A recently quenched galaxy 700 million years after the Big Bang

The evolution of galaxies, particularly their star formation activity, is a central theme in astrophysics. At low redshifts (z < 3), galaxies exhibit a bimodal distribution: actively star-forming galaxies and passive systems. However, theoretical models predict a more stochastic star formation history at early cosmic times (high redshift) and in low-mass systems, with rapid transitions between starburst phases and periods of suppressed star formation, potentially leading to temporary quiescence – mini-quenching events. Directly observing mini-quenched galaxies in the early universe is crucial for constraining models of galaxy formation and evolution. While previous studies have identified early quenched galaxies at z < 5, these are massive and relatively old. This research aims to directly address the gap in our understanding by identifying and characterizing a mini-quenched galaxy at a much higher redshift, offering invaluable insight into the processes governing early galaxy evolution and potentially shedding light on the role of feedback mechanisms in shaping the properties of early galaxies. This study leverages the unparalleled capabilities of the James Webb Space Telescope (JWST) to perform unprecedentedly deep observations, enabling the detection and spectroscopic characterization of faint and distant objects.

Previous studies have identified early quenched galaxies, mainly at redshifts z < 5. These galaxies were massive (M* > 10¹⁰ M⊙) and relatively old, suggesting a different evolutionary pathway compared to the mini-quenching events predicted by theory. Studies have explored Balmer-break galaxies in the epoch of reionization, indicating the existence of evolved stellar populations and quiescent phases. However, without spectroscopic data, it was challenging to rule out low-equivalent-width emission lines or the presence of strong emission lines masking as Balmer breaks. Ground-based observations were limited by atmospheric transmission, preventing the investigation of Balmer breaks at z > 5. JWST's capabilities enabled direct confirmation of the absence of ongoing star formation, greatly advancing our ability to study the early universe.

The study focuses on JADES-GS-z7-01-QU, a galaxy initially identified as a Lyman-break galaxy. Deep (28-hour) JWST/NIRSpec-MSA observations with a prism were conducted as part of the JADES survey. The galaxy's redshift (z = 7.29 ± 0.01) was determined using the BEAGLE code, based on the Lyα drop and Balmer break. Strict upper limits on the Hβ and [O III] emission-line fluxes were obtained, implying a very low star formation rate (<0.65 M⊙ yr⁻¹). The UV slope (β = −2.09 ± 0.09) indicated strong star formation activity in the preceding 100 Myr. A clear Balmer break and Hβ absorption were observed, fulfilling the spectroscopic definition of a post-starburst galaxy. To estimate physical properties like stellar mass, star formation rate, star formation history (SFH), dust attenuation, and stellar metallicity, four different SED-fitting codes (pPXF, BAGPIPES, Prospector, and BEAGLE) were employed. The analysis examined alternative explanations, such as high Lyman-continuum escape fractions and obscured star formation, finding them less likely given the observed data. The codes produced consistent results regarding the low stellar mass (4–6 × 10⁹ M⊙), low star formation rate, and recent quenching.

The primary finding is the discovery and spectroscopic confirmation of a mini-quenched galaxy (JADES-GS-z7-01-QU) at redshift z = 7.3. This galaxy, with a stellar mass of 4–6 × 10⁹ M⊙, exhibits a Balmer break and lacks detectable nebular emission lines, indicating a rapid transition from starburst activity to quiescence. The four independent SED-fitting codes used in the analysis consistently showed that the galaxy experienced a short (20–100 Myr) starburst followed by rapid quenching within the last 20-50 Myr. The star formation rate at the time of observation is extremely low, between two and three orders of magnitude below the star-forming main sequence at this redshift, confirming the quenched state. The average stellar metallicity is tentatively low (log₁₀(Z/Z⊙) ≈ −2), except for a small, more enriched population detected by one code. Analysis of the possibility of a high Lyman-continuum escape fraction or heavily obscured star formation deemed these scenarios less probable than a recently quenched galaxy. The galaxy's mass rules out UV background quenching. A lack of nearby massive galaxies suggests that environmental effects were not the primary quenching mechanism.

The findings strongly support the hypothesis of mini-quenching in the early universe. The rapid transition to quiescence and the galaxy's mass suggest powerful outflow events driven by either star formation feedback (potentially radiation pressure) or accretion onto a primeval supermassive black hole. These outflows might have ejected most of the star-forming gas, leading to temporary quenching until new material replenishes the gas supply. This scenario aligns with cosmological simulations predicting periodic starbursts and quiescence in early galaxies. The galaxy's properties lie in a region of theoretical uncertainty where simulations struggle to reproduce the observed complete quenching. The lack of inclusion of AGN feedback in existing models further highlights the importance of this discovery. JADES-GS-z7-01-QU provides a critical benchmark for refining models of galaxy evolution and feedback mechanisms.

The discovery and spectroscopic analysis of JADES-GS-z7-01-QU, a mini-quenched galaxy at z = 7.3, marks a significant advance in understanding early galaxy evolution. This research demonstrates the power of JWST in directly observing and characterizing such elusive objects in the primordial Universe. Future JWST observations will be crucial in statistically characterizing the properties of these early mini-quenched galaxies and further refining theoretical models.

The low resolution of the prism spectroscopy introduces uncertainties in the stellar metallicity measurements. While alternative scenarios like high Lyman-continuum escape fractions and obscured star formation are considered and deemed less likely, they cannot be entirely ruled out. The analysis relies on several SED-fitting codes with different priors, but the consistent findings across these models strengthen the conclusions.