Recent observations made using the James Webb Space Telescope have surprisingly revealed an excess of massive galaxies in the universe, already appearing in the first half-billion years after the Big Bang. While these findings challenge existing knowledge about the formation of the first galaxies, Hebrew University researchers now propose a simple solution to this conundrum.
According to the proposed model, the unique conditions that prevailed in the first galaxies, characterized by high density and low concentration of heavy elements, allowed for efficient star formation without interference from other stars.
The new theoretical model, published by Professor Avishai Dekel together with his colleagues Professor Yuval Birnboim, Dr. Nir Mandelker, Dr. Kartick Sarkar and Dr. Zhaozhou Li, all of the Racah Institute of Physics at the Hebrew University of Jerusalem, was recently accepted for publication in the journal of the Royal Astronomical Society.
“As early as the first half-billion years, we identified galaxies, each containing about 10 billion stars, such as our sun,” Dekel said. “This startling discovery has led researchers to consider various explanations for this phenomenon, ranging from the possibility that estimates of the number of stars in galaxies are exaggerated, to the need for critical changes in the standard cosmological model of the Big Bang.”
According to the accepted theory of galaxy formation, supported by observations of the latest universe and computer simulations, gravitational forces cause gas dispersed throughout the universe to collapse into huge spherical clouds of dark matter, where it transforms into bright stars, such as our sun. This is how galaxies were formed throughout the universe, including the Milky Way galaxy, where our solar system is located. However, theory and observations have so far shown that the efficiency of star formation in galaxies is low, with only about 10% of the gas flowing in the clouds actually turning into stars.
“Most of the gas doesn’t form stars because it heats up and is even blown out of galaxies by winds and supernova explosions from stars that manage to form first,” Dekel explained.
If so, how is it possible that there were so many stars in galaxies during the early universe?
According to the Hebrew University researchers, the unique conditions that prevailed in the first galaxies allowed all the gas to efficiently transform into stars without being disturbed by stellar winds or supernova explosions. That’s because such outbursts occur only two million years after the outbreak of star formation, after massive stars have exhausted their central fuel (mostly hydrogen) and ended their lives in explosions. When the abundance of heavy elements is low, pre-explosion winds also allow for a similar “window of opportunity” of about two million years before they become effective.
‘In the early stages of the universe, star-forming clouds were denser, allowing for rapid collapse of the gas within them and its transformation into stars in less than a million years, within the window of opportunity,’ he said. said Dekel. “This has naturally led to the high efficiency of star formation from gas without disruption from explosions or winds from older stars.”
The researchers show that this phenomenon occurred naturally only during the earliest period, less than 600 million years after the Big Bang, and was limited to galaxies with more than 10 billion solar masses. According to Dekel, each of these galaxies should contain about 10,000 clusters of one million solar masses each. The researchers refer to the proposed process as a “Feedback-Free Starburst” (FFB). The new model offers clear observational predictions, which are already starting to be confirmed by new observations from space telescopes.
The research findings also have significant implications for the evolution of galaxies across all time periods.
For example, the proposed scenario explains the efficient formation of black holes of intermediate size (about a thousand solar masses) in the centers of the first clusters. These black holes can later merge and grow into supermassive black holes (up to a billion solar masses), as observed in the centers of large galaxies about half a billion years later. This scenario can also explain the mystery of the existence of supermassive black holes, which is an intriguing topic awaiting its resolution in cosmological research.
“Our intention is to examine these implications and other important consequences in future studies,” concluded Dekel.
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