BEIJING — Scientists have lengthy grappled with a perplexing query: within the early universe, new stars appeared with dazzling frequency, but at the moment, star formation has clearly slowed. Now, a brand new examine led by Chinese language scientists is providing contemporary and essential clues to assist unravel this cosmic thriller.
A global crew — together with the Nationwide Astronomical Observatories of the Chinese language Academy of Sciences (NAOC), the Shanghai Astronomical Observatory (SHAO) of the Chinese language Academy of Sciences, Shanghai Jiao Tong College, and different establishments — has made important progress on this entrance. Utilizing China”s 5-hundred-meter Aperture Spherical radio Telescope (FAST) along with the Darkish Vitality Spectroscopic Instrument (DESI), a world venture involving greater than 70 establishments, the researchers performed high-precision measurements of impartial hydrogen evolution within the universe over the previous 4.5 billion years. Their work is shedding new mild on this long-standing puzzle.
“Why is it changing into more and more tough for the universe to kind new stars? This can be a core query within the subject of galaxy formation and evolution,” mentioned Jiang Peng, a researcher at NAOC. A broadly held and intuitive clarification is that because the universe evolves, the chilly gasoline that nurtures stars is progressively consumed, ultimately resulting in the decline in star formation. If this state of affairs holds, then the drop within the star formation fee ought to inevitably be accompanied by a commensurate depletion of chilly gasoline reservoirs.
Impartial atomic hydrogen is among the most essential chilly gasoline reservoirs in galaxies and serves as a important hyperlink between large-scale gasoline biking and inside star formation. Nevertheless, impartial hydrogen is primarily detected by means of its extraordinarily faint 21-centimeter spectral line. And for distant galaxies, particular person alerts are sometimes buried in background noises, leading to a persistent lack of dependable, direct observational proof, mentioned Guo Hong, a researcher at SHAO.
By combining FAST’s ultra-sensitive radio observations with DESI’s in depth spectroscopic sky survey, the examine analyzed about 2.5 million galaxies throughout roughly one-third of the sky. This method allowed researchers to hint the evolutionary historical past of cosmic impartial hydrogen with unprecedented statistical precision and pattern dimension.
The exact measurements reveal that 4.5 billion years in the past, the cosmic star formation fee was about 2.5 occasions that of at the moment, whereas the density of impartial atomic hydrogen at the moment was solely about 1.4 occasions its present worth.
In different phrases, whilst star-forming exercise plummeted, the universe’s impartial hydrogen reserves didn’t dry up in tandem. This discovering immediately guidelines out the simplistic state of affairs that “a fast exhaustion of impartial hydrogen induced the decline in star formation,” Jiang mentioned.
So if impartial hydrogen reserves are nonetheless considerable, what’s making star formation more and more tough?
The researchers counsel that stars are primarily born inside denser molecular gasoline clouds. As the general cosmic gasoline density drops, the effectivity of changing impartial hydrogen into molecular hydrogen decreases accordingly, inflicting the molecular gasoline that may immediately incubate stars to progressively diminish.
The findings have been revealed Tuesday within the worldwide journal Nature Astronomy.
