China’s FAST telescope maps neutral hydrogen, the cosmic gas for making stars

China's FAST telescope has compiled the world's largest catalogue of neutral hydrogen beyond the Milky Way, surpassing the US Arecibo Telescope's survey. The FAST All-Sky H I Survey (FASHI) released its second batch of data, containing 156,411 extragalactic sources – nearly five times the number detected by the Arecibo Telescope.

This publicly available data allows astronomers worldwide to investigate some of the universe's biggest questions, such as the formation of cosmic structures and the evolution of galaxies. Compared to the Arecibo survey, FASHI covered about three times as much sky and detected smaller, fainter, and more distant galaxies. FAST, a 500-metre radio dish in southwest China's Guizhou province, has greater sensitivity and can observe 19 patches of sky simultaneously, scanning large areas faster than Arecibo.

The telescope's primary focus is neutral hydrogen, the basic form of the universe's most abundant element and the raw material for star formation. FASHI began systematically searching for this signal in 2020, with the first data release in January 2024 cataloguing 41,741 galaxies, surpassing the Arecibo Telescope's final tally. The second release, based on observations through July 2025, nearly quadrupled the number of detected galaxies.

This vast sample could shed light on mysteries such as the distribution of ordinary matter within dark matter. The second release is now open to researchers worldwide, serving as a benchmark for future radio surveys, including those planned with the upcoming Square Kilometre Array.