Scientists have uncovered the reason for slowing down the process of star birth in the universe

Over the past 4.5 billion years, the rate of new star formation in the universe has decreased dramatically, although the supply of neutral atomic hydrogen, one of the main sources of matter for star formation, has changed much less. An international team of astronomers came to this conclusion by comparing data from the Chinese FAST radio telescope and observations of 2.5 million galaxies. Why galaxies stop effectively creating new stars is in the Izvestia article.

There was more hydrogen than expected

Scientists have long been trying to understand why, as the universe ages, the process of star formation slows down. One obvious explanation was that galaxies were gradually using up the cold gas needed to produce new stars. However, if the depletion of gas reserves were the main reason, its amount would have to decrease at about the same rate as the rate of star formation. New data shows that this has not happened.

The researchers focused on neutral atomic hydrogen (HI). It is an important reservoir of cold gas inside galaxies and connects matter coming from the cosmic environment with processes that eventually lead to the formation of stars. It is not easy to observe HI directly: astronomers register extremely weak radio emission at a wavelength of 21 cm, which can be lost amid interference.

To obtain the necessary data, the scientists combined the sensitivity of the Chinese FAST radio telescope with a huge array of spectroscopic observations from the DESI project. The study included about 2.5 million galaxies located in almost a third of the sky. Using the spectral addition method, weak signals from individual galaxies were combined to obtain an average HI signal, which could not be reliably measured for each galaxy separately. The results of the study are published in the journal Nature Astronomy.

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There are 2.5 times fewer stars

Calculations have shown a noticeable gap between the amount of hydrogen and the rate of star formation. About 4.5 billion years ago, that is, around the time of the formation of the Solar system, the cosmic rate of formation of new stars was about 2.5 times higher than the current one. Over the same period, the density of neutral atomic hydrogen decreased significantly less — now it is about 1.4 times lower than at that time.

Thus, the rate of star formation slowed down much more than the HI stock decreased. According to the researchers, this means that the simple explanation "the universe has run out of gas" is not enough. Much of the necessary hydrogen still remains in galaxies, but it is not so efficiently converted into the material from which stars are directly formed.

At the same time, neutral atomic hydrogen itself is not an immediate building material for most new stars. They are mostly born inside much denser clouds of molecular gas. The authors claim that HI occupies an intermediate position between the total supply of gas and molecular hydrogen, which can already participate in star formation.

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Galaxies may have forgotten how to turn gas into stars

The researchers suggest that the key to the solution may lie not so much in the total amount of hydrogen, but in how the gas moves inside and between the galaxies. As the universe evolves, the flow of matter from the cosmic web may become weaker, and the density of gas may decrease. Under such conditions, it becomes more difficult for galaxies to convert HI into molecular hydrogen.

If this scenario is correct, a relatively stable supply of atomic hydrogen may exist simultaneously with a gradual reduction in the amount of molecular gas directly associated with star formation. Then the main reason for the slowdown in space "star production" is not the disappearance of raw materials, but a decrease in the efficiency of its processing.

According to the scientists, the results allow us to shift the main question: instead of finding out why there is less hydrogen in the universe, it is necessary to understand why, with the remaining HI reserves, it becomes increasingly difficult for galaxies to form molecular gas and launch new stars. The authors note that the FAST and DESI data may become a new guideline for studying the gas cycle in galaxies and the causes of a long-term decrease in star formation activity.