Extremely ultraviolet-bright starbursts at the end of cosmic reionization

We present a study of 27 very UV-bright (−22.0 ≲ M UV ≲ −24.4) star-forming galaxies at z ∼ 6 identified in the Subaru High- z Exploration of Low-Luminosity Quasars (SHELLQs) survey. Stacking their rest-frame UV spectra revealed a prominent N V λ 1240 P Cygni feature, consistent with very young (∼6 Myr) stellar populations dominated by massive and hot stars. Ultraviolet-bright galaxies in the reionization epoch are thus powerful and efficient ionizing sources, with an average ionizing photon production efficiency of log( ξ ion /Hz erg −1 ) = 25.54 +0.09 −0.12 . For one representative source, J0217–0208 at z = 6.204 ( M UV = −23.4), we analyzed available JWST/NIRCam and NIRSpec observations. Its spectral energy distribution indicates a young (∼5 Myr) starburst with a stellar mass of 10 9 M ⊙ and a high specific star formation rate (∼100 Gyr −1 ). Together with its very compact NIRCam-measured size ( r eff ≃ 260 pc), this corresponds to stellar mass and star formation rate surface densities approximately one hundred times higher than those of typical galaxies at comparable redshifts. NIRSpec spectroscopy further revealed strong nebular emission, for which we derived a high electron density ( n e ≃ 10 3 cm −3 ), a metallicity of 12 + log(O/H) = 8.20 ± 0.11 (from the direct method), and a super-solar N/O ratio (log(N/O) ≃ −0.30). Furthermore, J0217–0208 shows broad components in several rest-optical emission lines, indicating powerful ionized outflows. From the Balmer decrement, these outflows appear to be heavily obscured ( E ( B − V ) out ≃ 0.6), in contrast to the nearly dust-free stellar continuum ( E ( B − V ) ★ = 0.01 ± 0.01) obtained from its steep UV slope, β UV ≃ −2.6. Combined with ALMA detections of a massive ( M dust ≃ 2 × 10 8 M ⊙ ), extended (∼1.4 kpc), and cold ( T dust ≃ 25 K) dust reservoir, these findings point to dusty feedback-driven outflows carrying and pushing dust well beyond the stellar core and likely boosting the observed UV luminosity. Taken together, our results suggest that UV-bright galaxies at high redshift represent short-lived but extreme phases of rapid stellar mass growth, efficient ionizing photon production, and strong feedback. The extreme properties of J0217–0208, such as a supersolar N/O, a steep UV slope, a compact size, and very high surface densities, closely mirror those of the brightest galaxies at z > 10, suggesting a shared evolutionary pathway.