FAST survey catalogs 156,411 hydrogen-rich galaxies and sets new local HI mass-function benchmark

Researchers using China’s Five-hundred-meter Aperture Spherical radio Telescope, or FAST, have reported a new catalog of 156,411 galaxies detected in neutral hydrogen, creating what appears to be the largest blind single-dish census of hydrogen-rich galaxies in the nearby universe to date. In the same study, the FAST Collaboration used the catalog to produce a new benchmark measurement of the local HI mass function, a key tally of how neutral hydrogen is distributed across galaxies of different sizes.

The survey, called the FAST All Sky HI Survey, or FASHI, mapped about 19,500 square degrees of sky north of declination minus 14 degrees and cataloged extragalactic sources out to redshift less than 0.09, according to the paper. HI means neutral atomic hydrogen, the cold gas in galaxies that serves as the raw material for future star formation.

For the mass-function analysis, the authors used a completeness-corrected sample of more than 109,000 sources, accounting for non-uniform survey sensitivity and the fact that broader HI signals can be harder to recover. They reported that the resulting HI mass function is robustly constrained down to about 10^6.2 times the mass of the sun in hydrogen. The team fit the result with a standard single-Schechter function and found characteristic parameters of log(M/h70^-2 solar masses) = 9.89 plus or minus 0.02, alpha = minus 1.31 plus or minus 0.02, and phi = (6.38 plus or minus 0.49) x 10^-3 h70^3 cubic megaparsecs per dex. The derived cosmic neutral-hydrogen density, Omega_HI, was (4.71 plus or minus 0.03 statistical plus or minus 0.40 systematic) x 10^-4 h70^-1. The paper also reports a median sensitivity of 0.57 millijanskys per beam at 6.4 kilometers per second velocity resolution.

The scale is what makes the release stand out. The earlier FASHI DR1 catalog reported 41,741 extragalactic HI sources. For wider context, the final ALFALFA catalog from the Arecibo Observatory reported about 31,500 HI sources over roughly 7,000 square degrees. That makes FASHI DR2 not just an incremental update, but a far larger nearby-universe inventory of hydrogen-bearing galaxies than previous blind HI surveys.

That matters because the HI mass function is a basic benchmark for galaxy evolution studies: It shows how the universe’s star-forming fuel is spread across low-mass and high-mass galaxies nearby. In the paper’s abstract, the authors wrote that “FASHI provides the most extensive and sensitive HI catalog to date, establishing an important benchmark for studies of gas accretion, galaxy evolution, and large-scale structure in the local universe.”

The manuscript, “The FAST All Sky HI Survey DR2: the FASHI Catalog and the HI Mass Function,” by Chuan-Peng Zhang and colleagues in the FAST Collaboration, is available on arXiv and says it was accepted by Science China Physics, Mechanics & Astronomy on June 30, 2026. FAST is a Chinese national radio observatory operated by the National Astronomical Observatories, Chinese Academy of Sciences, together with the Guizhou Radio Astronomical Observatory. The paper says the full DR2 catalog is scheduled to be made public on project web pages in August 2026.