A new James Webb Space Telescope study has dated two large, luminous galaxies to only 300 million years after the Big Bang. But how did they get so big and bright so quickly?

Image showing most distant galaxy known
Among the distant galaxies in this JWST Near-Infrared Camera image is JADES-GS-z14-0 (inset), determined to be at a redshift of 14.32, making it the current record-holder for the most distant known galaxy. It resided in a universe less than 300 million years old. (By chance, it appears to lie right next to another, unrelated galaxy that is much closer to Earth.)
NASA / ESA / CSA / STScI / Brant Robertson (UC Santa Cruz) / Ben Johnson (CfA) / Sandro Tacchella (Cambridge) / Phill Cargile (CfA)

It seems like there's nothing the James Webb Space Telescope (JWST) can’t do. Solar System Science, exoplanet atmospheres, and distant galaxies and stars are all within its purview. Just last year, an international collaboration announced the discovery of candidate galaxies in the first 400 million years of cosmic time. Now this same group has pushed the envelope even further, raising challenging questions about our current theories on galaxy formation.

Stefano Carniani (Scuola Normale Superiore, Italy), a member of the JWST Advanced Deep Extragalactic Survey (JADES) recently posted a study on arXiv, a non-peer reviewed site for preprint articles, describing the most distant galaxies ever observed. His team used JWST’s Near-infrared Spectrograph to study three galaxies previously identified as possibly lying at great distances. The latest observations confirm that the light from two of the galaxies has been in transit for more than 13.5 billion years, breaking previous distance records.

What’s more, both galaxies are surprisingly large and bright for their age, suggesting that luminous galaxies were already in place 300 million years after the Big Bang, and may be more common than expected for this early epoch of cosmic history, sometimes referred to as the cosmic dawn.

“There is something weird in this data,” says Carniani. “During the cosmic dawn we would expect the galaxies to look younger and not have so many stars inside them, because [lots of stars] take more time to form. So we thought.”

The most distant galaxy, called JADES-GS-z14-0, is also the most luminous between the two. Analysis of its spectrum suggests its light is not being emitted by an accreting black hole, but by stars — lots of them, with a combined mass of 500 million Suns.

Spectrum of most distant galaxy known to date
The James Webb Space Telescope’s Near-Infrared Spectrograph captured this spectrum of the distant galaxy JADES-GS-z14-0. The galaxy's redshift and therefore age can be determined from the location of a critical wavelength known as the Lyman-alpha break. (No other emission or absorption lines are visible in the spectrum.) This galaxy dates back to less than 300 million years after the Big Bang.
NASA / ESA / CSA / Joseph Olmsted (STScI)

The Mid-infrared Instrument onboard JWST also collected data that might indicate the presence of oxygen, created when a star explodes. (That data was presented in a separate study, also posted on the arXiv, led by Jakob Helton at University of Arizona.) The detection of oxygen would mean that, even at this early epoch, the galaxy is already experiencing its second or third generation of stars. Carniani says this turnaround is hard to understand; however, these observations don’t break any rules yet.

“Current formation models suggest that the more luminous galaxies will take a bit less than one billion years to form hundreds of millions of stars similar to our Sun,” he says. “But these galaxies are only around 300 million years old.” 

There are a few possible explanations for how these galaxies formed so many stars so quickly. The first is that we’ve just taken a “snapshot” of these galaxies during a particularly active star-forming phase. It’s possible that galaxies don’t evolve at steady rates but instead grow in fast bursts. It’s also possible that gas turns into stars on shorter timescales than we thought. Or maybe these galaxies are just strangely bright outliers, and we won’t find many others like them.

Bingjie Wang (Penn State), who was not involved in these studies, thinks the latter might be true, explaining that as big, bright galaxies are the easiest to see, that’s what any survey is most likely to pick up. But that doesn’t mean these two bright galaxies are representative of the population as a whole. The majority of galaxies at this early epoch might very well be as small and dim as expected.

Moving forward, if the majority of distant objects above a certain mass are found to be larger and more luminous than expected, this could pose a challenge to our current understanding of how galaxies form and evolve. Wang is excited to see how this latest mystery pans out.

“I think it's a bit early to say there’s something wrong in our standard model,” she says. “All we have now is a clue. We need a larger statistical sample to draw any robust conclusions.”


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