The universe is experiencing a notable decline in star formation, producing fewer stellar “babies” than in the past. Over 4.5 billion years, new star formation rates have dropped to less than half of previous levels. Interestingly, the supply of neutral atomic hydrogen, a critical fuel for star formation, has changed only modestly.
This research, led by the Chinese Academy of Sciences in collaboration with the Dark Energy Spectroscopic Instrument (DESI) project, utilized China’s Five hundred meter Aperture Spherical radio Telescope (FAST) to measure cosmic neutral atomic hydrogen across time. Findings showed that while star formation has significantly decreased, the amount of neutral atomic hydrogen has remained relatively stable.
This raises the question: Why is the universe producing fewer stars, despite having sufficient hydrogen? A common hypothesis—depleting cold gas—doesn’t fully explain the discrepancy, as a significant decline in star formation would typically accompany a sharp decrease in gas supply, which hasn’t been observed.
Neutral atomic hydrogen plays a key role as it connects the cosmic gas supply to star formation processes. However, detecting its faint 21-centimeter radio emissions poses challenges due to background noise.
Before this study, astronomers struggled with observational limitations, as deep surveys lacked extensive coverage, while wide surveys couldn’t detect faint signals. The new research combined FAST’s sensitivity with DESI’s optical data to study about 2.5 million galaxies across a significant area of the sky using an HI spectral stacking method. This approach allowed for precise tracking of cosmic hydrogen changes.
The results revealed a stark contrast between the decline in star formation and the stability of neutral hydrogen supplies. Approximately 4.5 billion years ago, the cosmic star formation rate was about 2.5 times higher than today, while neutral atomic hydrogen density was only about 1.4 times higher. This indicates that the depletion of neutral hydrogen alone cannot account for the dramatic fall in star formation.
Consequently, researchers shifted focus from whether gas is depleting to why star formation is decreasing despite abundant neutral hydrogen. They propose that changes in how gas moves—specifically, a weakening flow from the cosmic web and reduced gas densities—might affect the efficiency of converting neutral hydrogen into molecular hydrogen, which directly fuels star formation.
These findings provide crucial insights into the long-term decline of star formation and the broader processes shaping galaxies. Combining observations from FAST and DESI sets a new benchmark for analyzing the cosmic gas cycle and the evolution of star formation in the universe.
