FAST All Sky HI Survey completes "two-step leap" — from 41,741 to 156,411 galaxies, China's FAST telescope maps the largest cosmic gas atlas

Just two years after its first data release (DR1), the FASHI team — led by the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) — has published the second data release (FASHI DR2) as a cover story in Science China Physics, Mechanics & Astronomy on September 8, 2026.

The survey now covers ~19,500 square degrees, or 47.3% of the entire celestial sphere, detecting 156,411 extragalactic HI sources at redshifts z < 0.09. This is nearly four times the 41,741 sources reported in FASHI DR1 (which covered ~7,600 square degrees) and five times the ~31,500 sources from the Arecibo ALFALFA survey (~7,000 square degrees). FASHI DR2 achieves a median sensitivity of 0.57 mJy/beam at a velocity resolution of 6.4 km/s — approximately 0.7 times deeper than ALFALFA (where smaller values indicate higher sensitivity). The cover artwork features a warm sunset scene, symbolizing the 21cm signals received by FAST from distant galaxies as cosmic messengers traversing billions of light years, reflecting the powerful capability of China's Five-hundred-meter Aperture Spherical radio Telescope (FAST) in exploring the Universe.

The remarkable growth of FASHI DR2 was made possible by FAST's exceptional sensitivity and stability maintained over the five year observing period from August 2020 to July 2025, combined with the efficient drift scan strategy using the 19-beam receiver array. Cross matching with optical spectroscopic surveys — the Dark Energy Spectroscopic Instrument (DESI) and the Sloan Digital Sky Survey (SDSS) — yielded precise optical counterparts for approximately 64,000 sources. Using a completeness-corrected sample of over 109,000 galaxies, the team extended the reliable measurement range of the HI mass function down to about one million solar masses (10^6.2 M⊙). The derived cosmic HI density reaches Ω_HI = (4.71 ± 0.03_stat ± 0.40_sys) × 10^-4 h₇₀³Mpc^-3dex^-1, with a statistical uncertainty of just 0.6% — the most precise measurement in the local Universe to date.

The vast and high quality dataset will drive progress across multiple frontiers:

1. Galaxy evolution: providing direct observational constraints on gas accretion, star formation feedback, and galaxy transformation;
2. Cosmology: delivering precise HI mass function measurements that constrain the distribution of baryonic matter within dark matter halos and test galaxy formation models;
3. Large scale structure: complementing optical surveys to map the HI cosmic web and reveal gas distribution in filaments and voids;
4. Gravitational wave and transient science: enabling improved localization of host galaxies for gravitational wave events;
5. SKA pathfinder science: establishing a critical benchmark for sample design and system comparison for next generation interferometric surveys with the Square Kilometre Array (SKA).

From DR1 "breaking the record" to DR2 "setting the benchmark," FASHI marks China's transition from follower to leader in 21cm radio astronomy. The full source catalog is publicly available at https://fast.bao.ac.cn/cms/article/271/, providing a vital foundation for galaxy evolution studies in the SKA era.

The research is led by the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), in collaboration with the Guizhou Radio Astronomical Observatory, Shanghai Astronomical Observatory, CAS South America Center for Astronomy, Peking University, and other institutions. The work was supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences, and the Ministry of Science and Technology of China.

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