Study: Webb observes z=10.255 galaxy that may leak nearly all ionizing radiation

A new James Webb Space Telescope study reports that a galaxy seen just 430 million years after the Big Bang appears to let nearly all of its ionizing radiation escape — an indirect but unusually strong result that could reshape astronomers’ thinking about cosmic reionization if similar galaxies are even mildly common.

The caveat is central to the claim. Astronomers did not directly detect escaping Lyman-continuum radiation from the galaxy, known as UNCOVER-37126 or U37126. At this redshift, the intergalactic medium is opaque to those photons. Instead, the result is inferred indirectly from something missing: the nebular emission lines that would normally appear if ionizing radiation were being absorbed by gas inside the galaxy, plus modeling of the galaxy’s overall light.

The paper, “PRISMS. U37126, a very blue, ISM-naked starburst at z = 10.255 with nearly 100% Lyman continuum escape fraction,” is by R. Marques-Chaves and colleagues. It is posted on arXiv and listed as accepted for publication in Astronomy & Astrophysics. U37126 already had a spectroscopic redshift of 10.255, measured with Webb’s NIRSpec instrument.

The new analysis draws on about 11 hours of Webb MIRI low-resolution spectroscopy, combined with existing UNCOVER NIRCam imaging and NIRSpec PRISM spectroscopy. The galaxy’s continuum — its underlying spread of light across wavelengths — is clearly detected. But the team reports no detectable nebular recombination or metal emission lines in the rest-frame optical spectrum. It places 3-sigma upper limits of less than 300 angstroms for the combined equivalent width of H-beta plus [O III], and less than 400 angstroms for H-alpha.

Those missing lines are paired with other unusual traits. U37126 has an extremely blue ultraviolet slope, with beta-UV about minus 2.9, and a weak or flat Balmer break, another sign tied to the age and makeup of its stars. Comparing those measurements with models, the authors infer a stellar population dominated by very young, massive stars but with unusually little surrounding nebular emission. They estimate a very high ionizing-photon production efficiency, with log10(xi-ion) about 25.75.

From there, the paper arrives at two indirect escape-fraction estimates. One is a lower limit of at least 86% from the H-alpha non-detection under the authors’ fiducial assumptions. The other, from spectral-energy-distribution fitting, is even higher: “fesc = 0.94 ± 0.06,” the paper says. The main caution is that those numbers depend on stellar-population and nebular-model assumptions, including metallicity and gas conditions.

Even after correcting for gravitational lensing, the galaxy looks small and intense. The authors report a magnification of 2.19, a stellar mass of about 10^7.8 times the mass of the sun, a star-formation rate of about 9.6 solar masses per year and an effective radius of about 61 parsecs. Those numbers imply very high stellar-mass and star-formation-rate surface densities, consistent with a compact starburst that may have cleared gas from its core. In the authors’ phrasing, “U37126 is undergoing an ‘ISM-naked’ starburst phase.”

That matters because cosmic reionization was the era when ultraviolet radiation from the first luminous sources reionized the hydrogen gas filling the early universe, and a central open question is whether young galaxies produced and leaked enough ionizing photons to drive it. Many models assume average galaxy escape fractions closer to 10% to 20%. A galaxy with an escape fraction near unity would be exceptional, suggesting that a small minority of extreme objects could make an outsized contribution.

The paper stops short of claiming a population measurement. But as an illustrative estimate, it argues that if only about 3% to 6% of early galaxies were like U37126, they could supply roughly 50% to 100% of the ionizing-photon budget needed for reionization.

Webb has already revealed a mix of strong-line and weak-line galaxies at similarly early epochs. U37126 now stands out as one of the clearest cases yet for a nearly line-free, compact starburst that may be leaking most of its ionizing radiation. The result is intriguing, and the paper’s acceptance in A&A gives it added weight, but the broader implications still hinge on two unresolved questions: whether more galaxies like this turn up, and how well the underlying models hold up.