Extremely UV-bright starbursts at the end of cosmic reionization

R. Marques-Chaves, D. Schaerer, M. Dessauges-Zavadsky, J. Álvarez-Márquez, T. Hashimoto, L. Colina, A. K. Inoue, C. Blanco-Prieto, Y. Nakazato, L. Costantin, S. Arribas, T. J. L. C. Bakx, D. Ceverino, A. Crespo Gómez, Y. Fudamoto, M. Hagimoto, A. Hamada, Y. Matsuoka, K. Mawatari, M. Onoue, W. Osone, Y. W. Ren, Y. Sugahara, Y. Terui, N. Yoshida

First listed 2025-10-14 | Last updated 2025-10-14

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]

Short digest

A sample of 27 very UV-bright (-22<MUV<-24.4) z~6 SHELLQs sources shows a stacked rest-UV spectrum with a pronounced N V 1240 P-Cygni, implying ~6 Myr, massive-star–dominated populations and an efficient ionizing output (log xi_ion=25.54). A deep look at J0217-0208 (z=6.2, MUV=-23.4) with NIRCam+NIRSpec reveals a compact (reff~260 pc), ~5 Myr starburst with M*~10^9 Msun, sSFR~100 Gyr^-1, ne~10^3 cm^-3, 12+log(O/H)=8.20 (direct), and super-solar N/O (log N/O=-0.30). Broad components in rest-optical lines and a high Balmer-reddened outflow (E(B-V)~0.6) contrasted with a nearly dust-free stellar continuum (E(B-V)~0.01), together with ALMA detection of a massive, extended, cold dust reservoir, point to dusty, feedback-driven outflows that can amplify the UV luminosity. UV-bright galaxies at these epochs thus trace short-lived, extreme phases of rapid stellar mass build-up and copious ionizing production, with J0217-0208 mirroring the brightest z>10 systems and hinting at a shared evolutionary pathway.

Key figures to inspect

• Figure 2: Inspect the N V 1240 P-Cygni profile in the stacked UV spectrum and its BPASS fits to see the ~6 Myr stellar-wind origin and how it contrasts with BALQSO/type II AGN templates.
• Figure 3: Use the NIRCam cutouts to appreciate the compact morphology of J0217-0208 and the NIRSpec Hβ+[O III] map/spectrum to locate the strongest nebular zones and line IDs.
• Figure 4: Compare one- vs two-component fits around Hβ+[O III] and Hα+[N II]; quantify the broad-component widths/fluxes and derive the strong Balmer-reddening of the outflow relative to the narrow/stellar components.
• Figure 1: Place the SHELLQs-LAEs in MUV–z space and contrast with REBELS/BoRG/DAWN sources to gauge how extreme the UV luminosities of this sample are at z~6.