Extremely ultraviolet-bright starbursts at the end of cosmic reionization
Journal article, 2026
We present a study of 27 very UV-bright (-22.0 less than or similar to M-UV less than or similar to -24.4) star-forming galaxies at z similar to 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 lambda 1240 P Cygni feature, consistent with very young (similar to 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(xi(ion)/Hz erg(-1)) = 25.54(-0.12)(+0.09). 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 (similar to 5 Myr) starburst with a stellar mass of 10(9) M-circle dot and a high specific star formation rate (similar to 100 Gyr(-1)). Together with its very compact NIRCam-measured size (r(eff) similar or equal to 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) similar or equal to 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) similar or equal to -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) similar or equal to 0.6), in contrast to the nearly dust-free stellar continuum (E(B - V)(star) = 0.01 +/- 0.01) obtained from its steep UV slope, beta(UV) similar or equal to -2.6. Combined with ALMA detections of a massive (M-dust similar or equal to 2 x 10(8) M-circle dot), extended (similar to 1.4 kpc), and cold (T-dust similar or equal to 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.
dark ages
reionization
galaxies: high-redshift
galaxies: starburst
first stars
Author
R. Marques-Chaves
University of Geneva
D. Schaerer
Centre national de la recherche scientifique (CNRS)
University of Geneva
M. Dessauges-Zavadsky
University of Geneva
J. Alvarez-Marquez
Spanish National Research Council (CSIC)
T. Hashimoto
University of Tsukuba
L. Colina
Spanish National Research Council (CSIC)
A. K. Inoue
Waseda University
C. Blanco-Prieto
Spanish National Research Council (CSIC)
Y. Nakazato
Flatiron Institute
University of Tokyo
L. Costantin
Spanish National Research Council (CSIC)
S. Arribas
Spanish National Research Council (CSIC)
Tom Bakx
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
D. Ceverino
Universidad Autonoma de Madrid (UAM)
A. Crespo Gomez
Spanish National Research Council (CSIC)
Space Telescope Science Institute (STScI)
Y. Fudamoto
Chiba University
M. Hagimoto
Nagoya University
A. Hamada
University of Tsukuba
Y. Matsuoka
Ehime University
K. Mawatari
Waseda University
M. Onoue
University of Tokyo
Waseda University
W. Osone
University of Tsukuba
Y. W. Ren
Waseda University
Y. Sugahara
Waseda University
Y. Terui
University of Tsukuba
N. Yoshida
University of Tokyo
Flatiron Institute
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 711 A112Subject Categories (SSIF 2025)
Astronomy, Astrophysics, and Cosmology
DOI
10.1051/0004-6361/202557740