A recent study challenges the prevailing understanding of why star formation in the universe has steadily declined over the past several billion years. Contrary to the long-held assumption that galaxies are running out of the cold gas necessary for star birth, new evidence indicates that the fundamental cosmic gas supply has diminished only slightly, suggesting other factors are responsible for the slowdown in star formation.
The research, conducted by scientists from the National Astronomical Observatories of the Chinese Academy of Sciences, the CAS Shanghai Astronomical Observatory, and Shanghai Jiao Tong University, was published on September 1 in the journal Nature Astronomy. The team combined observations from two major astronomical instruments: the Five-hundred-meter Aperture Spherical radio Telescope (FAST) located in Guizhou province, China, and the Dark Energy Spectroscopic Instrument (DESI) at Kitt Peak National Observatory in Arizona, United States.
Astronomers have traditionally believed that star formation rates dropped because galaxies depleted their reservoirs of cold gas—primarily neutral atomic hydrogen, which serves as a precursor to the molecular gas needed to form stars. However, accurately measuring this gas across vast cosmic distances has been difficult due to weak radio signals from distant hydrogen being obscured by background noise.
To overcome this challenge, the researchers employed a technique called spectral stacking. This method involved aligning and combining the faint radio emissions from about 2.5 million galaxies—covering nearly one-third of the sky—based on each galaxy’s redshift, a measurement related to its distance and age. The process enabled the extraction of clear data from the noisy signals.
Analysis revealed that roughly 4.5 billion years ago, the star formation rate was approximately 2.5 times higher than it is today, whereas the density of neutral atomic hydrogen gas was only about 1.4 times greater. This discrepancy indicates that the decline in star formation cannot be attributed simply to a reduction in cosmic hydrogen reserves.
“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. He emphasized that research should focus on understanding why galaxies are less efficient at converting existing gas into stars, rather than whether the gas is running out.
According to Zhang Chuanpeng, an associate professor at the National Astronomical Observatories and the study’s first author, stars form not directly from atomic hydrogen but from molecular gas clouds that result from the condensation of atomic hydrogen. The study suggests that as gas inflow into galaxies slows with cosmic time, the gas density decreases, hampering the transformation from atomic to molecular hydrogen and thereby reducing star formation.
Yang Xiaohu, deputy director of the Tsung-Dao Lee Institute at Shanghai Jiao Tong University, described the findings as offering “crucial new clues” for understanding the gradual decline of the universe’s stellar production. The research points toward changes in the internal processes of galaxies as the primary factor behind the downturn in star formation, rather than a straightforward depletion of cold gas supplies.