Extremely UV-bright starbursts at the end of cosmic reionization
Abstract
We present a study of 27 very UV-bright (-22.0<MUV<-24.4) sources at z~6 identified in SHELLQs survey. Stacking their rest-frame UV spectra reveals a prominent NV1240A P-Cygni, consistent with very young (~6Myr) stellar populations dominated by massive stars. They are thus powerful and efficient ionizing starbursts with an average ionizing photon production efficiency of log(Xi_ion/Hz erg-1)=25.54. For one of these, J0217-0208 at z=6.2 (MUV=-23.4), we analyzed NIRCam and NIRSpec data. Its SED indicates a young (~5Myr), compact (reff~260pc) starburst with a stellar mass of 10^9Msun and high sSFR (~100Gyr^-1). This translates to high stellar mass and SFR surface densities, about 100x higher than typical galaxies at similar redshifts. NIRSpec further reveals strong nebular emission, for which we derive a high electron density (~10^3 cm^-3), a metallicity 12+log(O/H)=8.20 (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 lines, indicating powerful ionized outflows, which appear heavily obscured (E(B-V)~0.6), in contrast to the nearly dust-free stellar continuum (E(B-V)~0.01). Combined with ALMA detections of a massive, extended, and cold dust reservoir, these findings point to dusty, feedback-driven outflows carrying and pushing dust well beyond the stellar core, boosting the observed UV luminosity. 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 supersolar N/O, steep UV slope, compact size, and very high surface densities, closely mirror those of the brightest galaxies at z>10, suggesting a shared evolutionary pathway.[Abridged]
Cite
@article{arxiv.2510.12411,
title = {Extremely UV-bright starbursts at the end of cosmic reionization},
author = {R. Marques-Chaves and D. Schaerer and M. Dessauges-Zavadsky and J. Álvarez-Márquez and T. Hashimoto and L. Colina and A. K. Inoue and C. Blanco-Prieto and Y. Nakazato and L. Costantin and S. Arribas and T. J. L. C. Bakx and D. Ceverino and A. Crespo Gómez and Y. Fudamoto and M. Hagimoto and A. Hamada and Y. Matsuoka and K. Mawatari and M. Onoue and W. Osone and Y. W. Ren and Y. Sugahara and Y. Terui and N. Yoshida},
journal= {arXiv preprint arXiv:2510.12411},
year = {2026}
}
Comments
Accepted for publication in Astronomy & Astrophysics