Star formation across the universe has dropped by a factor of 2.5 over the past 4.5 billion years, yet the fundamental cosmic gas supply has decreased only slightly, according to a breakthrough study published Sept. 1 in Nature Astronomy. Led by researchers from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), the CAS Shanghai Astronomical Observatory, and Shanghai Jiao Tong University, the findings challenge the long-held astronomical assumption that galaxies simply run out of cold gas to build new stars.
How Spectral Stacking Revealed Hidden Cosmic Hydrogen
For decades, astronomers believed star formation slowed down because galaxies were running out of cold gas, which serves as the essential raw material for new stars. If this traditional view were correct, a sharp drop in star formation should match a sharp drop in cold gas reserves. To test this theory, researchers focused on neutral atomic hydrogen—a primary form of cold gas acting as an intermediate link between raw cosmic gas and star production. Measuring this gas over huge cosmic distances has historically been difficult because weak radio signals from distant hydrogen easily drown out in background space noise.
To overcome this technical bottleneck, the research team combined data from two world-class astronomical tools: the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in Southwest China’s Guizhou province, and the Dark Energy Spectroscopic Instrument (DESI) at Kitt Peak National Observatory in Arizona. FAST is the world’s largest single-dish radio telescope, while DESI maps the overall three-dimensional structure of the universe. Using a technique called spectral stacking, the team analyzed roughly 2.5 million galaxies mapping nearly one-third of the sky. By aligning and combining millions of faint radio signals based on each galaxy’s precise redshift—a metric measuring how much light stretches as the universe expands—the researchers successfully extracted clear data from background noise.
Why the Universe’s Star Formation is Shutting Down
The precise measurements revealed a striking contrast. Universally, the pace of star creation stood at approximately 2.5 times its current levels roughly 4.5 billion years ago. However, the density of neutral atomic hydrogen gas was only 1.4 times higher than present levels. This stark difference directly contradicts the idea that simple fuel depletion drives the slowdown. “This indicates that while star formation activity plummeted, the cosmic hydrogen reservoir did not dry up at the same pace,” said Guo Hong, a professor at the Shanghai Astronomical Observatory. Guo pointed out that attention must now shift away from fuel depletion and instead investigate why galaxies struggle to convert existing gas reserves into stars.
Did you know? FAST is the world’s largest single-dish radio telescope in Southwest China’s Guizhou province, providing unprecedented sensitivity for detecting faint neutral hydrogen signals across the cosmos.
Stars are not made directly from atomic hydrogen, according to Zhang Chuanpeng, an associate professor at NAOC and first author of the study. Before forming fresh stars under gravitational collapse, atomic hydrogen must initially gather into much heavier molecular gas clouds. The study suggests that the problem lies in this conversion process. As gas flowing into galaxies slows down over time, density drops, making it much harder for neutral hydrogen to transform into dense molecular gas. Consequently, ultimate raw fuel reserves stay relatively stable while active fuel factories shut down.
Broader Implications for Galaxy Evolution
The research provides vital new context for understanding galactic lifecycles. “The study provides crucial new clues for understanding why the massive stellar engines of the universe are gradually shutting down,” said Yang Xiaohu, deputy director of the Tsung-Dao Lee Institute at Shanghai Jiao Tong University. By ruling out simple fuel exhaustion, astronomers must now build more sophisticated models of how galaxies process gas as they age over billions of years.
Frequently Asked Questions
Why is star formation declining in the universe?
While scientists once assumed galaxies were simply running out of fuel, new research shows the cosmic gas supply remains relatively stable. The decline stems from galaxies struggling to convert neutral atomic hydrogen into the dense molecular gas clouds required to ignite new stars.
What instruments were used in this breakthrough study?
Researchers combined data from the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China and the Dark Energy Spectroscopic Instrument (DESI) in Arizona.
What is spectral stacking?
Spectral stacking is an analytical technique where researchers align and combine millions of faint radio signals from galaxies based on their precise redshift, allowing them to extract clear data from background space noise.
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