When the James Webb Space Telescope launched in late 2021, we expected stunning images and illuminating science results. So far, the powerful space telescope has met our expectations. The JWST has shown us things about the early universe that we never expected.
Some of these results force a rewriting of astronomy textbooks.
Textbooks are updated regularly as new evidence works its way through the scientific process. But rarely does new evidence come at the speed at which the JWST provides it. Chapters on the early universe need a major update.
At the recent International Space Science Institute (ISSI) Breakthrough Workshop in Bern, Switzerland, a group of scientists summarized some of the telescope’s results to date. Their work is featured in a new article titled “The First Billion Years, According to JWST.” The list of authors is long, and those authors are quick to point out that an even larger group of international scientists played a role. It will take an international scientific community to use JWST observations and advance the “collective understanding of the evolution of the early universe,” as the authors write.
The early universe is one of the main scientific goals of the JWST. Its infrared capabilities allow it to see the light from ancient galaxies more clearly than any other telescope. The telescope is designed to directly answer confusing questions about the high-redshift universe.
The following three broad questions are fundamental issues in the cosmology that the JWST addresses.
What are the physical properties of the earliest galaxies?
The early universe and its transformations are fundamental to our understanding of the universe around us. Galaxies were still in their infancy, stars were forming and black holes were forming and becoming more and more massive.
The Hubble Space Telescope was limited to observations at about z = 11. The JWST pushed that limit aside. The current high redshift observations have reached z=14.32. Astronomers believe that the JWST will eventually observe galaxies at z=20.
The first few hundred million years after the Big Bang are called the Cosmic Dawn. JWST showed us that ancient galaxies were much brighter and therefore larger than we expected during the Cosmic Dawn. The galaxy that the telescope found at z=14.32, called JADES-GS-z14-0, has several hundred million solar masses. “This begs the question: How can nature create such a bright, massive, and large galaxy in less than 300 million years?” scientists involved in JWST Advanced Deep Extragalactic Survey (JADES) said in a NASA post.
It also showed us that they were a different shape, that they contained more dust than expected and that oxygen was present. The presence of oxygen indicates that generations of stars have already lived and died. “The presence of oxygen so early in the life of this galaxy is a surprise and suggests that several generations of very massive stars had already lived out their lives before we observed the galaxy,” the researchers wrote in the post.
“Taken together, these observations tell us that JADES-GS-z14-0 does not resemble the kind of galaxies predicted by theoretical models and computer simulations to exist in the very early Universe,” they continued.
What is the nature of active galactic nuclei in early galaxies?
Active galactic nuclei (AGN) are supermassive black holes (SMBHs) that are actively collecting material and emitting jets and wind.
Quasars are a sub-type of AGN that are extremely luminous and distant, and quasar observations show that SMBHs were present at the centers of galaxies as early as 700 million years after the Big Bang. But their origin was a mystery. Astrophysicists think these early SMBHs formed from “seeds” of black holes that were “light” or “heavy.” Light seeds had a mass of about 10 to 100 solar masses and were stellar remnants. Heavy seeds had 10 to 105 solar masses and arose from the direct collapse of gas clouds.
The JWST’s ability to effectively look back in time has made it possible to discover an ancient black hole at about z=10.3 that is between 107 to 108 solar masses. The Hubble Space Telescope did not allow astronomers to measure the stellar mass of entire galaxies in the way the JWST does. Thanks to the power of the JWST, astronomers know that the black hole at z=10.3 has approximately the same mass as the stellar mass of the entire galaxy. This is in stark contrast to modern galaxies, where the black hole mass is only about 0.1% of the total stellar mass.
Such a massive black hole that existed only about 500 million years after the Big Bang is evidence that early BHs originated in heavy seeds. This is actually consistent with theoretical predictions. So the authors of the textbook are now in a position to remove the uncertainty.
When and how was the early universe ionized?
“We know that hydrogen reionization occurred, but exactly when and how it happened is an important missing piece in our understanding of the first billion years.”
From ‘The first billion years according to the JWST.’
We know that hydrogen was ionized in the early universe during the epoch of reionization (EoR). Light from the first stars, black holes, and galaxies that heated and reionized the hydrogen gas in the intergalactic medium (IGM), removing the dense, hot, primordial fog that shrouded the early universe.
Young stars were the main source of light for the reionization. They created expanding bubbles of ionized hydrogen that overlapped. Eventually the bubbles expanded until the entire universe was ionized.
This was a crucial stage in the development of the universe. It allowed future galaxies, especially dwarf galaxies, to cool their gas and form stars. But scientists aren’t sure how black holes, stars and galaxies contributed to the reionization or exactly what time frame it took place. “We know that hydrogen reionization occurred, but exactly when and how it occurred has been an important missing piece in our understanding of the first billion years,” the authors of the new paper write.
Astronomers knew that reionization ended about a billion years after the Big Bang, at about redshift z=5-6. But before JWST, it was difficult to measure the properties of the UV light that caused this. With the advanced spectroscopic capabilities of the JWST, astronomers have narrowed down the parameters of reionization. “We found spectroscopically confirmed galaxies up to z = 13.2, implying that reionization started only a few hundred million years after the Big Bang,” the authors write.
JWST results also show that accreting black holes and their AGN likely contributed no more than 25% of the UV light that caused reionization.
These results require some rewriting of the textbook chapters on ERA, although questions remain. “There is still much debate about the main sources of reionization, in particular the contribution of faint galaxies,” the authors write. Although the JWST is extremely powerful, some distant, faint objects are beyond its reach.
The JWST is not even halfway through its mission and has already transformed our understanding of the first billion years of the universe. It was built to answer questions surrounding the era of reionization, the first black holes and the first galaxies and stars. There is certainly much more to come. Who knows what the sum of his contributions will be?
As an astronomy writer, I am deeply grateful to all the people who made the JWST come to fruition. Construction took a long time, cost much more than expected, and was almost canceled by Congress. The perilous path to its completion makes me even more grateful to be able to discuss its results. The researchers using JWST data are also clearly grateful.
“We dedicate this article to the 20,000 people who have worked for decades to make JWST an incredible discovery engine,” they write.