Could the first galaxies in the universe have burned much brighter than theory allows? That’s the new question fascinating astronomers with the James Webb Space Telescope (JWST) sighting of 300 very bright objects some 13 billion years ago, when the universe’s first stars and galaxies were beginning to emerge.
This result, led by researchers at the University of Missouri, is based on JWST’s record-breaking ability to probe deep cosmic time. With its Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI), the researchers employed the “dropout” technique, wherein objects very distant are identified as the ones that shine in redder wavelengths but not in bluer wavelengths. This activity signals the Lyman break, a steep decline of a galaxy’s spectrum from ultraviolet light absorption of neutral hydrogen a signature that shifts into the infrared with higher redshift. As study co-author Haojing Yan put it, “The higher the redshift, the closer the galaxy is to the beginning of the universe.”
By comparing the spectral energy distributions of these sources at different wavelengths, the researchers estimated their redshifts, ages, and masses. And the results were a surprise: candidates that, based on current theory of galaxy formation, would be low-mass and weak actually were wildly luminous. Already, one object has been spectroscopically identified as an early galaxy, but the team is quick to point out that spectroscopy the “gold standard” for determining distance will be required to verify the others.
The brightness of these objects is part of a broader trend JWST has revealed since its launch in 2022. Again and again, deep-field surveys have found galaxies at redshifts larger than 9 that are brighter and more massive than the Lambda Cold Dark Matter (ΛCDM) cosmological model anticipates. For some AGN, however, such as the spectroscopically confirmed z ≈ 10 AGN GHZ9 in the GLASS field, active galactic nuclei may be contributing to the ultraviolet emission, rendering starlight alone problematic to interpret.
Astrophysicists are exploring different possibilities. Star-based models include a “top-heavy” initial mass function comprising many short-lived massive Population III stars, which would produce prodigious ultraviolet radiation. Others propose episodic, burst-like star formation ten times more intense than in later ages or incredibly high star-formation efficiency in the dense, gas-rich early universe environments. But these ideas are refuted: more star formation would create more dust, which would redden and obscure galaxies, contrary to JWST’s view of many dustless systems.
A second possibility is that some of these early beacons are supermassive black hole-powered, at least partly. Simulations have suggested that black holes that form within the first 50–100 million years since the Big Bang could energize powerful outflows that compress gas and fuel rapid star formation. Others invoke more exotic mechanisms, such as intense ultraviolet radiation from hydrogen recombination due to high-energy photons in a black hole’s photon sphere light that could mimic stellar light in JWST images at infrared wavelengths.
Redshift is applied in all this. It ages such objects within a few hundred million years of the Big Bang not only but also gives input into intrinsic luminosity estimates. At z ~ 10, for example, the universe was less than 500 million years old, and ΛCDM’s halo mass function ought to deliver very many fewer bright galaxies than JWST is discovering. The actually confirmed number density of z ~ 10 galaxies in some fields, more than three times other JWST estimates, implies either proto-clusters overlapping or large-scale clustering.
Spectroscopic follow-up will be key. By dissecting light into its component wavelengths, the NIRSpec instrument of JWST can record emission lines such as C III] λ1908 or Lyα breaks, providing precise redshifts and clues to the physical processes at work. In the GLASS field, these observations have already linked some bright high-z sources to AGN activity, while others have extended, dust-poor stellar populations.
Whatever these 300 shining objects prove to be, they show the power of JWST to test and perhaps overthrow hundreds of years of cosmic dawn assumptions. As Yan summarized, “Even if only a few of these objects are confirmed to be in the early universe, they will force us to modify the existing theories of galaxy formation.”
