Declining star formation not caused by simple 'fuel crisis,' new measurements suggest

The study found that the rate of star formation in the universe has declined by more than half over the past 4.5 billion years, but the essential fuel for star formation—neutral atomic hydrogen (HI)—has not decreased significantly. This was achieved through high-precision measurements using China's FAST telescope and the DESI project. The researchers analyzed 2.5 million galaxies, uncovering a significant disparity: while star formation dropped, HI density remained about 1.4 times current levels. This suggests that the decline in star formation is not due to a simple "fuel crisis."

• The study was led by researchers from the Chinese Academy of Sciences and the DESI project.
• The team used innovative HI spectral stacking techniques to overcome the challenge of detecting faint signals.
• The results indicate that the cosmic HI reservoir has not dried up, despite the drop in star formation.

This finding challenges the previous assumption that the depletion of cold gas would be matched by a corresponding drop in HI. Instead, it suggests that the efficiency of converting HI into molecular gas may be decreasing, allowing the HI reservoir to stay stable even as molecular gas dwindles. The research provides new insights into the mechanisms behind the decline in star formation and the broader evolution of galaxies.

The study is a significant advancement in understanding the cosmic gas cycle and the factors affecting star formation. It establishes a new observational benchmark for studying the late-stage evolution of the universe and the role of HI in galaxy formation. The collaboration between FAST and DESI highlights the power of combining sensitive radio observations with large-scale optical spectroscopy.

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