Over the past 4.5 billion years, cosmic star formation has dropped by 59 per cent, yet neutral atomic hydrogen reserves declined by only 11 to 26 per cent. A massive survey of nearly 2.5 million galaxies reveals a stark mismatch, ruling out rapid fuel exhaustion as the main driver of declining stellar birth.
The Great Mismatch Between Hydrogen Fuel and Star Production
The universe did not simply run out of raw gas as its rate of star formation declined. That is the clearest implication of a new measurement spanning almost 2.5 million galaxies and the past 4.5 billion years. Over that interval, the cosmic star-formation rate density fell by a factor of 2.46.
Neutral atomic hydrogen, written H I by astronomers, followed a very different path. Its cosmic density fell by a raw factor of 1.35, equivalent to about 26 per cent. After researchers applied conservative corrections for likely systematic effects, the factor became only 1.12, or a decline of roughly 11 per cent.
The mismatch does not identify one new mechanism that shut down star formation. It does rule out a simple version of the fuel-shortage story in which galaxies rapidly exhausted or lost their atomic hydrogen. Much of that reservoir remained. The harder question is why less of it reached the cold molecular phase from which stars form. Atomic hydrogen is fuel, but not the fuel inside the engine.
How FAST and DESI Tracked Millions of Galaxies
Gathering these numbers required a massive observational effort. The research team used observations from China’s Five-hundred-meter Aperture Spherical radio Telescope, or FAST. Specifically, the data came from the FAST All Sky H I survey, known as FASHI. Neutral hydrogen emits a faint radio signature with a rest wavelength of about 21 centimetres. It is produced when the relative spin configuration of the proton and electron changes. The transition is extraordinarily unlikely for any one atom, but galaxies contain enough hydrogen for the collective signal to become measurable.
Radio data alone were not enough. To know where each weak hydrogen line should appear after cosmic expansion stretched its wavelength, the researchers needed accurate galaxy distances. They matched the FAST footprint with optical spectroscopy from the Dark Energy Spectroscopic Instrument’s Bright Galaxy Survey. The result was a sample of 2,473,945 galaxies over roughly 12,000 square degrees of sky. The usable galaxies occupied four redshift intervals with mean redshifts of 0.033, 0.069, 0.281 and 0.358. Radio-frequency interference prevented a simple uninterrupted sequence through the middle of the range. Most of the galaxies were measured together, not one by one.
Spectral Stacking and the Mechanics of Stellar Nurseries
At the greater distances in the study, the 21-centimetre emission of a typical individual galaxy is too faint for a secure direct detection. The researchers used spectral stacking. They shifted many radio spectra into a common rest frame using DESI redshifts, then combined them so a shared hydrogen signal accumulated while unrelated background noise averaged down.
Most ordinary matter in the universe is hydrogen, yet hydrogen inside a galaxy occupies several physical states. It can be ionised, with electrons separated from protons. It can be neutral and atomic, with one electron bound to each nucleus. In sufficiently cold, shielded regions, two hydrogen atoms can pair to form molecular hydrogen, H2. Stars form inside dense molecular clouds. Gravity draws parts of those clouds inward until cores become hot and compressed enough for nuclear fusion. A galaxy may therefore possess a large extended envelope of H I without converting much of it into the compact molecular structures that actually make stars.
This distinction has already appeared on smaller scales. SpaceDaily previously reported that the location and concentration of atomic gas within galaxies can matter more than total gas volume. H I close to the stellar disc is more relevant to current star formation than diffuse gas spread far outside it. The new work asks the corresponding cosmic question. Across an enormous population and several billion years, did the universe’s stock of atomic hydrogen collapse alongside its production of new stars?
What Stays Hidden in the Cosmic Gas Supply
While the new survey maps average atomic-hydrogen properties across stellar-mass bins, it leaves open the precise mechanics of why molecular cloud formation slows down. Researchers must now determine why atomic hydrogen capable of directly nurturing stars gradually shut down even while massive gas envelopes remain suspended across the cosmos.
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