China's FAST Telescope Releases Largest Hydrogen Galaxy Catalog

On September 8, 2026, the National Astronomical Observatories of the Chinese Academy of Sciences (NAO/CAS) released the second data release of the FAST All Sky H I Survey (FASHI DR2). The catalog, published as a cover article in Science China Physics, Mechanics & Astronomy, contains 156,411 extragalactic neutral hydrogen sources observed with the Five-hundred-meter Aperture Spherical Telescope (FAST) in Guizhou Province. This represents the largest single-dish neutral hydrogen galaxy catalog ever assembled, supplanting the long-standing benchmark set by the Arecibo-based ALFALFA survey.

From 41,000 to 156,000 sources

FASHI DR1, released in January 2024, cataloged 41,741 sources across roughly 7,600 square degrees of sky. In less than two years, DR2 has grown the source count to 156,411 — a nearly fourfold increase — while expanding sky coverage to approximately 19,500 square degrees, close to half of the entire sky. The median sensitivity has improved from 0.76 to 0.57 mJy per beam, and the frequency coverage now spans 1305.7 to 1422.8 MHz, corresponding to redshifts below z = 0.0878. Observations for DR2 were accumulated between August 2020 and July 2025 using FAST's 19-beam receiver array in drift-scan mode.

For comparison, the ALFALFA survey — conducted with the 305-meter Arecibo telescope between 2005 and 2012 — cataloged approximately 31,500 sources across about 7,000 square degrees and served as the field's standard reference for nearly two decades. FASHI DR2 now contains roughly five times as many sources as ALFALFA and covers nearly three times the area. Arecibo itself collapsed in 2020, leaving FAST as the sole single-dish facility capable of this scale of H I survey.

The physics: measuring the universe's raw material

Neutral hydrogen — the simplest atom in existence, a single proton and electron — is the most abundant element in the cosmos and the fundamental fuel for star formation. It emits radiation at the 21-centimeter line (1420.4058 MHz), a faint but penetrating signal that passes through interstellar dust, allowing astronomers to detect gas in galaxies that optical telescopes cannot see. Mapping this gas across large volumes of the nearby universe provides the raw data for studying how galaxies accrete material, how star formation proceeds and declines, how dark matter halos are populated with baryonic matter, and how the cosmic web is structured on the largest scales.

FASHI DR2 enabled two precision measurements of fundamental cosmological interest. The cosmic neutral hydrogen mass density, written as Omega_HI, was pinned down to (4.71 plus or minus 0.03 statistical plus or minus 0.40 systematic) times 10 to the minus 4 per inverse hubble constant — a relative precision of 0.6 percent, the tightest constraint ever achieved. The survey also extended the reliable measurement of the H I mass function down to approximately 10 to the 6.2 solar masses, about one million solar masses, providing the best handle to date on the low-mass end of the galaxy gas distribution, where the slope of the mass function was measured at minus 1.31 plus or minus 0.02.

A facility without peer

FAST, located in a karst depression in Pingtang County, Guizhou, has an aperture of 500 meters, making it the world's largest single-aperture radio telescope. Its 19-beam receiver array, installed in 2019, enables the kind of wide-field drift-scan survey that FASHI requires. The receiver achieves a half-power beam width of 2.9 arcminutes at 1420 MHz and a system temperature of 16 to 19 kelvin. With 65,536 spectral channels at 7.629 kHz spacing — corresponding to about 1.6 km/s velocity resolution — the system resolves the narrow H I emission lines of external galaxies with fidelity that smaller dishes cannot match.

The FASHI collaboration is led by Jiang Peng's group at NAO/CAS, with contributions from the Shanghai Astronomical Observatory, the Guizhou Radio Astronomy Observatory, the CAS South America Center for Astronomy, and Peking University, among others. The international peer review process reportedly described the dataset as a long-awaited milestone for the astronomical community and a likely standard reference.

Looking ahead: SKA and beyond

FASHI DR2 covers about half of what FAST can ultimately survey. The full FASHI survey, when completed, is expected to map the entire accessible sky from FAST's latitude. The data are publicly available, and the DR1 catalog has already been cited more than 100 times. Multiple international teams have begun using FASHI data for studies ranging from galaxy group dynamics to gravitational wave host identification.

The survey also serves as a bridge to the Square Kilometre Array (SKA), the international interferometric array now under construction in South Africa and Australia. When complete, SKA is expected to detect millions of H I sources rather than hundreds of thousands. FASHI provides the benchmark sample against which SKA's early results will be calibrated, and it establishes the survey design parameters — sensitivity, area coverage, source selection — that SKA's H I programs will build upon.

A modest caveat

It is worth noting that the DR2 catalog, while the largest of its kind, is not the final word. The main DR2 paper appears in Science China Physics, Mechanics & Astronomy, a reputable journal but one with a lower international profile than Nature or The Astrophysical Journal. Independent Western follow-up analyses of specific DR2 results — mass function fits, Omega_HI measurements, individual source confirmations — will take time to appear. The data quality, however, has been validated through internal consistency checks and cross-matching with optical and infrared surveys. The gap between FAST's growing catalog and the next-generation SKA survey will be measured in years, and FASHI DR2 now stands as the definitive bridge.