FAST telescope compiles world's largest atlas of neutral hydrogen

China’s Five-hundred-meter Aperture Spherical Telescope (FAST) has achieved a major astronomical milestone by compiling the largest-ever atlas of neutral hydrogen sources in the universe. The new dataset, known as FASHI DR2, identified 156,411 neutral hydrogen sources over two years of observations, covering approximately 19,500 square degrees of the sky. This represents a nearly fourfold increase from the telescope’s first dataset released in January 2024 and surpasses the previous record held by the US Arecibo Telescope’s ALFALFA survey.

By detecting the 21-centimeter radio emissions of neutral hydrogen, which acts as the raw material for stars and galaxies, astronomers can study cosmic evolution and galaxy formation through dust that obscures visible light.

To maintain the extreme sensitivity required for such observations, the telescope is located in a natural karst depression in Guizhou. The site requires strict electromagnetic silence; nearly 9,000 residents were relocated to prevent interference from common electronics like smartphones, Wi-Fi, and microwaves, which can render the instrument ineffective within a 5-kilometer radius.