Galaxies don't just accumulate gas from the intergalactic medium — they actively return it. These 'exhalations', or galactic outflows, carry heavy elements, regulate growth, and ultimately determine when a galaxy stops forming stars. The epoch of cosmic noon (z ~ 2–3), when star formation rates peaked, is especially intriguing. Yet it remained unclear whether feedback mechanisms operated so early, in the first 2 billion years after the Big Bang, as the first massive 'dead' galaxies were taking shape. The answer was hiding in cold neutral gas — the main reservoir for outflows — but observing it directly at large distances was considered nearly impossible.
The JWST space telescope has for the first time enabled measurements of the Na I D doublet absorption — a spectral indicator of cold neutral gas — in distant galaxies. The Sapori et al. team used 811 publicly available low-resolution spectra (NIRSpec prism) with signal-to-noise ratios above 5, obtained from the DAWN archives and the MoM survey. For each galaxy, they built a stellar continuum model using spectral energy distribution (SED) fitting with the Bayesian code Prospector, incorporating both spectra and multi-band photometry (16 bands from HST and JWST). Then, from the difference between observations and model, they extracted residual Na I D absorption — a signal not explained by stellar photospheres. A key aspect: stellar contributions to Na I D are already accounted for in the model, so the excess is attributed to interstellar medium gas. The low prism resolution (~100) couldn't resolve kinematics, but equivalent width (EW) was preserved.
Out of 811 galaxies, robust Na I D absorption (S/N > 3) was detected in 20 — just ~2.5%, but with a striking dependence on properties. Among massive systems with star formation (M⋆ > $10^{10}$ M⊙), the fraction rose to 4%, and for dusty ones (AV > 1.5) to 7%. The signal was even more pronounced in quenched galaxies with suppressed star formation: among massive 'quenchers', the detection rate hit 13%, and when selecting by specific star formation rate sSFR < $10^{-11}$ yr^{-1}, it leaped to 43%. Measured EW Na I D ranged from 4 to 16 Å, with the highest values (above 10 Å) found in dusty star formation systems, while quiescent ones were more modest, 4–9 Å. Spectral stacking confirmed the pattern: in the bulk of galaxies, neutral gas is indeed present but drowned out by He I λ5876 emission, especially in actively star-forming ones. Nevertheless, even after excluding strong sources, in the subpopulation of massive and dusty systems the mean EW was 1.7 Å. A literature analysis showed that at moderate resolutions, equivalent widths above 5 Å are accompanied by a blueshift in 80% of cases — a telltale sign of Doppler effect from an outflow. Thus, the 20 identified objects very likely host neutral winds.
The physics of galaxy evolution takes a new turn. Estimates of outflow masses (several ×$10^7$ M⊙) and ejection rates (4–12 M⊙/yr), assuming conservative region sizes (1 kpc) and velocities (200 km/s), turn out to be comparable to the star formation rate, and for 30% of the golden sample galaxies they exceed it. This means that neutral exhalations can influence the gas reservoir already in the early Universe. Notably, strong absorption occurs both in dusty ultra-luminous infrared galaxies and in already quenched systems. Perhaps we are witnessing two stages: a powerful wind driven by star formation paves the way for quenching, and then a residual outflow, possibly sustained by weak activity from the active galactic nucleus, completes the cleaning. However, the low resolution cannot rule out emission filling effects, so the true prevalence could be higher.
The next step is medium- and high-resolution spectroscopy with JWST for thousands of galaxies at z > 3. This will allow direct measurements of line shifts and separation of interstellar medium absorption from He I emission, as well as comparing outflows with internal dynamics. At the same time, numerical models will be needed to trace the fate of ejected gas all the way to filling the circumgalactic medium (CGM) and intergalactic medium (IGM). It will be interesting to test the hypothesis that neutral winds in early massive galaxies were launched not only by superstarbursts but also by low-power active galactic nucleus that currently evade direct detection.
The results impact models of chemical evolution: neutral ejecta are the primary way to enrich intergalactic space with metals. For galaxy formation theories, this means a need to rethink quenching mechanisms in the first 2 billion years.
They plan to request medium-resolution JWST observations of confirmed candidates to measure outflow velocities and map the ejection regions. In parallel, data from deep NIRCam fields will be analyzed to look for morphological signs of interactions and tidal tails associated with the winds.
This work touches on the problem of feedback in the early Universe: why do massive galaxies stop star formation so quickly? Neutral outflows are one of the main suspects, alongside gas heating by active galactic nucleus. They also shed light on the origin of metals in the Lyα forest phase of the IGM. The question of which mechanism drives the winds — stellar or accretion — remains open.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The authors used 811 archival low-resolution spectra from JWST/NIRSpec to isolate Na I D absorption as a tracer of cold neutral gas in distant galaxies (z ~ 2–6). They built stellar continuum models and subtracted them, uncovering excess absorption in 20 objects, predominantly massive, dusty, and with suppressed star formation. The resulting equivalent widths and detection rates point to powerful cold-gas outflows in the early Universe.
Where its strength is
This work is the first to systematically measure Na I D in distant galaxies across large samples, linking cold outflows to mass, dust, and the quenching of star formation already at z > 2. It provides a direct observational bridge between early quenching phases and the cold phase of galactic winds.
Where it could go next
Worth trying
Why this may matter
Understanding the cold phase of outflows in the early Universe is critical for galaxy evolution models: cold gas can efficiently remove mass and metals, regulating the quenching of the first massive systems. Direct Na I D observations open a path to calibrating these processes in simulations.
Nearby works in our archive
The archive is crowded around this work: 2 reviews nearby, closest at 0.75. This ground is well walked — and there is plenty to compare against.
The Na I D doublet — those very sodium lines that give street lamps their yellow glow. In galaxies, they become an indicator of cold gas hidden from ultraviolet light behind dusty screens.