The James Webb Space Telescope, the most powerful space observatory ever built, has made a surprising discovery in its first glimpse of the distant universe. It has spotted six galaxies that are so massive and so old that they challenge our current understanding of how galaxies formed in the early cosmos.
The six galaxies were observed as they were when the universe was only about 5% of its current age, about 13.5 billion years ago. They formed within the first 700 million years after the Big Bang, a period known as the cosmic dawn, when the first stars and galaxies lit up the dark and cold universe.
“These galaxies are so massive that they break our understanding of how structure forms in the early universe,” said Ivo Labbé, an astronomer at Swinburne University of Technology in Australia and lead author of a study published in Nature on February 22, 2023.
The galaxies are also very bright, emitting more light than 100 trillion suns. This suggests that they were undergoing intense star formation, producing new stars at a rate hundreds or thousands of times faster than our own Milky Way galaxy.
But how did these galaxies grow so big and so fast in such a short time? One possible explanation is that they were located in regions of high density of dark matter, the mysterious invisible substance that makes up most of the matter in the universe. Dark matter acts as a gravitational seed for star formation, attracting normal matter and gas to form stars and galaxies.
“These observations are giving us our first glimpse into how galaxies looked when they were forming some of the first stars and black holes,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate.
However, another study from the Cosmic Dawn Center suggests that there may be more to these galaxies than meets the eye. The study, led by Clara Giménez Arteaga, a Ph.D. student at the center, found that if the galaxies are not treated as one big blob of stars, but as an entity made up of multiple clumps, they may be even more massive than previously estimated.
“We used the standard procedure to calculate stellar masses from the images that James Webb has taken, but on a pixel-by-pixel basis rather than looking at the whole galaxy,” said Giménez Arteaga. “In principle, one might expect the results to be the same: Adding the light from all pixels and finding the total stellar mass, versus calculating the mass of each pixel and adding all individual stellar masses.”
But what she found was that some pixels had much more light than others, indicating that some regions of the galaxies were more compact and dense than typical galaxies at that time. This could imply that these galaxies had more stars than expected from their light emission, and thus were more massive.
“This is a very exciting discovery because it shows that we can use Webb to study these ancient galaxies in unprecedented detail,” said Rychard Bouwens, an astronomer at Leiden University in the Netherlands and a co-author of both studies.
The James Webb Space Telescope, which launched in December 2021, is designed to observe the infrared light from the most distant and oldest objects in the universe. It has a primary mirror that is 6.5 meters (21 feet) across, about seven times larger than that of its predecessor, the Hubble Space Telescope. It also has a suite of four sophisticated instruments that can capture images and spectra of cosmic phenomena.
“We’re seeing things we didn’t expect to see,” said Garth Illingworth, an astronomer at the University of California, Santa Cruz and a member of the Webb team. “These galaxies are opening a new window into the early universe and revealing new mysteries to explore.”