Atomic hydrogen remained while star formation fell

Galaxies form far fewer stars today than they did several billion years ago, but the cosmic reservoir of atomic hydrogen has not shrunk at the same pace. An analysis of about 2.5 million galaxies found only weak evolution in this gas over the past 4.5 billion years. The mismatch reframes the question: the recent bottleneck may be less about the total supply of initial fuel and more about the route it takes to reach star-forming regions.

Chuan-Peng Zhang and colleagues combined spectra from China’s FAST radio telescope with positions and distances from the optical DESI survey. The sample covers roughly 12,000 square degrees of sky and extends from the nearby Universe to redshift 0.41. Because neutral-hydrogen emission from a distant galaxy is usually too faint to appear alone, the team aligned and stacked many spectra. Their sum reveals the average 21-centimeter emission produced by hydrogen atoms across large populations.

In the uncorrected data, cosmic atomic-hydrogen density declined by a factor of 1.35, with an uncertainty of 0.10, between 4.5 billion years ago and today. The researchers then built a forward model of the observations to estimate biases from optical selection and stacking. After conservative corrections, the inferred decline was only a factor of 1.12, also with an uncertainty of 0.10. Over the same interval, cosmic star-formation-rate density fell by a factor of 2.46.

The team also separated galaxies by stellar mass. At a fixed stellar mass, the average atomic-hydrogen fraction changed by less than 0.2 dex—less than about 60% on a logarithmic scale—over the period studied. That result contrasts with measurements of molecular gas, whose abundance tracks the decline in star formation more closely. Molecular hydrogen is the cold, dense phase from which star-forming clouds are organized.

The data alone do not identify which process slowed stellar production. They do, however, disfavor the simple explanation that recent galaxies form fewer stars because they rapidly exhausted the cosmic stock of atomic hydrogen. Between a diffuse reservoir and a star lies a chain: gas must enter galaxies, cool, concentrate, turn into molecules, and withstand flows that heat or expel it. The observed mismatch directs attention to those intermediate stages.

By measuring a population across an enormous area of sky, the study sets a benchmark for models of galaxy fueling and regulation. Progress now requires distinguishing how atomic gas enters, circulates, and becomes molecular gas in different environments. The raw fuel appears to have remained relatively abundant; the central problem is why a smaller share reached the factories that make stars.