of The James Webb Space Telescope (JWST) has discovered what may be the earliest star clusters in the universe.
JWST spotted the five proto-globular clusters – bundles of millions of stars bound together by gravity – within the Cosmic Gems arc, a galaxy that formed just 460 million years after Big Bang.
The Cosmic Gems arc gets its name from its appearance: When seen from our solar system, the star-studded galaxy looks like a hair-thin crescent due to the powerful gravitational pull of a foreground galaxy, which magnifies and distorts the view of the distant galaxy.
The galaxy is the most massive region seen in the first 500 million years of our universe, giving astronomers an unprecedented window into how the tumult of the first stars sculpted galaxies during the cosmic dawn.
The cosmic dawn is the time spanning the first billion years of the universe. Approximately 400 million years after the Big Bang, the era of reionization began, in which light from newborn stars stripped hydrogen of their electrons, leading to a fundamental reshaping of galactic structures.
“The early universe is nothing like we expected,” the first author of the study Angela Adamo, an astronomer at Stockholm University, told Live Science. “Galaxies are brighter, they form stars at high rates, and they do so in massive and dense star clusters. We are building a new understanding of how early galaxies formed.”
The researchers published their findings June 24 in the journal Nature.
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Lights flash in cosmic fall
As stars form, they push the material out in the form of winds and jets of ionized plasma – a process known as stellar feedback.
“To form these 5-star clusters, this small galaxy had to do so with very high efficiency,” Adamo said. “Stellar reactions from stars in star clusters must have been extraordinary.”
Scientists discovered the Cosmic Gems arc in 2018 using the Hubble Space Telescope. Usually, galaxies from such an early time emit light that is too faint to be detected by telescopes. But a phenomenon called gravitational lensing can help astronomers see them.
As described by Einstein in his the theory of general relativity, gravity is the bending and distortion of space-time in the presence of matter and energy. This curved space, in turn, sets the rules for the movement of energy and matter.
This means that even though light travels in a straight line, light can be bent and magnified by the presence of gravity. In this case, the galaxy SPT-CL J0615-5746 sits between the Cosmic Gem Arc and our solar system, bending and magnifying the early galaxy’s light so it can be seen with a telescope.
By guiding JWST into this curved region of space, astronomers observed the arc of cosmic gems in unprecedented detail, resolving the five globular clusters nestled within. They found that the clusters were incredibly dense, being roughly three orders of magnitude denser than the star-forming regions observed closer to Earth.
The clusters are among the first ever observed. But it is still unclear whether they are the first to exist, Adamo said.
“In principle, I would expect star formation to occur in a clustered manner even in fairly ancient galaxies,” she added. “But to form [massive] proto-globular clusters, the host galaxy must be able to create and retain sufficient mass in the gas. So it all depends on how fast the primordial galaxies can grow.”
To learn more about the first cosmic embers in the region, the researchers will continue with a spectroscopic analysis using JWST. This will enable astronomers to reconstruct the physical properties of the clusters, further constrain their ages, and trace the impact the cluster’s stars had on their wider galaxy.
