When galaxies collide, new stars often stop forming. This effect has been predicted by simulations for more than a decade, but observational evidence for it has been sparse and more recent theoretical work has brought it into question. Now, researchers have demonstrated that freshly-merged galaxies are far more likely than peaceful ones to shut down star formation.
at the University of Victoria in Canada and her colleagues assembled a sample of 508 galaxies that had experienced mergers within the last billion years or so. They then compared the sample with a set of isolated, quiet galaxies that didn’t appear to have undergone mergers recently or to be heading towards a collision.
They found that the merged galaxies were up to 60 times more likely than the isolated ones to have undergone a period of rapid star formation called a starburst, followed by near-total cessation of star birth. While the basics of this process are understood – the two galaxies smash together, gas flows towards the centre of the new, merged galaxy, and that sudden influx causes a blast of energy to flow outwards through the galaxy – the specifics remain fuzzy.
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“We don’t even really know what’s stopping star formation,” says Ellison. “It could be because the gas has been blown out of the galaxy, it could be because it’s heated and buzzing around too fast to collapse into stars, it could be because the molecules are being destroyed.”
However, the researchers did spot a correlation between how likely each galaxy was to have stopped star formation and how warped and messy its shape was after the merger. This may hint that when galaxies of similar sizes merge, star formation is more likely to shut off than when a larger galaxy devours a small one.
“If you have a merger between a huge galaxy and a small one, it doesn’t mess anything up very much – it’s like bumping into a toddler,” says Ellison. “But if they’re of equal mass, that messes things up a lot more.”
To figure out what is really going on in these merged galaxies, astronomers will need to perform more detailed studies of their gas, she says. That could also illuminate why this work is in disagreement with recent simulations which have suggested that the merger process might not blast out enough energy to tamp down star formation.
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Article amended on 30 September 2022
We have corrected how much more likely merged galaxies were than isolated ones to have undergone a period of rapid star formation, followed by near-total cessation.
