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Astronomy and space

Astronomy and space

JWST spectrometer refines redshifts of distant galaxies

18 Apr 2023
NIRSpec
Before lift-off The NIRSpec instrument being tested on the ground prior to the launch of the JWST.(Courtesy:NASA/Chris Gunn)

The NIRSpec instrument on the James Webb Space Telescope (JWST) has revealed that a far away galaxy previously thought to be at a record-breaking redshift of 16.4 is actually much closer to Earth. The study has also confirmed that some other objects observed by the JWST are among the most distant galaxies ever seen.

Cosmological redshift is a measure of how much a galaxy’s light has been stretched to longer, redder wavelengths by the expansion of the universe. The higher the redshift, the more time that light from a galaxy must have spent moving through the expanding cosmos. This means that we see high-redshift objects as they appeared a very long time ago – and that the objects are very far away.

Astronomers are very keen on studying high-redshift galaxies because they provide a window into the early universe. Indeed, recent observations support an emerging picture that galaxies in the early universe were more massive, more developed and more luminous than had been previously predicted.

Several faint galaxies

In the summer of 2022, the JWST’s first deep surveys of the distant universe turned up several faint galaxies that were estimated to be the highest-redshift galaxies ever seen.

One object, called Maisie’s Galaxy, was discovered in the JWST data by a team led by Steve Finkelstein of the University of Texas at Austin. The galaxy was initially thought to be at redshift 14.3, which would have placed it just 280 million years after the Big Bang. Another candidate, CEERS-93316, found by a team led by Callum Donnan of the University of Edinburgh, appeared to be at a redshift of 16.4, which equates to just 250 million years after the Big Bang.

For comparison, the most distant confirmed galaxy known prior to the launch of the JWST was Gn-z11, which has a redshift of 11.6.

Revised redshifts

These early JWST measurements were made using a photometric technique, which gauges the overall redness of a galaxy. While this technique can be used on faint, distant objects, it can be affected by the presence of dust and is not as accurate as measuring the shifts of individual spectral lines. Now, a team of astronomers has used the JWST’s Near-Infrared Spectrometer (NIRSpec) to observe the galaxies and has refined the redshift estimates – with mixed results.

“Unfortunately, the redshift 16.4 candidate [CEERS-93316] turned out to be low-redshift,” says Donnan, who is a member of the team led by Pablo Arrabal Haro of NOIRLab in Arizona. Because the NIRSpec data were immediately made public with no proprietary time for the scientists who proposed the observations, Haro and team had to write their paper in less than three days to avoid being beaten to the punch.

Rather than being at 16.4, CEERS-93316 was found to be a dusty galaxy at a redshift of 4.9, meaning that we see it as it existed 12.5 billion years ago. Donnan’s team had previously thought it had a strong case for a record-breaking redshift, particularly as the galaxy displayed strong blue and ultraviolet emission in its rest frame (as it appears with the redshift removed).

However, the combination of very strong emission lines coupled with the fact that one of those lines, of the hydrogen-alpha wavelength, was in a position where three of NIRSpec’s filters overlap so that the emission line contributes to all three, erroneously gave the impression that CEERS-93316 was an intrinsically luminous galaxy at a much higher redshift.

Maisie’s Galaxy

There was better news in the redshift stakes for Maisie’s Galaxy, which was revealed to be at a redshift of 11.4. This is still a very high redshift and indicates a galaxy that is dust-free. The galaxy also has a relatively high star-formation rate and a total stellar mass of 250 million times the mass of the Sun. This mass had grown over a period of 30–120 million years prior to the time we see Maisie’s Galaxy.

A further eight galaxies have now also been shown by NIRSpec to have redshifts greater than 10. The current record holder is JADES-GS-z13-0, which has a spectroscopically confirmed redshift of 13.2 and which we see as it existed just 350 million years after the Big Bang.

Donnan is still hopeful that the JWST will be able to discover galaxies with spectroscopic redshifts greater than 14. “It’s possible, especially in deeper imaging,” he tells Physics World.

Dust production

Not that a well-studied galaxy at a redshift of 4.9 is anything to be sniffed at. Studying the properties of galaxies that existed when the universe was just over a billion years old is crucial in understanding how galaxies have developed in terms of their star formation. This can be inferred from the amount of dust that successive generations of stars produce – the same dust that causes CEERS-93316 to appear redder.

“We need to do a more detailed analysis of the properties of CEERS-93316, but it appears to be dusty,” says Donnan. “We need to look into its star-formation history if we want to understand how it came to be.”

Meanwhile, further observations are planned for the very-high-redshift galaxies such as Maisie’s Galaxy according to Finkelstein, who is also involved with the NIRSpec study.

Deeper spectroscopy

“The next step is definitely deeper spectroscopy, to probe exactly what is causing [Maisie’s Galaxy] to be so blue,” he says, referring to is its rest-frame colour. The leading theory is that early galaxies such as Maisie’s Galaxy had a higher proportion of luminous, blue, massive stars compared with galaxies today. Observations using one of the Keck 10 m telescopes in Hawaii are already under way, and Finkelstein hopes to follow up with the JWST in the future.

“We’ll be looking for weaker rest-UV emission line features, which are diagnostics for things including the presence of very massive stars and also how intense the starlight is from the stars we see,” says Finkelstein.

Ultimately, the findings are a reminder of the need for the spectroscopic confirmation of galaxy redshifts and that until such measurements are made, we should take claims of record-breaking photometric redshifts with caution.

The research is described in a prerint on arXiv.

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