Star Formation Drops While Hydrogen Levels Stay Steady

Astronomers have found that the decline in star‑forming activity over the last 4.5 billion years is far steeper than the reduction in the raw material that fuels it. A study combining observations from China’s Five‑Hundred‑Meter Aperture Spherical Telescope (FAST) with spectroscopic data for about 2.5 million galaxies from the Dark Energy Spectroscopic Instrument (DESI) shows that the rate of star formation was roughly 2.5 times higher 4.5 billion years ago, whereas the abundance of neutral hydrogen— the primary reservoir for new stars—was only about 1.4 times greater.

The analysis reveals that while the supply of neutral hydrogen has remained relatively steady, the efficiency with which galaxies convert that gas into stars has dropped dramatically. This suggests that additional processes, such as changes in galactic dynamics, feedback from supernovae or active galactic nuclei, or variations in the interstellar medium, may be suppressing star formation in the modern universe. By mapping the distribution of hydrogen across millions of galaxies, the research provides a new benchmark for models of galaxy evolution and the cosmic star‑formation history.