Faintest early-Universe galaxy confirmed

Gravitational lensing enables astronomers to use massive objects like galaxy clusters as natural magnifying glasses to spot more distant objects lurking behind them. The technique has now been used to detect and confirm the faintest early-Universe galaxy ever observed.

A colour image of the cluster showing the three spectra of the gravitationally lensed images. The graphs show peaks at the same wavelength, indicating they all belong to the same source.

Astronomers have confirmed the faintest early-Universe galaxy ever detected, by using an observing technique whereby the light from a distant object is magnified by the gravitational pull of a closer object.

The galaxy was detected as it existed 13 billion years ago. It was observed using the W. M. Keck Observatory in Hawaii, but could not have been detected at all without using a technique known as gravitational lensing.

Gravitational lensing was predicted by Einstein and is used by astronomers today to observe distant objects that would otherwise be out of sight. It involves light from the target object being magnified by the gravitational pull of another massive object lying between it and the observer. As this light is bent around the in-between object, it often finds multiple paths and can therefore produce multiple images of the same object.

In the case of this discovery, the galaxy detected was magnified by the gravitational pull of galaxy cluster MACS2129.4-0741, which is massive enough to have created three different images of the target object.

“Because you see three of them and the characteristics are exactly the same, that means it was lensed,” says Marc Kassis, staff astronomer at Keck Observatory. “The other thing that is particularly interesting is that it is small. The only way (the team) would have seen it is through lensing. This allowed them to identify it as an ordinary galaxy near the edge of the visible Universe.”

The galaxy is so far away it lies near the end of the epoch of reionisation, which is when the hydrogen gas of the early Universe was ionised and lit up the stars for the first time.

“We now have good constraints on when the reionisation process ends – at redshift around 6 or 12.5 billion years ago – but we don’t yet know a lot of details about how it happened,” says Kuang-Han Huang, lead author of the paper. “The galaxy detected in our work is likely a member of the faint galaxy population that drives the reionisation process.”

“This galaxy is exciting because the team infers a very low stellar mass, or only one percent of one percent of the Milky Way,” Kassis says. “It’s a very, very small galaxy and at such a great distance, it’s a clue in answering one of the fundamental questions astronomy is trying to understand: What is causing the hydrogen gas at the very beginning of the Universe to go from neutral to ionized about 13 billion years ago. That’s when stars turned on and matter became more complex.” 


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