A recently quenched galaxy 700 million years after the Big Bang

Abstract

Local and low-redshift (z < 3) galaxies are known to broadly follow a bimodal distribution: actively star-forming galaxies with relatively stable star-formation rates and passive systems. These two populations are connected by galaxies in relatively slow transition. By contrast, theory predicts that star formation was stochastic at early cosmic times and in low-mass systems^1-4. These galaxies transitioned rapidly between starburst episodes and phases of suppressed star formation, potentially even causing temporary quiescence-so-called mini-quenching events^5,6. However, the regime of star-formation burstiness is observationally highly unconstrained. Directly observing mini-quenched galaxies in the primordial Universe is therefore of utmost importance to constrain models of galaxy formation and transformation^7,8. Early quenched galaxies have been identified out to redshift z < 5 (refs. ^9-12) and these are all found to be massive (M_⋆ > 10¹⁰ M_⊙) and relatively old. Here we report a (mini-)quenched galaxy at z = 7.3, when the Universe was only 700 Myr old. The JWST/NIRSpec spectrum is very blue (U-V = 0.16 ± 0.03 mag) but exhibits a Balmer break and no nebular emission lines. The galaxy experienced a short starburst followed by rapid quenching; its stellar mass (4-6 × 10⁸ M_⊙) falls in a range that is sensitive to various feedback mechanisms, which can result in perhaps only temporary quenching.

Fig. 1. NIRSpec R100/prism spectrum of JADES-GS-z7-01-QU.

The absence of emission lines, together with the Balmer break, reveals that this is a—temporarily or permanently—(mini-)quenched, post-starburst galaxy. The clearly detected Lyα drop and the Balmer break unambiguously give a redshift of z = 7.3. The vertical dashed lines indicate the rest-frame wavelengths of the strongest nebular emission lines. The red line indicates the pPXF spectral fit. The upper panel shows the signal-to-noise ratio (S/N) in the 2D prism spectrum. The bottom panel shows the ratio between the residuals of the fit and the noise. For reference, the flux in the F200W NIRCam filter is 3.33 ± 0.08 × 10⁻¹⁷ erg cm⁻² s⁻¹, fully consistent with the spectrum.

Fig. 2. The SFH of the galaxy as inferred by four different full spectral fitting codes, which use different (effective) priors on the SFH of the galaxy.

All four codes confirm that the galaxy is quenched at the epoch of observation and reconstruct comparable SFHs. a, The stellar age–metallicity grid resulting from the pPXF fit. The code reconstructs dominant metal-poor populations forming from approximately 100 Myr to approximately 20 Myr before observation. The colour bar represents the fractional mass distribution over the SSP grid. b, The SFH inferred by BAGPIPES. The solid green line shows the median posterior, the shaded region shows the 1σ range, indicating a single star-formation burst lasting approximately 20 Myr and quenching approximately 20 Myr before observation. c, The SFH inferred by BEAGLE, which suggests that the galaxy formed approximately 100 Myr before the epoch of observation and quenched approximately 10–20 Myr before observation. d, The SFH inferred by Prospector, which suggests that the galaxy quenched approximately 20–30 Myr before observation after a starburst lasting approximately 50 Myr.

Extended Data Fig. 1. NIRSpec R1000/grating spectrum of the (mini-)quenched galaxy JADES-GS-z7-01-QU at z = 7.3.

The spectrum confirms the absence of emission lines. The blue-shaded region shows the 1D noise level. The upper panel shows the signal-to-noise ratio (S/N) in the 2D grating spectrum. The spectrum is median-smoothed, for visualization.

Extended Data Fig. 2. JWST/NIRCam image covering JADES-GS-z7-01-QU and its nearby projected environment.

The NIRCam F444W-F200W-F090W rgb colour image is created from cutouts of the mosaics at wavelengths λ ≈ 0.8–5 μm. The five NIRSpec microshutter positions used for this target are overlaid in white.

Extended Data Fig. 3. Summary of key outputs by BAGPIPES.

Bottom left, corner plot. Top right, spectrophotometric BAGPIPES fit of the JADES-GS-z7-01-QU R100/prism spectrum.

Extended Data Fig. 4. Summary of key outputs by BEAGLE.

Bottom left, corner plot with free f_esc. Top right, BEAGLE maximum a posteriori model of the R100 spectrum.

Extended Data Fig. 5. Summary of key outputs by Prospector.

Bottom left, corner plot with stellar mass M_⋆, SFR, t_form, t_quench, dust attenuation A_V, A_UV/A_V and stellar metallicity Z. Top right, Prospector maximum a posteriori model of the R100 spectrum.