The measurement could hardly have been made with ordinary instruments. The 21-centimeter radio emission line of neutral atomic hydrogen, written H I by astronomers, is intrinsically faint, and individual galaxies beyond the local universe are simply too dim to detect directly in this line. The team therefore combined two of the most powerful survey machines ever built. FAST, the largest single-dish radio telescope on Earth, provided exquisitely sensitive H I spectra drawn largely from the FAST All Sky H I Survey, or FASHI. DESI, mounted on the Nicholas U. Mayall Telescope at Kitt Peak in Arizona, supplied optical spectroscopy for approximately 2.5 million galaxies covering roughly 12,000 square degrees of sky. By cross-matching the two datasets and statistically stacking the faint radio signals of thousands of galaxies at once, the researchers could recover an average H I signal even for galaxies whose individual emission would be buried in the noise forever.

The technique, known as spectral stacking, works on a simple principle: while random noise averages down as observations are added, a real signal consistently aligns with each galaxy’s known redshift from the DESI catalogue and therefore builds up coherently. Stacking galaxies in bins of redshift and stellar mass allowed the team to measure the average atomic gas fraction as a function of stellar mass out to redshift 0.41, which corresponds to a lookback time of about 4.5 billion years. Converting these stacked signals into a cosmic H I density, denoted Omega_HI, required careful calibration of every systematic effect that could masquerade as cosmic evolution, from radio frequency interference and baseline ripples in the FAST spectra to source confusion, luminosity bias and the detailed selection function of the DESI Bright Galaxy Survey.

The headline result is a raw decline in Omega_HI by a factor of 1.35 plus or minus 0.10 over the past 4.5 billion years. After the team applied conservative systematic corrections through their forward model, the inferred decline shrank further, to just a factor of 1.12 plus or minus 0.10. That number may sound technical, but its implications are profound. Over the same interval, the cosmic star formation rate density has collapsed by a factor of roughly 2.46, more than twice as steep a drop. In other words, while star formation across the universe has been falling off a cliff, the raw atomic hydrogen fuel that feeds star formation has barely diminished at all.

This mismatch strikes at the heart of a decades-old debate. In the standard picture of galaxy evolution, stars form from cold molecular gas, but that molecular gas must be assembled from the more diffuse atomic hydrogen that dominates the cold gas reservoir of most galaxies, particularly in their extended outer discs. If galaxies had been rapidly draining their atomic reservoirs over cosmic time, that depletion could plausibly explain the dying star formation rate. The new measurement rules out this scenario. Atomic hydrogen, it turns out, has been remarkably stable, evolving at most modestly while the star formation it supposedly fuels has collapsed. The study instead finds, consistent with prior work, that the molecular gas density evolves far more closely in step with star formation, pointing the finger at the conversion of atomic gas into molecular gas, and at the regulation of star formation within molecular clouds, as the real bottleneck.

The weak evolution is not confined to a particular class of galaxy. At fixed stellar mass, the team found that the average H I gas fraction of galaxies evolves by less than 0.2 dex, an astronomer’s shorthand for a change of less than about 60 percent, across the full 4.5-billion-year window. This means the flatness of the cosmic H I density is not an artifact of the galaxy population shifting its composition over time, for example through the growth of massive, gas-poor galaxies. Rather, the stability of atomic hydrogen reservoirs appears to be a property of the galaxy population as a whole, from moderate spirals to the most massive systems in the DESI sample. The result aligns with, and substantially tightens, earlier stacking measurements from instruments such as the Giant Metrewave Radio Telescope, the Arecibo Observatory, MeerKAT and the Very Large Array, which individually traced smaller redshift ranges with larger uncertainties.

Reaching this precision demanded an unusually rigorous treatment of systematic uncertainties. The authors built a forward model that simulates how observational effects distort the true underlying signal, testing their measurements against mock catalogues built from cosmological simulations such as IllustrisTNG and SIMBA. They quantified how spectral confusion, in which multiple faint galaxies blend into a single radio feature, inflates the apparent gas fraction at the low-mass end, and corrected for the luminosity bias that arises because brighter galaxies are preferentially detected in the optical parent sample. They also addressed the frequency-dependent environment of FAST, masking regions of strong radio frequency interference and verifying that stacked noise scaled with the expected inverse square root of the number of stacked spectra. The consistency of three independent correction methods for luminosity bias gave the team confidence that the measured trend reflects the universe rather than the instrument.

