In one of the most ambitious mapping efforts in the history of radio astronomy, China’s Five-hundred-meter Aperture Spherical radio Telescope — better known as FAST — has delivered a cosmic atlas of staggering proportions. The FASHI team, headquartered at the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), has announced the second data release of the FAST All Sky HI Survey, published as a cover story in the journal Science China Physics, Mechanics & Astronomy. Just two years after the survey’s first data release catalogued 41,741 galaxies, the new edition explodes that figure to 156,411 extragalactic sources detected in neutral hydrogen — nearly a fourfold leap that firmly establishes FASHI as the largest census of atomic hydrogen gas in galaxies ever compiled.

The scale of the achievement is difficult to overstate. FASHI DR2 now spans roughly 19,500 square degrees of sky, equivalent to 47.3 percent of the entire celestial sphere, and reaches out to redshifts below z = 0.09 — distances of well over a billion light years. For comparison, the previous benchmark in this field, the ALFALFA survey conducted with the now-collapsed Arecibo telescope in Puerto Rico, charted approximately 31,500 hydrogen-bearing galaxies across about 7,000 square degrees. FASHI DR2 has thus quintupled the galaxy count of its Arecibo predecessor while covering nearly three times the sky area, and it has done so with a sensitivity roughly 0.7 times deeper than ALFALFA’s, measured as a median noise level of 0.57 mJy per beam at a velocity resolution of 6.4 kilometers per second. In radio astronomy, lower numbers mean better sensitivity — and FASHI’s numbers are among the best ever achieved in a blind, wide-area survey.

The engine behind this performance is FAST itself. With its 500-meter dish nestled in the karst hills of Guizhou Province, the telescope remains the most sensitive single-dish radio instrument on Earth at the 21-centimeter wavelength of neutral hydrogen. The survey exploited a 19-beam receiver array, allowing the telescope to sweep the sky in an efficient drift-scan mode that turned routine transits of the sky above the dish into a torrent of data. Crucially, the instrument maintained exceptional stability across five full years of observing, from August 2020 through July 2025. In surveys of this kind, calibration drifts and instrumental fluctuations can quietly corrupt weak extragalactic signals; FAST’s long-term steadiness meant that faint hydrogen emission from distant galactic disks could be stacked, averaged and trusted.

Neutral hydrogen — the primordial fuel of star formation — is invisible to optical telescopes. The 21-centimeter spectral line, produced when the electron in a hydrogen atom flips its spin relative to the proton’s, provides the only direct tracer of the cold atomic gas from which galaxies build their stars. Detecting this feeble signal across cosmological distances requires both enormous collecting area and exquisite receiver stability. A blind HI survey, unlike targeted observations, simply listens to every direction on the sky and catalogs whatever whispers back, making it an unbiased inventory of gas-rich galaxies regardless of their optical brightness, dust content or morphology. That is precisely why such surveys are prized: they catch the dim, gas-dominated dwarfs and bloated hydrogen clouds that optical catalogs systematically miss.

To give the hydrogen detections physical meaning, the FASHI team cross-matched their radio catalog with the two great optical spectroscopic endeavors of the current era: the Dark Energy Spectroscopic Instrument survey and the Sloan Digital Sky Survey. This cross-identification yielded precise optical counterparts for approximately 64,000 sources, anchoring the hydrogen detections to galaxies with measured distances, stellar masses and star-formation rates. From these matches the team constructed a completeness-corrected statistical sample of more than 109,000 galaxies — a sample robust enough to push fundamental measurements into territory previously beyond reach.

The flagship result of the release is a new measurement of the HI mass function, the statistical distribution describing how many galaxies of a given hydrogen mass populate the local universe. Thanks to the sheer sample size and FAST’s depth, the team extended the reliable measurement range of this function down to hydrogen masses of about one million solar masses — roughly 10 to the power 6.2 in solar mass units — a regime where only the faintest dwarf galaxies live. These smallest gas reservoirs are the building blocks of galaxy evolution, the raw material that hierarchically assembles larger systems, and their census has long been fragmentary. Equally striking is the derived cosmic density of neutral hydrogen: Ω_HI equals (4.71 ± 0.03 statistical ± 0.40 systematic) × 10⁻⁴ in units of h₇₀³ Mpc⁻³ dex⁻¹. The statistical uncertainty is a mere 0.6 percent — the most precise determination of the universe’s atomic hydrogen budget ever made at low redshift.

Why does such a number matter? The cosmic HI density tracks how much raw fuel exists for future star formation, and comparing its evolution across cosmic time constrains how galaxies acquire and exhaust their gas. Because atomic hydrogen sits at the interface between the intergalactic medium and the star-forming interstellar medium, its abundance is a sensitive probe of gas accretion onto dark matter halos and of the feedback processes that heat or expel gas from galaxies. Baryonic matter, after all, is hidden in far more places than stars: cold gas, warm gas, plasma in and around halos. A precision measurement of the hydrogen component anchors models of where ordinary matter resides within the cosmic web of dark matter.

The applications of the new catalog ripple far beyond that single number. For galaxy evolution studies, the 156,411 detections provide direct observational constraints on gas accretion, star formation feedback and galaxy transformation — the processes that turn blue, gas-rich spirals into passive red systems. For cosmology, the HI mass function tests galaxy formation models within the prevailing dark matter framework. For large-scale structure work, the survey complements optical galaxy catalogs to trace the so-called HI cosmic web, revealing how diffuse gas threads through filaments and pools in voids — architecture that optical surveys alone can only infer. The dataset will also aid gravitational-wave and transient science: rapid localization of the host galaxies of gravitational-wave events depends on accurate galaxy catalogs, and an inventory of hundreds of thousands of nearby galaxies sharpens those host identifications. Finally, FASHI serves as a pathfinder for the Square Kilometre Array, the giant interferometric observatory under construction in Australia and South Africa, providing benchmark samples against which next-generation interferometric surveys will calibrate their systems and select their targets.

