A recently quenched galaxy 700 million years after the Big Bang (2302.14155v3)

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.

• The paper identifies a quenched galaxy at redshift 7.3, demonstrating a rapid halt of star formation after a brief starburst phase.
• It employs multiple spectroscopic models (ppxf, bagpipes, BEAGLE, and Prospector) to detail the galaxy's star-formation history and physical conditions.
• The analysis challenges conventional galaxy evolution models by highlighting early and rapid quenching processes in low-mass systems during the cosmic dawn.

Overview of the Recent Observation of a Quenched Galaxy at High Redshift

This paper presents an analysis of a recently identified quenched galaxy located at a redshift of 7.3, when the universe was merely 700 million years old. Using data from the James Webb Space Telescope's NIRSpec instrument, the paper offers a comprehensive spectroscopic view that reveals unique characteristics indicative of galaxies during the cosmic dawn. The primary focus is on a galaxy labeled JADES-GS-z7-01-QU, discovered through deep spectral analysis facilitated by the JADES program.

Key Observations and Findings

1. Spectroscopic Analysis:
   • The galaxy exhibits a significantly blue spectral continuum, with a $U-V$ color index of 0.16 ± 0.03 mag.
   • A pronounced Balmer break is detected, without accompanying strong nebular emission lines, indicating a cessation of star-formation activities.
2. Star Formation and Quenching:
   • The galaxy demonstrates a short-lived starburst phase followed by a rapid quenching period. The absence of nebular lines supports an upper limit on the current star formation rate (SFR), suggesting ongoing star formation is currently negligible.
   • The stellar mass of this galaxy is estimated to be about 4-6 × 10^8 M, placing it in the dwarf-galaxy regime, albeit at an early cosmic epoch.
3. Methodological Approach:
   • Multiple spectroscopic models, including ppxf, bagpipes, BEAGLE, and Prospector, were employed to infer the galaxy's star-formation history (SFH), metallicity, and dust content. All methods indicated the galaxy had quenched star formation just a few tens of millions of years prior to observation.
4. Alternative Interpretations:
   • Although a high escape fraction of ionizing photons could potentially mimic lack of emission, the analysis disfavors this due to normal UV slope and presence of strong Balmer absorption, emphasizing the quenched nature.
5. Galaxy Evolution Context:
   • The rapid transition from a star-forming to a quenched state in such a low-mass galaxy provides critical insight into early galaxy evolution mechanisms, contrasting with the typically more massive, older quenched systems observed at high redshifts.

Implications and Future Directions

The discovery and analysis of JADES-GS-z7-01-QU underscore the potential for detailed high-redshift galaxy studies to refine our understanding of galaxy formation and transformation processes. In particular, this galaxy's attributes challenge traditional models of galaxy growth, suggesting the necessity to consider intermittent or mini-quenching events, potentially driven by feedback processes or environmental factors not yet fully understood in the cosmological context.

Additionally, these results from JWST exemplify the critical role of spectroscopic surveys in confirming galaxy quenching and determining precise stellar ages at such early epochs. This capability enhances constraints on theoretical models of feedback and star formation regulation in the primordial universe. Future observations with JWST are anticipated to build on these findings, providing larger statistical samples to elucidate the frequency and nature of such rapid quenching events across the early universe.

In summary, the paper of JADES-GS-z7-01-QU marks a significant step toward understanding the diverse pathways of galaxy evolution during the critical first billion years after the Big Bang, emphasizing the evolving complexity of star formation and feedback processes in shaping the observable universe.