Over the past 4.5 billion years, cosmic star formation has dropped by roughly 59 per cent, while neutral atomic hydrogen reserves declined by only 11 to 26 per cent. A new study analyzing nearly 2.5 million galaxies shows this unexpected mismatch rules out simple fuel depletion as the primary cause for slowing star birth.
The universe is producing fewer stars than it once did, but the standard explanation of running out of fuel no longer holds up against new data. Astronomers examining a massive dataset spanning nearly 2.5 million galaxies and the past 4.5 billion years have uncovered a profound mismatch between the rate at which stars are born and the amount of available gas remaining in space.
Over that vast stretch of time, the cosmic star-formation rate density fell by a factor of 2.46. Put another way, the present universe forms stars at about 41 per cent of the rate it managed at the beginning of the period, marking a decline of roughly 59 per cent. Yet the stock of neutral atomic hydrogen did not experience a parallel collapse.
How FAST and DESI Tracked Millions of Galaxies
To measure changes in atomic hydrogen across billions of years, researchers relied on radio observations and optical spectroscopy. Neutral hydrogen emits a faint radio signature with a rest wavelength of about 21 centimetres when the relative spin configuration of its proton and electron changes. While an individual transition is extraordinarily rare, galaxies contain enough hydrogen for the collective signal to be detected.
Data came from China’s Five-hundred-meter Aperture Spherical radio Telescope, specifically utilizing the FAST All Sky H I survey, known as FASHI. Because distant radio signals are too faint for direct individual detection, the team applied a technique called spectral stacking. They combined numerous radio spectra into a common rest frame using optical spectroscopy from the Dark Energy Spectroscopic Instrument’s Bright Galaxy Survey.
This combination yielded a sample of 2,473,945 galaxies spread across roughly 12,000 square degrees of sky. The galaxies occupied four redshift intervals with mean redshifts of 0.033, 0.069, 0.281, and 0.358, allowing researchers to calculate average atomic-hydrogen properties across various stellar masses.
The Gas Mismatch That Defies Simple Explanations
The core finding centers on the stark difference between stellar output and hydrogen reserves. While star formation plummeted by about 59 per cent, the cosmic density of neutral atomic hydrogen fell by a raw factor of 1.35, which equates to roughly 26 per cent. When researchers applied conservative corrections for likely systematic effects, that reduction narrowed further to a factor of 1.12, or a decline of roughly 11 per cent.
This significant gap rules out a straightforward fuel-shortage narrative where galaxies rapidly exhausted or lost their atomic hydrogen supply. Much of the original reservoir remains intact. The deeper mystery involves why less of that available gas successfully transitions into the cold molecular phase required for stellar nurseries.
The study notes that future investigations must determine whether the gas is depleting or why it is increasingly difficult to form stars despite abundant neutral hydrogen reserves.
From Atomic Fuel to Molecular Clouds
Hydrogen inside a galaxy exists in multiple physical states. It can be ionized, neutral and atomic, or paired into molecular hydrogen in sufficiently cold, shielded regions where gravity pulls material inward until nuclear fusion begins. A galaxy may hold a large extended envelope of atomic hydrogen without actively converting it into the compact molecular structures that generate new stars.
The analysis also demonstrated that the ratio between atomic hydrogen mass and stellar mass declined strongly as galaxy mass increased, meaning massive galaxies generally carried less atomic gas relative to their stars. However, at any fixed stellar mass, the evolution of that ratio over the 4.5-billion-year interval remained minimal. Researchers noted that future investigations must determine whether the gas is depleting or why it is increasingly difficult to form stars despite abundant neutral hydrogen reserves.
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