A recently quenched galaxy 700 million years after the Big Bang

TL;DR

The study identifies a mini-quenched galaxy at $z=7.3$ (age ~700 Myr after the Big Bang) with stellar mass about a few times $10^{8}\,M_\odot$, exhibiting a strong Balmer break and no nebular emission lines. Using JWST/NIRSpec spectroscopy plus JWST/NIRCam imaging, the authors perform joint spectro-photometric modelling with four independent codes to reconstruct a bursty star-formation history: a short initial star-formation episode lasting ~20–100 Myr, followed by rapid quenching within ~10–50 Myr, yielding a current SFR effectively near zero. All codes converge on a quenched state at the epoch of observation, with a formation redshift around $z\sim7.6$ and a low metallicity, suggesting explosive feedback (stellar winds or early AGN outflows) as a likely quenching mechanism in this dwarf galaxy. This object demonstrates that mini-quenching in the early Universe occurred in low-mass systems, providing pivotal constraints for theories of feedback, gas removal, and reionization during cosmic dawn.

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. In contrast, theory predicts that star formation was stochastic at early cosmic times and in low-mass systems: these galaxies transitioned rapidly between starburst episodes and phases of suppressed star formation, potentially even causing temporary quiescence -- so-called mini-quenching events. 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. Early quenched galaxies have been identified out to redshift $z<5$, and these are all found to be massive ($M_{*}>10^{10}~M_{\odot}$) 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$$=$0.16$\pm0.03$~mag), but exhibits a Balmer break and no nebular emission lines. The galaxy experienced a short starburst followed by rapid quenching; its stellar mass (4-6$\times 10^8~M_\odot$) falls in a range that is sensitive to various feedback mechanisms, which can result in perhaps only temporary quenching.

Figures (7)

• Figure 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 Ly 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 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 $3.33 \pm 0.08 \times 10^{-17}~{\rm erg} \ {\rm cm}^{-2} \ {\rm s}^{-1}$, fully consistent with the spectrum.
• Figure 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 in the 2D grating spectrum. The spectrum is median-smoothed, for visualisation.
• Figure 2: The galaxy's star-formation history as inferred by four different full spectral fitting codes, using different -- effective -- priors. All four codes confirm that the galaxy is quenched at the epoch of observation and reconstruct comparable SFHs. Left: The stellar age-metallicity grid inferred by ppxf. The code reconstructs dominant metal-poor populations forming from $\sim$150 to $\sim$50 Myr prior to observation. The colour-bar represents the fractional mass distribution over the SSP grid. Right: The star formation histories inferred by bagpipes, beagle and prospector. bagpipes infers that the galaxy formed $\sim$40 Myr before the epoch of observation and quenched $\sim$20 Myr before observation. beagle infers that the galaxy quenched $\sim$20 Myr before observation after a star-burst lasting $\sim$70 Myr. prospector infers that the galaxy quenched $\sim$40 Myr before observation after a star-burst lasting $\sim$80 Myr.
• Figure 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 $\approx\!0.8$--5 µ ms. The five NIRSpec microshutter positions used for this target are overlaid in white.
• Figure 3: Summary of key outputs by bagpipes. Bottom left: corner plot. Top right: spectro-photometric bagpipes fit of the JADES-GS-z7-01-QU R100/prism spectrum.