Star birth slows while neutral hydrogen remains abundant, new study finds

A recent study appearing in Nature Astronomy reports that the universe’s supply of neutral atomic hydrogen (HI) has changed little over the past 4.5 billion years, despite a steep fall in the cosmic star-formation rate to roughly one-quarter of its former level. About 4.5 billion years ago, stars were being born at roughly 2.5 times the current pace, while HI density was only about 1.4 times higher. Researchers combined observations from the Chinese FAST telescope with data from the Dark Energy Spectroscopic Instrument, examining roughly 2.5 million galaxies across nearly a third of the sky and stacking weak HI signals to obtain a statistical measure.

The results challenge the notion that galaxies simply run out of gas, pointing instead to a reduced efficiency in converting HI into the dense molecular clouds that directly fuel star formation. As the inflow of gas from the cosmic web weakens and overall gas density drops, the transformation to molecular hydrogen may become less effective, leaving abundant HI but fewer new stars. The work highlights the need to understand the processes governing gas-phase changes within galaxies.