In the vast cosmic tapestry of the early universe, galaxies forged their existence amid the dark matter haloes that served as gravitational anchors. Stars ignite within these galaxies as baryonic matter cools radiatively and collapses into these deep potential wells. Yet, this process of star formation does not perpetuate indefinitely; it halts, or “quenches,” due to complex interactions involving gas depletion and feedback mechanisms that disrupt the collapse of gaseous matter. Now, with the advent of the James Webb Space Telescope (JWST) and its groundbreaking instrumentation, astronomers are peering back over 12 billion years to observe a remarkable example of massive galaxy quenching in action.
Utilizing the JWST’s NIRSpec integral field unit (IFU), a team of researchers has conducted spatially resolved spectroscopic observations of a rare massive, quiescent galaxy named Jekyll and its close companion, Hyde, which resides in the same cosmic neighborhood at redshift z = 3.714. To put this in perspective, the universe was less than 1.7 billion years old at this epoch—just 10% of its current age. Providing an unprecedented glimpse into galaxy evolution during the first two billion years, these observations expose complex interactions in a gas-rich environment located inside a massive dark matter halo exceeding 10¹² solar masses.
Jekyll’s counterpart, Hyde, reveals a sharp contrast. While Jekyll is quiescent and devoid of ongoing star formation, Hyde thrives as a powerhouse of dust-enshrouded stellar production, boasting a star formation rate on the order of 300 solar masses per year. This dichotomy within a single halo presents a natural laboratory to unravel the processes behind accelerated quenching and chemical enrichment in massive early universe galaxies. Both galaxies are embedded in a massive reservoir of gas, yet only Hyde is actively forming stars, raising puzzles about the mechanisms that suppress star formation in Jekyll despite its ample fuel supply.
The data reveal a vast circumgalactic medium (CGM) teeming with ionized and neutral gas that exhibits signs of vigorous kinematic disturbances. These perturbed gas dynamics suggest that the interactions are anything but quiescent and hint at energetic feedback processes at play. Photoionization consistent with intense radiation exposure or shock waves driven by energetic outflows could explain these signatures. However, intriguingly, deep observations do not detect an active galactic nucleus (AGN) in either galaxy, neither obscured nor unobscured, which challenges conventional models of quenching that often invoke AGN-driven feedback as a quenching agent.
Jekyll’s stellar population paints a compelling picture. It assembled a stellar mass on the order of 10¹¹ solar masses—akin to massive ellipticals in the nearby universe—within a remarkably short timespan of 200 to 300 million years. Furthermore, its stars show signs of chemical enrichment indicative of early and efficient star formation. Despite hosting such a matured stellar system, the galaxy has remained quiescent for over 500 million years, a silence long enough to signal a robust and sustained quenching mechanism operating soon after its formation epoch.
The implications of these observations shake up prevailing paradigms of early massive galaxy evolution. The classical “open-box” models, in which galaxies continuously accrete fresh cold gas and reignite star formation, seem inadequate here. Instead, this study supports a “closed-box” scenario where Jekyll’s gas reservoir was consumed or rendered inaccessible early on, followed by a phase of preventive feedback that stopped fresh gas inflow and prevented the cooling necessary for new star formation episodes. This feedback may arise from processes yet to be fully understood, possibly related to environmental effects from neighboring galaxies or energetics within the halo itself—without the direct presence of an AGN.
Moreover, the coexistence of Jekyll and the intensely star-forming Hyde within the same dark matter halo suggests diverse evolutionary pathways conditioned by local dynamics and feedback. Hyde’s sustained star formation amidst enriched, turbulent circumgalactic gas challenges traditional expectations. It indicates that galaxy-scale feedback mechanisms may act locally and anisotropically, quenching some galaxies while sparing others, even in close proximity. This heterogeneity highlights the complex interplay between baryonic physics and dark matter gravitational scaffolding in shaping early cosmic structures.
The JWST’s high spatial and spectral resolution via the NIRSpec IFU enables detailed dissection of the ionized and neutral gas phases, revealing velocity profiles and line diagnostics previously inaccessible. Such granularity allows astronomers to infer the physical state of the gas, including temperature, density, ionization mechanisms, and kinematics—critical parameters to understand the nature of the feedback processes involved. These observations mark a new frontier in galaxy formation studies, pushing beyond imaging to decipher the dynamical and chemical fingerprints of cosmic evolution.
From the chemical perspective, Jekyll’s stellar population reveals enrichment patterns that imply rapid recycling of metals within the interstellar medium during its formation burst. The early enrichment points toward efficient, massive star formation episodes accompanied by strong supernova feedback, rapidly elevating metallicity before quenching halted further stellar birth. This chemical maturity aligns intriguingly with its quiescence, suggesting that early feedback linked to stellar processes may have played a substantial role in arresting star formation alongside or instead of AGN activity.
Despite the absence of AGN signatures, the possibility remains that faint or highly obscured nuclei could exist below current detection thresholds. The intense ionization and shock-like conditions within the CGM hint at energetic processes potentially related to hidden AGN or other mechanisms such as galactic winds driven by stellar feedback. Future deeper observations—possibly including alternative wavelengths such as X-rays or radio—may further constrain or reveal subtle AGN contributions.
The massive dark matter halo hosting these galaxies underscores the importance of environment in early galaxy evolution. Halo mass exceeding 10¹² solar masses provides the gravitational potential to retain large gas reservoirs and foster interactions between baryonic components and dark matter. Such halos may promote mechanisms that stabilize gas against collapse or trigger feedback loops capable of sustaining quenching. Environment-driven preventive feedback appears crucial in regulating baryonic assembly and star formation cessation in massive early universe systems.
This observational milestone illustrates how the combination of JWST’s capabilities and targeted spectroscopic analyses can revolutionize our understanding of galaxy assembly and transformation. By capturing the simultaneous presence of a quenched, chemically mature galaxy alongside an intensely star-forming neighbor embedded in a turbulent and enriched medium, this study provides key insights into the complex mechanisms driving accelerated quenching during the universe’s formative epochs.
Ultimately, this research advocates for the widespread adoption of closed-box plus preventive feedback frameworks to model massive galaxy evolution during the first two billion years. It highlights the delicate balance between gas inflows, star formation, feedback, and environmental pressures that shape the lifecycle of the earliest massive galaxies. As JWST continues to unveil the hidden universe, new revelations about the emergence of cosmic structures will refine these models and deepen our understanding of the cosmos’ grand evolutionary narrative.
Subject of Research: Galaxy formation and quenching mechanisms in the early universe
Article Title: Accelerated quenching and chemical enhancement of massive galaxies in a z ≈ 4 gas-rich halo
Article References:
Pérez-González, P.G., D’Eugenio, F., Rodríguez del Pino, B. et al. Accelerated quenching and chemical enhancement of massive galaxies in a z ≈ 4 gas-rich halo. Nat Astron (2025). https://doi.org/10.1038/s41550-025-02586-8
Image Credits: AI Generated
