JWST Spots Galaxy 530 Million Years After Big Bang That May Leak 85% of Its Ionizing Light
If the preprint holds up, EBG-1 would be an unusually efficient source of the radiation that cleared the early universe's fog, but it is one object and the estimate rests on model assumptions.
The James Webb Space Telescope has found a galaxy from the dawn of the universe that may be letting most of its ultraviolet light escape into space, a trait that could help explain how the early cosmos was transformed from neutral gas into the ionized state it has today. The result comes from a preprint posted on arXiv on October 16 that has not yet been peer reviewed.
The object, called Extreme Blue Galaxy 1 or EBG-1, sits at redshift 9.244, which corresponds to about 530 million years after the Big Bang. A foreground galaxy cluster, MACS0647, bends and magnifies its light through gravitational lensing, letting astronomers study it in detail. The team is led by Yunjing Wu of the University of Tokyo.
The question behind the work is what drove cosmic reionization, the period roughly 600 million to one billion years after the Big Bang when ultraviolet radiation from early stars and galaxies stripped electrons from the universe's neutral hydrogen. Reionization was complete by about redshift 5.5. The key photons are Lyman continuum photons, with wavelengths shorter than 912 angstroms. Much of this light is absorbed inside a galaxy, so what matters is the escape fraction, the share that reaches intergalactic space. Researchers still debate which galaxies supplied most of the photons, with some models favoring dwarf galaxies and others favoring more massive, efficiently star-forming ones.
EBG-1 stood out because it has one of the bluest ultraviolet slopes yet measured at that redshift. A very blue slope usually signals little dust, very young stars and low metal content, and it can hint that ionizing photons are escaping. To test that, the team took a deep, 14-hour spectrum with Webb's NIRSpec instrument and looked for emission lines from carbon, hydrogen and oxygen.
The spectrum was weak overall. The oxygen line was prominent, the hydrogen line was faint, and carbon was not detected. From the oxygen-to-hydrogen ratio, the team estimated an oxygen abundance of about 3 percent of the Sun's, consistent with a young galaxy early in cosmic history. When they compared the data with synthetic spectra, models with an escape fraction of around 85 percent fit best. The paper's figures give a model-based lower limit of about 0.75. The authors also report that EBG-1 lies roughly 0.5 dex above an extrapolated relation from earlier work by Matthee and colleagues, which they read as evidence of an exceptionally efficient ionizing source.
The caveats are significant. EBG-1 is a single object, and its surroundings have not been studied, so astronomers cannot yet say how much it contributes to ionizing the gas around it. The escape-fraction estimate depends on assumptions, including that the galaxy has no dust and forms stars at a constant rate. The authors say a census of neighboring galaxies would show whether EBG-1 or its neighbors dominate the local ionizing budget.
The preprint is titled "Can Blue UV-Bright Galaxies Drive Reionization at Cosmic Dawn? An MUV = -19.6 Galaxy with fesc ≈ 0.85 at z = 9.244" and carries the identifier 10.48550/arXiv.2610.24092.
