One of the chief functions of the JWST is to find the earliest galaxies, expanding our exploration of the universe in both space and time. In the few weeks since the space telescope’s first data release, astronomers have mined its output to reveal a galaxy older than any we had spotted before, and then possibly broke that record again. The title of oldest and most distant galaxy may have already been broken for a third time, but in this case, the situation − dubbed "Schrödinger's galaxy candidate" − is a lot less clear.
A preprint of a yet-to-be peer-reviewed paper considers the case of CEERS-1749. On one measure we are seeing this galaxy as it was around 220 million years after the Big Bang. If so, it would not merely comfortably beat the record for the most distant galaxy, it would refute almost every model of early galaxy evolution ever found. Galaxies this evolved just shouldn’t have formed that quickly. On the other hand, some other measures put CEERS-1749 so much closer to us it would be unusual in an entirely different way.
When it comes to exploring the early universe, the most important measurement is z, the redshift. This measures how fast an object is moving away from us, causing spectral lines to be shifted towards the lower end of the spectrum, just as sirens sound deeper once they pass us. The expansion of the universe means the more redshifted a galaxy or star is, the more distant it is likely to be, and the further back in time we are seeing it.
Before this year the record for highest redshift galaxy we had seen was z=11. Shortly after the first JWST release a team led by Dr Rohan Naidu of the Harvard and Smithsonian Center for Astrophysics reported a z=13 galaxy, subsequently topped by a candidate at z=14. Meanwhile, a recent paper exploring the most distant galaxies visible enough to measure their metal content dealt with values around 8.
Consequently, if the 0.6 − 5µm photometry measurement of CERES-1749 as having a redshift of z≈17 is correct, it is a very dramatic breakthrough indeed. On the other hand, CEERS-1749 has three apparent neighbors with z≈5 (12.5 billion years old). It’s plausible it’s actually part of a cluster with this group, and the apparent redshift more than three times greater is an illusion. It’s this uncertainty that has led Naidu and co-authors to label CERES-1749 “Shrödinger’s galaxy candidate”, currently both the oldest galaxy known and billions of years younger.
Even if CEERS-1749 is part of the z=5 cluster, it may still set a different record. To look as it does at a z=5 distance it must either be a galaxy where star formation started, and then largely ceased, or a smaller one unusually thick with dust (a preprint by another team also proposes the small and dusty scenario). In those cases; “It will be the highest-redshift quiescent galaxy, or one of the lowest mass dusty galaxies of the early Universe detected to date,” Naidu and co-authors write. Either would also challenge existing models of galactic evolution, if not quite as profoundly.
It’s probably the second possibility that led Naidu to tweet crypticly:
Nine days later he shared the preprint. As a field, astronomy has probably never moved as fast as in recent weeks.
If galaxies with redshifts of 5 are impersonating something much older it would pose quite a challenge to cosmologists. However, the authors conclude: “Such a perfectly disguised contaminant is possible only in a narrow redshift window (∆z < 0.1).” One co-author commented it also requires specific telescope filter combinations, so is unlikely to be a factor we have to worry about often.
“Spectroscopic follow-up of this remarkable galaxy is of critical urgency to JWST’s mission of expanding the cosmic frontier… if this source does lie at z ≈ 17, we may embark on the grand enterprise of revising the physics of galaxy evolution at the earliest epochs,” the authors write. Unfortunately, the world’s most valuable telescope time is not easy to come by.