“Webb” rewrites astronomy textbooks

When the James Webb Space Telescope launched in late 2021, we expected stunning images and dramatic scientific results. So far, this powerful space telescope has lived up to our expectations. Webb showed us things about the early universe that we never imagined.

Some of these results are forcing astronomy textbooks to be rewritten.

Textbooks are updated regularly as new data comes through the scientific process. But rarely does new data arrive at the speed with which Webb produces it. The chapters on the early universe need significant updating.

At a recent seminar in Bern (Switzerland) International Institute of Space Science (ISSI) “Breakthrough 2024” a group of scientists summed up the work of the telescope to date. Their work is published in a new paper entitled “Webb's First Billion Years.” The list of authors is very long, and these authors did not fail to note that an even larger group of international scientists played a role. To build on Webb's observations and advance “a collective understanding of the evolution of the early Universe,” an international scientific community is needed, the authors write.

The early Universe is one of Webb's main scientific goals. Its infrared images reveal the light of ancient galaxies with greater clarity than any other telescope. The telescope was designed to directly address complex questions related to high-redshift objects in the Universe.

Webb helps us understand the following three big questions fundamental to cosmology.

What are the physical properties of the earliest galaxies?

The early Universe and its transformations are fundamental to our understanding of the modern Universe. 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 at z=11. Webb pushed back that boundary. Currently, observations of highly luminous objects have reached z=14.32. Astronomers believe that Webb will eventually be able to observe galaxies at z=20.

The first few hundred million years after the Big Bang are called the Cosmic Dawn. Webb showed us that ancient galaxies during the cosmic dawn were much more luminous and therefore larger than we expected. The galaxy discovered by the telescope at z=14.32, named JADES-GS-z14-0, has several hundred million solar masses. “This raises the question: How was nature able to create such a bright, massive and large galaxy in less than 300 million years?” — scientists participating in the JWST Advanced Deep Extragalactic Survey (JADES) program write in a NASA message.

It also turned out that they have different shapes, that they contain more dust than expected, and that they contain oxygen. 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 surprising and suggests that several generations of very massive stars had already lived out their lives before we saw this galaxy,” the researchers write in their report.

“Taken together, these observations tell us that JADES-GS-z14-0 is not like the types of galaxies predicted to exist in the early Universe by theoretical models and computer simulations,” they continued.

What is the nature of active galactic nuclei in early galaxies?

Active galactic nuclei (AGNs) are supermassive black holes (SMBHs) that actively accrete material and emit jets and winds.

Quasars are a subtype of AGNs that are extremely bright and distant, and observations of quasars indicate that SMBHs were present at the centers of galaxies as early as 700 million years after the Big Bang. But their origin remained a mystery. Astrophysicists believe that these early SMBHs were created from black hole “seeds” that were either “light” or “heavy.” The light seeds had a mass of 10 to 100 solar masses and were stellar remnants. The heavy seeds had a mass of 10 to 10^5 solar masses and resulted from the direct collapse of gas clouds.

Webb's ability to effectively peer into the past allowed it to detect an ancient black hole at z=10.3 containing between 10^7 and 10^8 solar masses. The Hubble Space Telescope did not allow astronomers to measure the stellar mass of entire galaxies the way Webb does. Thanks to Webb's capabilities, astronomers know that a black hole with z=10.3 has approximately the same mass as the entire stellar mass of its entire galaxy. This is in stark contrast to modern galaxies, where the black hole's mass makes up only about 0.1% of the total stellar mass.

Such a massive black hole, which existed only about 500 million years after the Big Bang, is evidence that early black holes arose from heavy seeds. This is consistent with theoretical predictions. Thus, textbook authors can now eliminate uncertainty.

When and how did the early Universe become ionized?

We know that in the early Universe, hydrogen became ionized during the era of reionization. Light from the first stars, accreting black holes, and galaxies heated and reionized hydrogen gas in the intergalactic medium (IGM), eliminating the dense, hot, primordial fog that filled 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 began to expand until the entire Universe was ionized.

This was a critical stage in the development of the Universe. It allowed future galaxies, especially dwarf ones, to cool their gas and form stars. But scientists do not know exactly how black holes, stars and galaxies contributed to reionization and in what time frame it occurred. “We know that hydrogen reionization occurred, but exactly when and how it occurred is the major 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, at redshift z=5-6. But before the advent of Webb, it was difficult to measure the properties of the ultraviolet radiation that caused it. Thanks to Webb's enhanced spectroscopic capabilities, astronomers were able to narrow down the reionization parameters. “We found spectroscopically confirmed galaxies down to z = 13.2, suggesting that reionization may have begun just a few hundred million years after the Big Bang,” the authors write.

Webb's results also indicate that accreting black holes and their AGNs likely contributed no more than 25% of the ultraviolet radiation that caused the reionization.

These results will require some revision of textbook chapters on reionization epochs, although questions about this still remain. “There is still considerable debate about the primary sources of reionization, in particular about the contribution of faint galaxies,” the authors write. Even despite the Webb's extraordinary power, some distant and weak objects remain beyond its reach.

The Webb telescope is not even halfway through its journey and has already changed our understanding of the first billions of years of the Universe. It was created to solve questions related to the era of reionization, the first black holes, the first galaxies and stars. Of course, there is still a lot of interesting things ahead. Who knows what the total amount of his contribution will be?

As an astronomy writer, I am extremely grateful to all the people who brought Webb to life. It took a long time to build, cost far more than expected, and was almost canceled by Congress. Its perilous path to completion makes me even more grateful to be covering its results. The researchers using Webb's data are certainly grateful, too.

“We dedicate this work to the 20,000 people who have spent decades making Webb an incredible discovery machine,” they write.