Faint distant galaxies spotted by JWST may be closer than they appear

Because of a calibration update for one of the James Webb Space Telescope’s main cameras, research using the first few weeks of data from the observatory may be flawed.

The James Webb Space Telescope (JWST) is performing better than expected, and that might be a problem for some of the early results. An update to our understanding of how one of its cameras is working may mean that many galaxies spotted in the early data are not as distant as they seem.

When JWST sends data back to Earth, it doesn’t come as complete images. Astronomers have to process it to make it usable, which requires understanding the sensitivity of the telescope’s scientific instruments. As JWST takes more data, we gain a better understanding of that sensitivity. But new information on the performance of an infrared camera caused the telescope’s operators to update its data-processing algorithms in July – well after the first images were released – and this threw some astronomers into a tizzy.

“When the first images came out, it was a bit of an ‘astronomers at Christmas’ scenario with everyone diving in to see what they could find,” said Nathan Adams at the University of Manchester, UK, in a statement. “What I think flew under the radar of a lot of astronomers was a part of that report mentions that NIRCam (one of the main cameras on the telescope) was overperforming in its reddest wavelengths.”

This is important because astronomers use the colour of an object’s light to measure its distance. The faster a galaxy is moving away from us, the redder its light appears. Because of the expansion of the universe, more distant galaxies move away from us faster, making them appear redder. Adams and his colleagues re-analysed some early JWST data after the update and found that some galaxies are actually less red – and correspondingly less distant – than they initially appeared.

“It’s potentially a very big deal,” says Guido Roberts-Borsani at the University of California, Los Angeles. “The data that we got is revolutionary and is great, but our understanding of the data is not.” It may mean that some of the early science coming from JWST data is incorrect, especially for the faintest galaxies, where luminosity and distance are determined with fewer data points than brighter galaxies. Some faint galaxies may be more than 10 times closer than we thought.

However, that doesn’t apply to everything. “This whole clamour of ‘oh my god, everything that everyone has written in the last few weeks is wrong, throw it out the window’ is really not the case,” says Rohan Naidu at the Harvard-Smithsonian Center for Astrophysics. “This is not unexpected, and many researchers made conservative choices in their work to account for it.” For work focusing on very bright galaxies in particular, the corrections that need to be made to researchers’ calculations may not be all that extreme, he says.

It may even solve one mystery from the early data: the fact that JWST’s first observations seemed to show far more extremely distant galaxies than we expected based on models of galaxy evolution. “This unaccounted for error might be the reason why,” said Adams. If those galaxies aren’t quite so far away, the tension between theory and observation evaporates.

“This helps with part of the tension, but doesn’t completely solve it,” says Roberts-Borsani. “There are still these bright monsters at high distances where we don’t really expect them to be forming stars like crazy.” Those bright galaxies aren’t affected as much by the updated NIRCam calibration.

More calibrations will come through from the JWST team in the coming months, but they’re all expected to be less severe, said Adams. Meanwhile, astronomers who analysed JWST data before this update are going back to double-check their conclusions. “Of course in the long run we’re going to iron all this out,” says Naidu.


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