Ancient serene galaxy suggests the early universe was eerily calm

By Leah Crane

Gravitational lensing makes a distant galaxy look like a coffee ring

ALMA (ESO/NAOJ/NRAO), Rizzo et al.

A strangely sedate galaxy in the early universe may upend our ideas about how galaxies form. We think most young galaxies in the early cosmos ought to be chaotic and turbulent, but galaxy SPT-SJ041839-4751.9, the earliest galaxy of its type we’ve been able to see with high resolution, isn’t.

We see this galaxy as it was when the universe was just 1.4 billion years old and it is much too dim and distant to observe in detail using a regular telescope. Simona Vegetti at the Max Planck Institute for Astrophysics in Germany and her colleagues used a phenomenon called gravitational lensing to take a picture of it and measure how the gas it contains was moving.

Gravitational lensing occurs when light from a distant object bends around a massive, more nearby object. The light is also magnified as it bends, so by turning that light back into an image of the farther object we can reconstruct a relatively detailed picture. Vegetti and her colleagues spotted the light from this galaxy spread into a near-perfect ring by the lensing effect of a closer galaxy. They nicknamed SPT-SJ041839-4751.9 the coffee stain galaxy because it looked like the mark left behind when a mug of coffee is set on a table.

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“This is the first time that a galaxy so far away could be seen with such precision, and also the results were quite surprising to us given what we thought about galaxy evolution,” says Vegetti. Because young galaxies like the coffee stain galaxy are still in the process of explosively forming new stars, we expect them to be churning and turbulent. But that’s not the case here.

“This galaxy is very quiet, it’s rotating in this very boring way, with not much chaos going on,” says Vegetti. It looks more like the type of mature galaxy we would see nearby than the sort we would expect in the early universe. If galaxies like this one are common, it may mean that we don’t understand galaxy formation or the early universe as well as we thought – things might have been less chaotic than our models predicted.

Journal reference: Nature, DOI: 10.1038/s41586-020-2572-6

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