BEIJING, Sept. 1 (Xinhua) — Scientists have lengthy grappled with a perplexing query: within the early universe, new stars appeared with dazzling frequency, but right now, star formation has clearly slowed. Now, a brand new examine led by Chinese language scientists is providing recent and essential clues to assist unravel this cosmic thriller.
A global group — 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 undertaking involving greater than 70 establishments, the researchers carried out 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 turning into more and more troublesome for the universe to type new stars? It is a core query within the subject of galaxy formation and evolution,” mentioned Jiang Peng, a researcher at NAOC. A extensively held and intuitive clarification is that because the universe evolves, the chilly gasoline that nurtures stars is steadily consumed, ultimately resulting in the decline in star formation. If this state of affairs holds, then the drop within the star formation price ought to inevitably be accompanied by a commensurate depletion of chilly gasoline reservoirs.
Impartial atomic hydrogen is among the most necessary chilly gasoline reservoirs in galaxies and serves as a crucial hyperlink between large-scale gasoline biking and inner star formation. Nonetheless, impartial hydrogen is primarily detected by way 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 price was about 2.5 occasions that of right now, 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 straight guidelines out the simplistic state of affairs that “a speedy exhaustion of impartial hydrogen precipitated the decline in star formation,” Jiang mentioned.
So if impartial hydrogen reserves are nonetheless ample, what’s making star formation more and more troublesome?
The researchers recommend 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 straight incubate stars to steadily diminish.
The findings have been revealed Tuesday within the worldwide journal Nature Astronomy. ■