The findings provide a stringent benchmark for theoretical models of how galaxies acquire and process gas. Simulations must now reproduce a universe in which the atomic hydrogen density varies by only tens of percent over nearly half the age of the Earth, even as star formation plummets. That constraint favors scenarios in which galaxies continuously replenish their atomic reservoirs through accretion of fresh gas from the cosmic web and the circumgalactic medium, while the conversion of atomic gas into molecules and the feedback processes that heat or expel gas become progressively less efficient. Models that allow atomic hydrogen to be rapidly depleted, or that tie star formation directly to the atomic gas supply, will need substantial revision. The result also connects to broader questions about the baryon cycle, the circulation of ordinary matter between galaxies and their surrounding halos that underpins all of galaxy evolution.

There is more to come. FAST continues to expand the FASHI catalogue toward its ultimate goal of a complete all-sky H I census, and DESI is amassing ever larger spectroscopic samples as its five-year survey proceeds. Together, these surveys promise to push H I stacking measurements to higher redshifts, potentially bridging toward the epoch around redshift one where earlier measurements hinted at more substantial evolution, when the universe was roughly half its current age. For now, the message of this study is clear and slightly humbling: the universe’s dimming is not for lack of fuel. Cold atomic hydrogen, the raw material from which galaxies knit their stars, has been there all along, patiently waiting, while the machinery that turns gas into stars has been quietly grinding to a halt.

Subject of Research: Weak evolution of the cosmic atomic hydrogen density over the past 4.5 billion years measured with FAST and DESI.

Article Title: Weak evolution of cosmic atomic hydrogen over the past 4.5 billion years

Article References: Weak evolution of cosmic atomic hydrogen over the past 4.5 billion years. (n.d.). https://doi.org/10.1038/s41550-026-02965-9

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02965-9

Keywords: cosmic atomic hydrogen, H I gas density, FAST telescope, DESI survey, spectral stacking, star formation decline, galaxy evolution, gas accretion, molecular gas, neutral hydrogen, redshift surveys, cosmology

Turbulence is not a cosmetic feature of galaxies; it is a controlling one. The random, supersonic motions of interstellar gas set the thickness of galactic disks, regulate how molecular clouds collapse into new stars, and determine how efficiently matter converts into luminous stellar populations. Without a continuous supply of energy, turbulence in a galaxy like Andromeda would decay on timescales of only a few tens of millions of years, far shorter than the age of the disk itself. Something must constantly re-energize the gas, and supernovae have topped the list of candidate drivers since the earliest theoretical treatments of the multiphase interstellar medium. Alternative mechanisms, including gravitational instabilities in the rotating disk and the magnetorotational instability, have also been proposed, and disentangling their contributions observationally has proven exceptionally difficult.

The key to the new result lies in superbubbles, the gigantic cavities that clusters of massive stars carve out of the neutral hydrogen gas around them. When a group of hot, short-lived stars forms together, the combined winds and subsequent core-collapse supernovae of its members blow a common expanding shell into the surrounding medium. These shells, sometimes hundreds of light-years across, act as fossil records of clustered stellar feedback: their sizes, expansion velocities, and ambient gas densities encode how much kinetic energy the parent star cluster deposited into the disk. Because supernovae in galaxies tend to occur in clusters rather than in isolation, superbubbles are the natural conduits through which stellar feedback feeds galactic-scale turbulence, making a complete census of them the decisive observational test.

Until now, no such dynamically complete, galaxy-wide census existed. Earlier surveys of hydrogen holes and shells in Andromeda and other nearby galaxies, dating back to work in the 1980s, identified cavities in the gas but lacked the sensitivity and velocity resolution to measure expansion reliably across a whole disk, leaving the energy budget unsettled. The new study overcame this limitation by combining the extraordinary sensitivity of FAST, the world’s largest single-dish radio telescope, with the fine angular resolution of the Jansky Very Large Array. The combination was essential: single-dish data recover the diffuse, large-scale hydrogen emission that interferometers miss, while interferometric data resolve the fine structure of individual shells. An image-fusion technique merged the two data sets into a single, dynamically complete hydrogen data cube of Andromeda, capturing structures on all relevant scales simultaneously.

