The Universe’s star-making boom period is already over, according to a new study, which suggests that the rate of star-formation in the Universe is around a hundred times lower than it was five billion years ago.
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Using observations made by the European Space Agency’s Herschel Space Observatory, Dr Dimitra Rigopoulou of the University of Oxford and her colleagues have found that most of the stars of our Universe were formed in a ‘baby boom’ period five to ten billion years ago.
“There is clear evidence that the galactic-scale physical processes that initiate the formation of stars in the most luminous galaxies in the Universe have changed,” Dr Rigopoulou said on the university’s blog.
Luminous galaxies far brighter than the Sun produce huge volumes of stars as they collide and merge with each other, compressing large quantities of gas into small compact regions to form new celestial bodies. But according to the study, just a few billion years ago, these luminous galaxies could make new stars without banging into each other.
“Normal disk galaxies are unperturbed systems that undergo a slow and steady evolution,” Rigopoulou explained. “So, by discovering normal disks with very high star formation rates, we have uncovered a fundamental change in the galactic-scale process of star formation in the most efficient star-forming galaxies of our Universe.”
Astronomers and physicists already have evidence that the Universe of ten billion years ago was vastly different to the one we observe today, but Herschel’s images suggest that things changed less than five billion years ago. The researchers searched the skies for ionised carbon, which is produced when the gas in a galaxy cools down and collapses, starting the star-making process.
Rigopoulou said they were surprised to discover that the level of ionised carbon in luminous galaxies five billion years ago was similar to the level of ten billion years ago, but completely different to today.
She has two potential explanations for the change – either the amount of gas available or the gas’ metallicity – the proportion of the matter made up of chemicals other than hydrogen and helium.
“As galaxies get older, they use up their gas to make stars so they run out of the raw material needed to create more stars,” she said.
“The availability of large gas reservoirs means that some galaxies can make stars efficiently without the need of interactions to trigger the star forming activity, as happens in local galaxies.
“Metallicity, on the other hand, is very closely related to star formation, so a change in the specific make-up of the gas can have a huge impact on the way star formation proceeds and hence affect a galaxy’s behaviour,” she added.
The team’s findings were published in the The Astrophysical Journal Letters.