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

TL;DR

PRISMS. U37126 analyzes an extremely blue, ISM-naked starburst galaxy at $z=10.255$ using deep JWST MIRI spectroscopy combined with NIRSpec/NIRCam data. The authors demonstrate a near-unity LyC escape fraction and a highly elevated ionizing photon production efficiency, driven by a very young stellar population with negligible dust. The compact morphology (de-lensed $r_{\rm eff} \approx 61$ pc), high stellar mass and SFR surface densities imply an ISM-depleted phase with strong feedback, enabling efficient LyC leakage. They argue that such rare sources, constituting a few percent of high-z galaxies, could disproportionately contribute to cosmic reionization, potentially dominating the ionizing emissivity.

Abstract

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 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.

Figures (8)

• Figure 1: NIRSpec/PRISM (black) and MIRI/LRS (red) spectra of U37126 with the 1$\sigma$ uncertainties shown in grey. The expected locations of the main rest-frame optical emission lines, H$\beta$, [O iii] $\lambda\lambda$4960,5008, and H$\alpha$, are indicated with red dashed lines. NIRCam photometry from broad- and medium-band filters is overplotted in blue and green, respectively.
• Figure 2: Comparison of the rest-frame Ly$\alpha$ break of U37126 (black) with that of MACS0647-JD ($z=10.170$) which shows a strong damped Ly$\alpha$ absorption feature ($N_{\rm HI} \simeq 2.5 \times 10^{22}$ cm$^{-2}$, Heintz2024Sci...384..890H).
• Figure 3: Predictions from BPASS synthetic stellar and nebular emission models ($Z/Z_{\odot}\simeq 0.15$, $T_{e}=1.5\times 10^{4}$ K, and $n_{e}=10^{3}\,$cm$^{-3}$) for the UV continuum slope ($\beta_{\rm UV}$), $EW_{0}$ (H$\alpha$), and the Balmer break strength ($F_{\nu}^{4200\AA}/F_{\nu}^{3400\AA}$) as a function of the age (top) and their combination (bottom). Solid and dashed lines correspond to constant star formation (CSFH) and instantaneous burst models (Burst), respectively. Model sequences are shown for LyC escape fractions $f_{\rm esc}$ ranging from 0% (red) to 90% (blue); additional models with $f_{\rm esc}=80\%$ and 95% are shown in the bottom panels only (light and dark blue, respectively). Violet dot-dashed lines (top) and circles (bottom) indicate the observational constraints for U37126, with the shaded regions representing the 1$\sigma$ uncertainties, except for $EW_{0}$ (H$\alpha$), which represents the $3\sigma$ upper limit.
• Figure 4: Best-fit spectral energy distribution (SED) of U37126 derived with CIGALE (black solid curve) and a pure stellar 5 Myr-old BPASS model (CSFH with $Z/Z_{\odot}=15\%$, violet dashed line). The NIRSpec/PRISM and MIRI/LRS spectra are shown in grey and red, respectively, while NIRCam photometric measurements and MIRI/LRS synthetic photometry are indicated by blue and red circles. The inset panel displays the posterior probability density function of the Lyman-continuum escape fraction.
• Figure 5: NIRCam F150W (top) and F200W (bottom) $1^{\prime\prime} \times 1^{\prime\prime}$ cutouts of U37126. Middle and right panels show the Sérsic best-fit models obtained with PySersic and the corresponding residuals, respectively.