Why Is Star Formation Declining Despite Abundant Hydrogen?

Why Star Formation Rates Are Falling

Understanding why star formation has become less active as the universe ages remains a central question in research on galaxy formation and evolution. One straightforward explanation assumes that galaxies have gradually consumed the cold gas needed to produce stars. If dwindling supplies of cold gas were primarily responsible, astronomers would expect the dramatic fall in star formation to be accompanied by a similarly large decline in available gas. So far, observations have not shown such a sharp depletion, according to researchers working with the Dark Energy Spectroscopic Instrument (DESI) project and China’s Five hundred meter Aperture Spherical radio Telescope (FAST).

Detecting Faint Hydrogen Signals Across Millions of Galaxies

Neutral atomic hydrogen, known as HI, plays a critical role inside galaxies by connecting the broader cosmic supply of gas with the processes that eventually produce new stars. Astronomers primarily detect HI through its extremely faint 21-centimeter radio emission line. Detecting that signal from distant galaxies is difficult because background noise often overwhelms it. For years, astronomers faced a major observational challenge: very deep surveys achieved necessary sensitivity but could not examine large regions of the sky, while wider surveys lacked the sensitivity needed to detect such faint radio signals.

To solve this, researchers combined the exceptional radio sensitivity of FAST with the enormous optical spectroscopy dataset provided by the DESI Bright Galaxy Survey. The team studied approximately 2.5 million galaxies spread across roughly 12,000 square degrees of sky. By using an HI spectral stacking method, researchers aligned the weak signals of individual galaxies using precise redshift measurements and stacked them together. This process allowed the average HI signal to emerge from the background noise, providing a sample of unprecedented size and high statistical precision.

Comparing Star Formation Versus Hydrogen Density

The analysis revealed a striking contrast in the progression between star formation activity and neutral hydrogen inventory.

Four and a half billion years ago, cosmic star formation proceeded at a pace roughly 2.5 times greater than it does at present. During that identical span, however, raw measurements show that neutral atomic hydrogen density was merely about 1.4 times higher than current levels, a figure that decreased to a mere 1.12 times greater once conservative adjustments for potential systematic variables were applied.

These outcomes demonstrate that the rapid depletion of neutral hydrogen cannot on its own account for the dramatic slowdown in star creation.

The Shifting Cosmic Mystery

The study’s authors note that these findings shift the primary inquiry away from whether gas supplies are running out and toward the puzzle of why star creation becomes increasingly arduous despite ample neutral atomic hydrogen stocks. Hydrogen in its neutral atomic state does not directly transform into stars; rather, star birth occurs predominantly within much denser reservoirs of molecular gas.

The team proposes that the most critical evolutionary shifts in the late universe concern pathways within the baryon cycle—specifically, how gas moves—rather than the total volume of available HI. As inflows originating from the cosmic web weaken and overall gas density diminishes, galaxies likely lose efficacy in transforming neutral atomic hydrogen into molecular hydrogen. Under this mechanism, the overarching reservoir of HI can remain relatively steady while the specific molecular gas supplies directly responsible for birthing stars steadily dwindle.

Frequently Asked Questions

Why is the universe making fewer stars?

While star formation has dropped by more than half over the past 4.5 billion years, research shows that galaxies retain a surprisingly stable reservoir of neutral atomic hydrogen. The decline is likely due to galaxies becoming less efficient at converting that atomic gas into dense molecular gas where stars actually form.

What instruments were used in this hydrogen study?

Researchers used China’s Five hundred meter Aperture Spherical radio Telescope (FAST) for sensitive radio observations and the Dark Energy Spectroscopic Instrument (DESI) for optical spectroscopy and precise galaxy redshifts.

How did scientists measure hydrogen in 2.5 million galaxies?

Because signals from individual distant galaxies are extremely faint, researchers used an HI spectral stacking method. By aligning the radio observations using each galaxy’s known redshift, they combined enormous numbers of weak signals so the shared hydrogen signal could emerge from background noise.