JWST Spots Metal-Rich Outflows Racing From Galaxies Just 600 Million Years After the Big Bang

Astronomers using the James Webb Space Telescope have caught the earliest direct glimpse yet of the cosmic recycling system that shapes how galaxies grow. In three luminous galaxies dating to between 500 and 750 million years after the Big Bang, a team led by Yongda Zhu and Zhiyuan Ji of the University of Arizona has detected metal-bearing gas streaming away from the galaxies at hundreds of kilometres per second, in multiple ionization states at once. The finding, published in Nature Astronomy, shows that the essential machinery of the baryon cycle, the circulation of gas, metals and energy between galaxies and their surroundings, was already running at full tilt before the universe had even reached the halfway point of reionization, the era when starlight first stripped electrons from the primordial hydrogen fog.

The baryon cycle is the engine of galactic evolution. Stars forge heavy elements such as carbon, oxygen and silicon, and stellar feedback in the form of supernova explosions and intense radiation drives those metals out of galaxies into the circumgalactic medium, from which the gas can eventually cool and fall back to fuel new generations of stars. Models have long predicted that this redistribution should begin almost as soon as star formation does, but direct observational constraints at the earliest cosmic epochs have been extraordinarily hard to obtain. Previous measurements of metal enrichment during reionization relied mostly on absorption lines imprinted on the light of distant quasars, which probe random sightlines through the intergalactic medium and are only indirectly connected to individual galaxies.

The new work takes a fundamentally different approach: measuring absorption directly against the ultraviolet glow of the galaxies themselves. The team exploited ultra-deep, medium-resolution near-infrared spectroscopy from JWST’s NIRSpec instrument, obtained as part of the Cycle 4 programme SPURS, the SPectroscopic Ultra-deep Reionization-era Survey, in the Abell 2744 field. Only three galaxies in the survey met the demanding selection criterion: their rest-frame ultraviolet continuum near 1,450 angstroms had to be detected at a signal-to-noise ratio above 10, allowing faint absorption lines to be measured against the stellar light. Crucially, the galaxies were chosen purely on continuum brightness, with no prior knowledge of whether metal absorption would be present.

The three targets span a remarkable range of cosmic time. Galaxy A sits at a systemic redshift of 9.31, corresponding to roughly 530 million years after the Big Bang, while Galaxies B and C lie at redshifts 7.88 and 7.29, near the midpoint of reionization at around 650 to 750 million years. In all three systems, the team identified a suite of absorption transitions spanning more than two orders of magnitude in ionization potential: neutral oxygen (O I), low-ionization silicon and carbon (Si II and C II), and highly ionized species (Si IV and C IV). The absorption features appear strikingly similar to those seen in the well-studied spectra of star-forming galaxies at redshift around 3, when the universe was more than three times older.

The kinematics tell the most compelling story. By anchoring each galaxy’s rest frame to the centroid of the strong nebular emission line [O III] 5008, the researchers found that the metal absorption is consistently blueshifted by roughly 50 to 250 kilometres per second relative to the systemic velocity. Because Hubble expansion cannot produce a velocity offset relative to a galaxy’s own rest frame, these blueshifts indicate gas that is physically moving, most plausibly in outflows driven by stellar feedback, although turbulent interstellar motions or more complex internal kinematics may also contribute. The neutral, low-ionization and high-ionization lines all share broadly overlapping velocity structure, typically extending to about minus 200 kilometres per second, pointing to a common reservoir of disturbed, metal-enriched gas.

The coexistence of ions spanning such a wide range of ionization energies within the same blueshifted complex is difficult to reconcile with a single quiescent component of cold interstellar gas. Instead, it points to a multiphase structure, the signature expected when feedback-driven winds shred and accelerate material across cold, warm and hot phases. The data even hint at ionization-dependent stratification: the high-ionization absorption appears more blueshifted than the neutral and low-ionization lines by roughly 60 to 210 kilometres per second depending on the system, a pattern consistent with simulations such as FIRE-2, in which more highly ionized gas traces a faster or more extended component of a multiphase outflow. Higher-resolution spectra would be needed to confirm this ordering in detail.

Independent lines of evidence reinforce the picture. Interpreting the measured line widths as purely thermal broadening yields upper limits on the gas temperature of order ten million kelvin, far above the temperatures at which the detected ions can survive, meaning non-thermal motions such as turbulence and unresolved velocity substructure must dominate the profiles. Equivalent-width ratios between high- and low-ionization species, and between neutral and low-ionization species, place the three galaxies in the same parameter space occupied by lower-redshift star-forming galaxies, with Galaxy A, the most distant, showing somewhat stronger high-ionization absorption relative to its low-ionization lines. The spectra show no broad emission lines or other definitive signatures of an active galactic nucleus, so the feedback is most naturally attributed to intense star formation.

The host galaxies themselves are extreme objects. Spectral energy distribution modelling of Hubble and JWST photometry yields stellar masses between about one and six billion solar masses, strikingly blue ultraviolet slopes between minus 2.12 and minus 2.26, and gas-phase metallicities of roughly one-third to one-tenth of the solar value, subsolar but far from pristine. The youngest stellar populations are forming stars at rates of 19 to 39 solar masses per year. That chemically enriched, outflowing gas could coexist with such blue, dust-poor ultraviolet continua connects these systems to the so-called blue monster galaxies recently identified at even higher redshifts, in which efficient star formation and feedback-driven redistribution of gas and dust shape the observed light.

Perhaps the most profound implication concerns the speed of chemical enrichment. Detecting carbon and oxygen absorption in a galaxy at redshift 9.3, barely half a billion years after the Big Bang, requires extremely rapid metal production, consistent with highly efficient early star formation. Recent theoretical work shows that such rapid enrichment does not demand a dominant contribution from the hypothetical first generation of Population III stars: ordinary Population II star formation with a high upper-mass cutoff of roughly 200 to 300 solar masses can produce sufficient metal yields on short timescales, in line with expectations that the maximum stellar mass rises at low metallicity. The relative abundance patterns of the three galaxies overlap those of previously studied high-redshift absorbers, without extreme offsets that would signal exotic nucleosynthetic channels.

The observations do not directly constrain which sources drove reionization, and the broader population of high-redshift absorbers remains diverse, ranging from enriched systems to extremely metal-poor gas, indicating that early enrichment proceeded in a patchy, inhomogeneous fashion. But they decisively rule out a picture in which galaxy environments stayed chemically pristine until late times. Within the first several hundred million years of cosmic history, at least a subset of luminous galaxies had already built the full toolkit of the baryon cycle: metals in multiple ionization phases, feedback-driven kinematics, and gas circulating between the interstellar and circumgalactic media. The same processes that today regulate star formation across the universe were evidently at work almost from the beginning.

Subject of Research: Multiphase metal-enriched gas outflows around galaxies during the epoch of reionization

Article Title: Early metal-enriched baryon cycling before the midpoint of cosmic reionization

Article References: Zhu, Y., Ji, Z., Becker, G. D., Ding, J., Egami, E., Fan, X., Jin, X., Liu, W., Lyu, J., Ma, Z., Narisetty, S., Rieke, G. H., Wu, Y., Yue, M., Zhang, J., & Rieke, M. J. (2026). Early metal-enriched baryon cycling before the midpoint of cosmic reionization. Nature Astronomy. https://doi.org/10.1038/s41550-026-02988-2

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02988-2

Keywords: JWST, early universe, cosmic reionization, baryon cycle, galactic outflows, metal enrichment, circumgalactic medium, NIRSpec, high-redshift galaxies, stellar feedback, absorption spectroscopy, Population II stars