Extremely UV-bright starbursts at the end of cosmic reionization

TL;DR

This study shows that UV-bright galaxies during the Epoch of Reionization are dominated by very young stellar populations that produce ionizing photons efficiently, as evidenced by a composite rest-frame UV spectrum with a clear N V $1240$ P-Cygni profile and a high $\xi_{\rm ion}$ of $\log(\xi_{\rm ion}/\mathrm{Hz\,erg^{-1}})\approx25.54$. JWST observations of a representative $z=6.2038$ galaxy, J0217-0208, reveal a compact, intensely star-forming system with a high SFR ($\sim142\ M_\odot$ yr$^{-1}$) and a young age ($\sim5.5$ Myr), along with a powerful, dusty ionized outflow extending to several hundred parsecs and a substantial dust reservoir situated at larger radii. The outflows carry and expel dust, potentially boosting the observed UV luminosity and enabling significant escape of ionizing photons ($f_{esc}^{\rm LyC}$ in the range $\sim18-75\%$). Together, these results imply that UV-bright EoR galaxies undergo short-lived but extreme bursts of mass growth and feedback, sharing properties with the brightest $z>10$ systems, and highlighting a cohesive picture of early galaxy formation where young, dense starbursts, efficient ionizing photon production, and feedback-driven dust clearing are closely linked to cosmic reionization. The findings underscore the importance of large, wide-field surveys and JWST-era spectroscopy to determine how common these phases are and their role in early galaxy evolution.

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]

Figures (9)

• Figure 1: UV absolute magnitudes and redshift distribution of the SHELLQs-LAEs analyzed in this work (dark blue circles). Other UV-bright galaxies selected from SHELLQs, but without or weak Ly$\alpha$ emission, are shown with light blue circles matsuoka2016matsuoka2018amatsuoka2018bmatsuoka2019matsuoka2022. In addition, we also show other UV-bright sources selected from REBELS bouwens2022 and BoRG roberts2024_borg surveys, along with a compilation of $z_{\rm spec}>5$ sources from DAWN/DJA archive (grey).
• Figure 2: Stacked rest-UV spectrum of UV-bright SHELLQs-LAEs. Panel A shows the median (black) and mean (grey) stacked spectrum of SHELLQs-LAEs which presents strong Ly$\alpha$ emission and a well-developed N v$\lambda$1240 P-Cygni profile (violet-shaded area). The location of low-ionization ISM absorption lines (Si ii, O i, and C ii) are also marked with orange dashed lines. The BPASS stellar population synthesis model that best reproduces the observed N v profile is shown in violet and is characterized by a continuous star formation with an age of $6.3^{+9}_{-3}$ Myr, assuming $Z/Z_{\odot}=0.3$ with the chabrier2003 IMF and $E(B-V)=0$. The right panels (B1-B4) show a zoom-in to the N v P-Cygni observed in SHELLQs-LAEs (black) and a comparison with B1: BPASS synthetic models with different ages ($1-100$ Myr, color-coded); B2: UV-bright galaxies at $z=2-4$Upadhyaya2024; B3: low- and high-ionization BALQSOs Brotherton2001; and B4: type II AGNs hainline2011b.
• Figure 3: Summary of the JWST observations of J0217-0208 at $z=6.20$. The top panels show JWST/NIRCam images centered on J0217-0208 at increasing wavelengths (from F115W to F444W, left to right, each with a $2^{\prime\prime}\times2^{\prime\prime}$ size). The bottom panels present JWST/NIRSpec spectroscopy: the left panel shows the integrated H$\beta$+[O iii] $\lambda\lambda$4960,5008 emission from J0217-0208 obtained with NIRSpec/IFS, while the right panel shows the 2D (top) and 1D (bottom) spectra of J0217-0208 obtained with NIRSpec/Fixed-slit. The locations of relevant emission lines are marked with dashed lines.
• Figure 4: NIRSpec/FS spectrum of J0217-0208 (black) around H$\beta$ + [O iii] $\lambda\lambda$4960,5008 (left) and H$\alpha$ + [N ii] $\lambda\lambda$6549,6585 (right) emission lines. Top panels show the best-fit model assuming one Gaussian component for each emission line (orange). The bottom panel shows the best-fit model assuming two Gaussian components. The total emission is shown as a blue dashed line, while the individual components are shown as dotted lines (green and red for the narrow and broad components). Note that the vertical axes are in logarithmic scales. The gray area represents the $\pm 1 \sigma$ uncertainty spectrum. The bottom panels show the residuals obtained for each best-fit model.
• Figure 5: Chemical abundance ratio N/O as a function of O/H. J0217-0208 is shown as a blue circle, while other $z\gtrsim 5$ nitrogen emitters are marked by violet squares (compiled by Schaerer2024; see also Ji2025 for a more extensive sample). For comparison, low-redshift star-forming galaxies and H ii regions from Izotov2023 are plotted in grey. The dashed line shows the average trend in low-$z$ galaxies, as parameterized by VCE1993.