Imagine a city where streetlights shine, but their glow isn't scattered by fog — instead, it streams into the sky almost entirely. That's what galaxy U37126 looks like, as seen by astronomers in the early universe with the James Webb Space Telescope. Its light has traveled for more than 13 billion years to reach us.
In typical galaxies, young hot stars are surrounded by lots of gas. This gas glows when heated by the stars, but it also absorbs some of the ultraviolet radiation. In U37126, however, astronomers saw none of the expected gas glow. Instead, the starlight itself was extremely bright and blue. This means the galaxy is almost devoid of a gaseous "fog"—it has been blown out by intense radiation and winds from massive stars.
Such a "bare" galaxy lets more than 86% of its ultraviolet light escape. For comparison, in typical galaxies this fraction is often less than 10%. It was this kind of light that heated and ionized intergalactic hydrogen in the early universe, making space transparent. Even if such galaxies were rare, just a few percent of all galaxies, they could have provided half or even all of the necessary light.
Astronomers built a model of the stellar population of U37126. They found that the galaxy hosts very young and massive stars, with an enormous star formation rate: about 10 solar masses per year. Yet the galaxy itself is tiny, so the density of stars and the rate of star birth per unit area are among the highest known. It seems that gas in the galaxy's center quickly turned into stars, and then powerful radiation swept the remaining gas outward. This is how a "naked" island of stars emerged, its light streaming unimpeded into space.
as written by the authors arXiv:2602.02322
We present very deep (~11h) JWST/MIRI low-resolution spectroscopy of the rest-frame optical emission of U37126, a UV-bright (M_UV ~ -20), mildly lensed ($$\mu\simeq 2.2$$) galaxy at z=10.255. The continuum emission is well detected in both NIRSpec and MIRI spectra, yet no nebular recombination or metal emission lines are observed (EW(Hbeta+[OIII])<300A and EW(Halpha)<400A, at 3sigma). Combined with the exceptionally blue UV continuum slope, beta_UV ~ -2.9, and weak/flat Balmer break, these constraints indicate a stellar population dominated by very young and massive stars with a strongly suppressed nebular contribution. Comparisons with synthetic stellar population models indicate that U37126 requires both a very high ionizing photon production efficiency, log(Xi_ion / Hz erg^-1) ~ 25.75, and a nearly unit LyC escape fraction, of fesc>86% (3sigma) based on Halpha flux limit and fesc=0.94+/-0.06 derived independently from SED fitting. The best-fit SED yields a (de-lensed) stellar mass of Mstar ~ 10^7.8 Msun and a star-formation rate of SFR~10Msun/yr (sSFR~160 Gyr^-1), that along with its very compact size, reff~61pc, yields very high stellar mass and star-formation-rate surface densities, Sigma_M ~ 3x10^3 Msun/pc^2 and Sigma_SFR ~ 400 Msun/yr/kpc^2. Together with the lack of detectable nebular emission, these properties suggest that U37126 is undergoing an ``ISM-naked'' starburst phase, possibly driven by an extremely efficient gas-to-star conversion followed by strong feedback that has cleared the remaining gas from its stellar core, allowing most LyC photons to escape. Finally, we show that even a small fraction of galaxies like U37126 (~ 3%-6%), with extreme LyC production and escape, could contribute disproportionately (~ 50%-100%) to the ionizing photon budget during cosmic reionization.
Light from U37126 has traveled for 13.1 billion years — almost the entire age of the universe. We see this galaxy as it was just 400 million years after the Big Bang.