The trajectory of the project tells its own story. When FASHI DR1 appeared, it broke ALFALFA’s long-standing record for the largest blind HI survey. With DR2, the team argues, FASHI has moved from record-breaking to benchmark-setting — a transition that observers of Chinese radio astronomy describe as the shift from follower to leader in 21-centimeter science. The cover of the journal issue reinforces the mood: a warm sunset scene evoking the faint 21-centimeter photons from distant galaxies arriving at FAST after journeys of billions of light years, messengers of the cold gas that permeates the universe.

The survey’s story is also one of institutional perseverance. The work is led by NAOC in collaboration with the Guizhou Radio Astronomical Observatory, the Shanghai Astronomical Observatory, the CAS South America Center for Astronomy, Peking University and other partner institutions, with funding from the National Natural Science Foundation of China, the Chinese Academy of Sciences and the Ministry of Science and Technology of China. Sustained national investment in a single flagship instrument has now produced a dataset whose statistical power no other facility can currently match at these wavelengths.

The full FASHI DR2 source catalog has been released to the public through the FAST data portal, a decision that will likely seed hundreds of follow-up studies in the coming years, from targeted observations of hydrogen-rich dwarfs to statistical analyses of gas in galaxy groups and environmental studies of gas stripping in clusters. As the SKA era approaches, FASHI DR2 stands as both a scientific foundation and a demonstration: that a single dish, used patiently and precisely over five years, can still redraw the map of the gaseous universe. With nearly half the sky covered and a quarter of a million more square degrees potentially within reach, the question is no longer whether FAST will extend the hydrogen frontier, but how far.

A key component of the Orion Nebula had escaped detection by astronomers until now, according to a new study in Astronomy & Astrophysics.

An international team led by Juan Diego Soler at the University of Vienna used two of the world’s most advanced radio telescopes to map neutral atomic hydrogen gas across the nebula’s outer shell at unprecedented resolution. The new maps provide a clearer view of the celestial feature, and reveal a previously uncatalogued second bubble—a shell with much less mass than earlier estimates—and a four-light-year filament of gas extending from the structure.

The Orion Nebula’s Miscalculated Bubble

Neutral hydrogen emits a faint radio signal at a wavelength of 21 centimeters, making it possible to trace otherwise invisible gas between the stars. Soler’s team solved this by combining data from the Karl G. Jansky Very Large Array in New Mexico and the FAST telescope in China. The VLA provided high resolution, while FAST picked up faint, widespread signals. Together, they produced the first detailed hydrogen map of the extended Orion Nebula, at the same scale as earlier ionized carbon maps.

Earlier calculations based on ionized carbon estimated that the front half of the shell was about 1,100 times the mass of our Sun. The hydrogen observations paint a different picture, suggesting the shell actually weighs in at closer to 100 solar masses. “Measuring mass is fundamental,” Soler said, “because it tells us about the efficiency of these newly formed stars shaping their environment with wind and radiation.”

A shell with only a tenth of the previously assumed mass significantly alters calculations of how efficiently newly formed stars shape their environment. The researchers note that molecular hydrogen, which is not detected in either the hydrogen or carbon data, may account for some of the missing mass.

A Second Bubble

The hydrogen maps also revealed an uneven distribution of gas, indicating a second, smaller cavity within the main shell. The team searched catalogs of known stars and young stellar objects in the region but found no candidate at the bubble’s center. The source responsible for creating the cavity may not be the type of object included in current catalogs.

Daniel Seifried at the University of Cologne, a co-author on the study, pointed to what that means for the models astronomers build to explain regions like this one. “These stunning observations serve as a reference for many modern astrophysical simulations investigating the evolution of gas and stars in the Milky Way. These are the kind of images that challenge the theoretical models and numerical simulations that we use to understand how massive stars affect their immediate surroundings.”

Previous models generally treated Orion’s shell as the product of a single expanding bubble. The discovery of a second cavity, with no obvious source at its center, instead suggests the nebula was shaped by multiple episodes of stellar feedback.

An Unexpected Filament

The new maps also uncovered a filament of hydrogen extending about four light-years from the shell’s western edge. Previous carbon-based observations saw only the base of this structure. The filament contains about 80 solar masses of hydrogen and does not appear to be associated with any known group of young stars. Its origin is unclear; it could be gas leaking through a weak spot in the shell or a leftover from the original cloud that existed before the bubble formed.

Claire Murray of the Space Telescope Science Institute, another co-author, framed the discovery within the broader push behind the observations. “This study is an exciting demonstration of the power of the latest-generation radio telescopes to uncover new pieces to the star formation puzzle.” Despite being one of the galaxy’s most extensively studied star-forming regions, the Orion Nebula continues to reveal discoveries.

This survey is the first scientific result from the NeAtHood project, which aims to map atomic hydrogen across nearby star-forming regions accessible to the VLA and FAST telescopes. Soler and his colleagues now plan to apply the same approach to additional targets. “Orion is only the beginning. Our newly developed methods show how future interferometers will reveal the hidden structure and dynamics of the interstellar medium—even in regions that astronomers already believed they understood well,” Soler explained.

If one of the sky’s best-studied star-forming regions still contains hidden structures, other nearby nebulae may prove equally surprising as the NeAtHood survey expands.

Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.