When the James Webb Space Telescope launched in late 2021, we expected stunning images and enlightening scientific results. So far, the powerful space telescope has lived up to our expectations. JWST has shown us things about the early Universe that we never anticipated.
Some of these results are forcing a rewrite of astronomy textbooks.
Textbooks are regularly updated as new evidence works its way through the scientific process. But new evidence rarely arrives at the rate JWST is delivering. The chapters on the Early Universe need significant updating.
At the recent International Space Science Institute (ISSI) Discovery Workshop 2024 in Bern, Switzerland, a group of scientists summarized some of the telescope’s results so far. Their work is in a new paper titled “The first billion years, according to JWST.” The list of contributors is long, and these authors are quick to point out that an even larger group of international scientists have played a role. An international scientific community is needed to use the JWST observations and advance “the collective understanding of the evolution of the Early Universe,” as the authors write.
The early universe is one of JWST’s primary science objectives. Its infrared capabilities allow it to see light from ancient galaxies with greater sharpness than any other telescope. The telescope was designed to directly address puzzling questions about the high-redshift Universe.
The following three broad questions are fundamental issues in cosmology that JWST is addressing.
What are the physical properties of the earliest galaxies?
The early universe and its transformations are fundamental to our understanding of the Universe that surrounds us today. Galaxies were in their infancy, stars were forming and black holes were forming and becoming more massive.
The Hubble Space Telescope was limited to observations around z=11. JWST has pushed that limit aside. Its current high-redshift observations have reached z=14.32. Astronomers think JWST will eventually observe galaxies at z=20.
The first few hundred million years after the Big Bang is called the Cosmic Dawn. JWST showed us that ancient galaxies during the Cosmic Dawn were much brighter and, therefore, larger than we expected. The galaxy found by the telescope at z=14.32, called JADES-GS-z14-0, has several hundred million solar masses. “This raises the question: How could nature create such a bright, massive and large galaxy in less than 300 million years?” Scientists involved with the JWST Advanced Deep Extragalactic Survey (JADES) said in a NASA post.
It also showed us that they had different shapes, that they contained more dust than expected, and that oxygen was present. The presence of oxygen indicates that generations of stars had already lived and died. “The presence of oxygen so early in the life of this galaxy is a surprise and suggests that multiple generations of very massive stars had already lived out their lives before we observed the galaxy,” the researchers wrote in the post.
“All these observations, together, tell us that JADES-GS-z14-0 is not like the types 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 accreting material and emitting jets and winds.
Quasars are a subtype of AGN that are extremely bright and distant, and observations of quasars show that SMBHs were present in the centers of galaxies as early as 700 million years after the Big Bang. But their origin was a mystery. Astrophysicists think that these early SMBHs were created from “seeds” of black holes that were either “light” or “heavy”. Light seeds were about 10 to 100 solar masses and were stellar debris. Heavy seeds were 10 to 105 solar masses and came from the direct collapse of gas clouds.
JWST’s ability to effectively look back in time has allowed it to spot an ancient black hole around z=10.3 that contains between 107 up to 108 solar masses. The Hubble Space Telescope did not allow astronomers to measure the stellar mass of entire galaxies the way JWST does. Thanks to the power of JWST, astronomers know that the black hole at z=10.3 has about the same mass as the stellar mass of its entire galaxy. This is in stark contrast to modern galaxies, where the mass of the black hole is only about 0.1% of the entire stellar mass.
A black hole this massive existing only about 500 million years after the Big Bang is proof that early BHs originated from heavy seeds. This is actually consistent with theoretical predictions. So the text writers are now in a position to remove the uncertainty.
When and how was the early universe ionized?
“We know hydrogen reionization happened, but exactly when and how it happened has been a big missing part of our understanding of the first billion years.”
From “The First Billion Years According to JWST”.
We know that in the early Universe, hydrogen was ionized during the Epoch of Reionization (EoR). Light from the first stars, accreting black holes, and galaxies heated and reionized the hydrogen gas in the intergalactic medium (IGM), removing the dense, hot, primordial haze that covered the early Universe.
Young stars were the main source of light for reionization. They created expanding bubbles of ionized hydrogen that overlapped each other. Eventually, the bubbles expanded until the entire Universe was ionized.
This was a critical 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 the exact time frame in which it happened. “We know hydrogen reionization happened, but exactly when and how it happened has been a big missing piece in our understanding of the first billion years,” write the authors of the new paper.
Astronomers knew that reionization ended about a billion years after the Big Bang, around redshift z=5-6. But before JWST, it was difficult to measure the properties of the UV light that caused it. With JWST’s advanced spectroscopic capabilities, astronomers have narrowed down the reionization parameters. “We have found spectroscopically confirmed galaxies up to z = 13.2, implying that reionization may have started only a few hundred million years after the Big Bang,” the authors write.
The JWST results also show that the accretion of black holes and their AGN likely contributed no more than 25% of the UV light that caused the reionization.
These results will require some rewriting of textbook chapters on EOR, although there are still lingering questions about it. “There is still significant debate about the main sources of reionization, in particular, the contribution of faint galaxies,” the authors write. Although JWST is extremely powerful, some distant and faint objects are beyond its reach.
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 address questions about the age of reionization, the first black holes, and the first galaxies and stars. Of course there is much more to come. Who knows what the total amount of her contributions will be?
As an astronomy writer, I am extremely grateful to all the people who made JWST happen. It took too long to build, cost much more than expected, and was almost canceled by Congress. Her perilous path to completion makes me even more grateful to be covering her results. Researchers using JWST data are also clearly grateful.
“We dedicate this paper to the 20,000 people who spent decades making JWST the incredible discovery machine it is,” they write.