Quasar Impostors: Two Extremely UV-Bright ($M_{\rm UV}\approx-23.5$) Reionisation-Epoch Galaxies Powered by Very Massive Stars

Daming Yang, Joseph F. Hennawi, Sarah E. I. Bosman, Frederick B. Davies, Rychard Bouwens, Timo Kist, Eduardo Bañados, Jiamu Huang, Alice E. Shapley, Marianne Vestergaard, Hiddo S. B. Algera, Silvia Belladitta, Anna-Christina Eilers, Xiaohui Fan, Francesco Guarneri, Xiangyu Jin, Romain A. Meyer, Elia Pizzati, Huub Röttgering, Jan-Torge Schindler, Mauro Stefanon, Feige Wang, Jinyi Yang

First listed 2026-08-20 | Last updated 2026-08-18

Abstract

The extreme bright end of the galaxy UV luminosity function during reionisation remains poorly constrained, particularly where the galaxy and quasar luminosity functions overlap and source classification becomes ambiguous. We present JWST/NIRSpec and ALMA Band-6 observations of J1450-0144 ($z=6.627$) and J1429-0104 ($z=6.796$), two $M_{\rm UV}\simeq-23.5$ sources originally classified as faint quasars by SHELLQs. NIRSpec reveals blue UV continua, strong P Cygni profiles in N V, Si IV, and C IV, broad He II $\lambda1640$ emission with rest-frame equivalent widths of $8.8\pm1.2$ and $3.7\pm1.1$ Å, respectively, and narrow nebular lines, reclassifying both as extremely UV-luminous galaxies. Standard population-synthesis models cannot simultaneously reproduce the strong He II and wind features, whereas models incorporating very massive stars (VMS; $M\gtrsim100\,M_\odot$) with dedicated wind prescriptions can. These models favor a star-formation duration of 2-4 Myr for J1450-0144, with a broader allowed range for J1429-0104, stellar masses of $\log(M_\star/M_\odot)\approx9.2$-$9.9$, and star-formation rates of $\simeq300$-$540\,M_\odot\,{\rm yr}^{-1}$. Under the same VMS wind models, equivalent-width diagnostics imply $M_{\rm up}\gtrsim225\,M_\odot$ for J1429-0104, while J1450-0144 lies beyond even the $M_{\rm up}=475\,M_\odot$ grid. ALMA detects luminous [C II] 158 $μ$m emission in both systems, with $L_{\rm [CII]}\approx0.8$ and $4.1\times10^{9}\,L_\odot$, respectively. J1429-0104 additionally shows bright dust continuum, with both [C II] and dust offset by $\sim5.4$ kpc from its UV emission. These sources demonstrate that VMS can power some of the most UV-luminous galaxies at cosmic dawn and show that source classifications, and hence the inferred demographics of both galaxies and quasars in the crossover regime, need revisiting.

Short digest

Yang et al. use JWST/NIRSpec and ALMA Band-6 observations to reclassify the SHELLQs faint-quasar candidates J1450-0144 at z=6.627 and J1429-0104 at z=6.796 as extraordinarily UV-luminous star-forming galaxies with MUV≈−23.5. Their blue continua, N V/Si IV/C IV P Cygni profiles, broad He II λ1640, and narrow nebular lines resemble cosmic-noon extreme UV-luminous galaxies and require population models with very massive stars and dedicated wind prescriptions rather than standard synthesis models. The VMS interpretation implies intense, young star formation, while the wind equivalent-width diagnostics demand upper IMF masses above 225 M⊙ for J1429-0104 and place J1450-0144 beyond the paper’s 475 M⊙ model grid. Luminous [C II] in both systems, plus a 5.4-kpc UV-to-dust/[C II] offset in J1429-0104, reinforce the view that VMS-powered galaxies can contaminate the galaxy–quasar crossover regime and alter inferred bright-end galaxy and faint-quasar demographics.

Key figures to inspect

• Figure 2. The central observational reclassification figure: the full NIRSpec spectra show blue UV slopes, stellar-wind P Cygni profiles, broad He II, narrow nebular lines, and the absence of broad Lyα or Mg II expected from a type-I quasar.
• Figure 5. This fitting map establishes the population-level inference from the wind diagnostics, showing that J1450-0144 prefers a very young, high-upper-mass IMF VMS solution while J1429-0104 retains a broader age–Mup degeneracy.
• Figure 9. The multi-wavelength imaging provides the key spatial caveat and ISM context: J1429-0104 has a clear offset between its UV source and the [C II]/dust-continuum peak, unlike the approximately co-spatial J1450-0144.
• Figure 13. The He II–N V and He II–C IV equivalent-width diagrams give the paper’s most compact VMS diagnostic, placing both galaxies among very young VMS-rich populations and quantifying the exceptionally high inferred IMF upper-mass limits.
• Figure 10. This synthesis figure connects the two reclassifications to the broader luminosity-function problem, showing how an EUVLG contribution at MUV≈−23.5 affects the overlap between the reionisation-era galaxy UV luminosity function and the quasar luminosity function.