicists hope to find out whether the Milky Way has done the same things as Andromeda or if it is different in some way. They will also endeavour to “come up with a more accurate clock” when Andromeda’s feeding events occurred.

First published on: 30-11-2022 at 14:00 IST
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