PRISMS. U37126, a very blue, ISM-naked starburst at z=10.255 with nearly 100% Lyman continuum escape fraction

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 ($μ\simeq 2.2$) 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 SFR10Msun/yr (sSFR160 Gyr^-1), that along with its very compact size, reff61pc, 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.

💡 Research Summary

This paper presents an in‑depth study of the galaxy U37126, a UV‑bright (M_UV ≈ ‑20) source at a spectroscopic redshift of z = 10.255 that is modestly magnified by a factor of μ ≈ 2.2. The authors obtained ultra‑deep JWST/MIRI low‑resolution spectroscopy (LRS) with a total on‑source integration time of ~11 hours, covering the observed wavelength range 4.8–14 µm (rest‑frame 0.43–1.20 µm) at a spectral resolution of R ≈ 100. Complementary NIRSpec PRISM data (~4.4 h) and extensive NIRCam imaging (broad‑ and medium‑band filters from F070W to F480M) were also used. Data reduction employed the JWST pipeline (v1.20.2) with custom background subtraction and artifact mitigation; the NIRSpec spectrum was scaled to the NIRCam photometry by a factor of 1.41, while the MIRI spectrum required no additional scaling.

The combined spectrum shows a clear stellar continuum in both NIRSpec and MIRI, but none of the key rest‑optical nebular lines (Hβ,