From this combined data set, the team identified 118 superbubbles distributed across the entire disk of M31, each appearing as a coherent shell in both space and velocity. The dynamical ages of the shells extend up to roughly 40 million years, a range that matches theoretical expectations for the duration of supernova activity within a star cluster, since the most massive members explode within a few million years while lower-mass stars detonate tens of millions of years later. This consistency between the observed age distribution and stellar-evolution models strengthens the interpretation that the shells are indeed powered by clustered supernovae rather than by other processes such as infalling clouds or galactic-scale instabilities.

The crucial quantitative step was an energy accounting performed two independent ways. First, from the measured sizes, expansion velocities, and surrounding gas densities of the superbubbles, the researchers calculated the rate at which supernovae inject kinetic energy into the neutral medium. The inferred injection rates span 10^49 to 10^51.5 erg per cubic kiloparsec per million years. Second, from the same hydrogen data, they measured the turbulent velocity dispersion of the gas and derived how quickly turbulent energy dissipates at each location in the disk, using established scaling relations for supersonic, magnetized turbulence. If supernovae truly power the turbulence, these two independently determined rates should agree, both in total magnitude and in how they vary from place to place across the galaxy.

They do agree, and remarkably well. The kinetic-energy-injection rates inferred from the superbubble population closely match the turbulence dissipation rates derived from the gas kinematics, not only in overall magnitude but also in their spatial distribution across the Andromeda disk. Regions where the shells inject more energy are precisely the regions where the gas exhibits the strongest turbulent motions. This point-by-point correspondence is far more constraining than a global average, because it rules out a coincidence in which supernovae supply the right amount of energy somewhere in the galaxy while a different mechanism actually drives the local turbulence. The result demonstrates that clustered supernova feedback alone is sufficient to sustain galactic-scale turbulence in a giant spiral galaxy.

The implications extend well beyond Andromeda. Because M31 is the nearest giant spiral and a close analogue of the Milky Way, the result provides the strongest direct evidence to date that our own galaxy’s turbulent interstellar medium is likewise maintained by the death throes of massive stars. Turbulence, in turn, feeds back into star formation: it both prevents gas from collapsing too quickly into stars and concentrates density enhancements that seed new star-forming clouds. A galaxy’s structure and evolutionary trajectory therefore depend on this feedback loop, and models of galaxy formation and evolution can now anchor their prescriptions for supernova-driven turbulence to a directly measured, observationally verified energy budget rather than to theoretical assumption alone.

The study also showcases the power of combining complementary radio facilities. FAST’s collecting area delivers sensitivity to faint, extended hydrogen emission at a level no other instrument can reach, while the Very Large Array contributes the sub-arcminute resolution needed to resolve individual shells hundreds of parsecs away in a neighboring galaxy. The data, the 118-object superbubble catalogue, and the analysis code have been released publicly, allowing other researchers to scrutinize the classification, refine the energy estimates, and extend the method to additional galaxies. As similar combined surveys target more spirals, astronomers will be able to test whether supernova-driven turbulence dominates universally or whether gravitational and magnetic mechanisms take over in particular environments, such as low-star-forming outer disks or violently interacting systems.

For decades, the image of galaxies as serene, slowly rotating pinwheels has coexisted with the reality that their gas is in constant, violent motion, churned by forces whose origin remained unproven. With 118 superbubbles now mapped across Andromeda and their energy output shown to balance the turbulent dissipation of the entire disk, that origin is no longer a hypothesis but a measurement. The explosions of massive stars, it turns out, are not merely the spectacular endings of stellar lives; they are the beating heart that keeps entire galaxies stirred, structured, and capable of forming the next generation of stars.

Subject of Research: Observational evidence that clustered supernova feedback, traced by hydrogen superbubbles, sustains galactic-scale turbulence in the Andromeda galaxy.

Article Title: Supernova origin of galactic turbulence revealed by superbubbles

Article References: Meng, F., Tsai, C.-W., Wu, J., Jiao, S., Mac Low, M.-M., Zhang, Z.-Y., Saintonge, A., Li, H., Li, Z., Wang, J., Wang, L., Xu, H., Yang, Y., Zhang, K., Li, R., & Li, D. (2026). Supernova origin of galactic turbulence revealed by superbubbles. Nature Astronomy. https://doi.org/10.1038/s41550-026-02981-9

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02981-9

Keywords: supernovae, superbubbles, galactic turbulence, Andromeda galaxy, M31, neutral hydrogen, FAST telescope, Jansky Very Large Array, interstellar medium, star formation, stellar feedback, Nature Astronomy