Astronomy

JWST Shows How the Early Universe Was Furiously Forming Stars

We can gaze out into regions in our neighbourhood of the Milky Way and find orgies of star birth. The closest region is in the Orion nebula, where astronomers have identified more than 700 young stars. They range from only 100,000 years—mere infancy for a star—to over a million years.

But we’re more than 13 billion years after the Big Bang now. What was star formation like way back when, when conditions in the Universe were so different?

The JWST was conceived, designed, and launched to answer questions about the early Universe. The JWST Advanced Deep Extragalactic Survey (JADES) program is an imaging and spectroscopic deep-field survey of galaxies from around z ~ 12 to z ~ 2, about 370 million to 3.3 billion years after the Big Bang. Its goal is to understand the formation and evolution of those galaxies and their stars in the Universe’s early days, one of astronomy’s big questions.

“Previously, the earliest galaxies we could see just looked like little smudges. And yet those smudges represent millions or even billions of stars at the beginning of the universe.”

Kevin Hainline, University of Arizona in Tucson

JADES is revisiting a region of the sky familiar to astronomy buffs: the Hubble Ultra Deep Field (HUDF.) The HUDF is a deep-field image of a small region of the sky that took over 11 days of observations spread over months by the Hubble Space Telescope to capture. It contains about 10,000 galaxies, and the smallest, reddest ones are from when the Universe was only about 800 million years old.

The JADES program uses two of the JWST’s instruments together—NIRSpec (Near-Infrared Spectrograph) and NIRCam (Near-Infrared Camera)—to revisit the region in and around the HUDF in greater depth and detail. The effort took 770 hours of observing time. The JADES team has already identified hundreds of galaxies from when the Universe was less than 600 million years old. Some of them are alive with multitudes of hot, young stars.

Marcia Rieke of the University of Arizona in Tucson is the co-lead of the JADES program. “With JADES, we want to answer a lot of questions, like: How did the earliest galaxies assemble themselves? How fast did they form stars? Why do some galaxies stop forming stars?” Rieke said.

The background for these questions concerns the Universe’s Epoch of Reionization (EOR).

The EOR occurred within one billion years after the Big Bang. Prior to it, the Universe was filled with neutral hydrogen, an opaque gas impervious to light for hundreds of millions of years. Only once stars formed and reionized the hydrogen did the Universe become transparent.

This diagram of the evolution of the universe from the big bang to the present shows the epoch of reionization. Credit: NASA, ESA, and A. Feild (STScI)

Part of JADES focuses on galaxies that existed 500 to 850 million years after the Big Bang and during the EOR. Ryan Endsley of the University of Texas at Austin led that investigation. Endsley and his colleagues used the JWST’s NIRSpec instrument to search for signs of star formation in those galaxies. They found it in abundance.

“Almost every single galaxy that we are finding shows these unusually strong emission line signatures indicating intense recent star formation. These early galaxies were very good at creating hot, massive stars,” said Endsley.

Massive stars burn hot and send out potent UV radiation. The radiation ionized the hydrogen atoms, removing the electron and leading to the EOR. The Universe is now composed mostly of low-density ionized hydrogen and remains transparent and open to observation with our telescopes. Since the Universe’s early galaxies contained vast numbers of hot massive stars, they likely drove the reionization process.

The JADES team created an interactive tool to explore the JWST’s images. Lose yourself in it as you gaze at some of the first galaxies to ever form in the Universe. Image Credit: JADES

Endsley also found that these galaxies had periods of intense star formation and periods of weak star formation. Star formation could have risen dramatically when galaxies consumed clouds of star-forming gas. And the periods of depressed star formation could’ve been the result of the massive stars themselves. Massive stars consume their fuel quickly and explode as supernovae, which can drive energy into the surrounding gas, heating it and preventing it from condensing and forming new stars.

To understand the early Universe, astronomers have to observe it. This is the JWST’s forté. Part of JADES focuses on galaxies that existed when the Universe was less than 400 million years old. These galaxies can help answer the question of how different star formation was then compared to now. The light from those galaxies is redshifted because of the Universe’s expansion, and astronomers measure the redshift to learn a galaxy’s age and distance. A redshift of 8 indicates that a galaxy was around when the Universe was less than 650 million years old, only a couple hundred million years or so after the first stars and galaxies formed.

This slide from an AAS presentation highlights the region of study by the JWST Advanced Deep Extragalactic Survey (JADES). This area is in and around the Hubble Space Telescope’s Ultra Deep Field. The JWST images are both larger and deeper. Scientists used Webb’s NIRCam instrument to observe the field in nine different infrared wavelength ranges. From these images, the team searched for faint galaxies that are visible in the infrared but whose spectra abruptly cut off at a critical wavelength. Image Credit: Kevin Hainline/JADES

Prior to JADES, astronomers had identified only a few dozen galaxies from this time, but the JWST has now found nearly a thousand of them. This came as a surprise to astronomers because they had no idea that there were this many early galaxies. In fact, JADES has found 717 galaxies from when the universe was between 370 million and 650 years old, although many of them need further confirmation. It also found the most distant galaxy observed by humans so far, JADES-GS-z13-0, which has a redshift of 13.2 and a light travel time of 13.395 billion years. Not only has it found them, but JWST’s view of them is far more detailed than any previous observations.

“Over 93% of the sources in this study have never been seen before.”

Kevin Hainline, JADES, University of Arizona in Tucson

“Previously, the earliest galaxies we could see just looked like little smudges. And yet those smudges represent millions or even billions of stars at the beginning of the universe,” said Kevin Hainline, part of the team that used the JWST’s NIRCam to measure the galaxies’ redshifts. “Now, we can see that some of them are actually extended objects with visible structure. We can see groupings of stars being born only a few hundred million years after the beginning of time.”

Kevin Hainline presented some of JADES’ results at the AAS 242 meeting this week. His presentation begins at the 17:00 minute mark. He talks about the first 600 million years after the Big Bang. “It’s an amazing thing that we can even talk about that at all, thanks to the JWST,” Hainline said. “Over 93% of the sources in this study have never been seen before.”

What is the JWST’s view of these early stars telling astronomers? “We’re finding star formation in the early universe is much more complicated than we thought,” added Rieke.

One of JADES’ surprising results is a quiescent galaxy only 730 million years after the Big Bang. Quiescent galaxies no longer form stars after they run out of gas. How did this happen so soon?

What about black holes? What effect did they have on early galaxies? JADES will help find active galactic nuclei (AGN,) which are highly luminous when a galaxy’s black hole accretes material. What role did black hole feedback play in the early Universe? “Finding the connection between galaxy growth and black hole growth is a key driver, which we hope to elucidate when our samples are more complete,” the JADES team writes.

The Universe’s first stars and galaxies shaped the Universe into what it is now. By reionizing hydrogen, the first stars triggered the transition from opaque to transparent during the Epoch of Reionization, ending the Universe’s Dark Ages. But there are things we don’t know.

Exactly when did the first stars form? Why were there so many massive, hot stars compared to now? The first galaxies may have consisted mostly of dark matter, but not in haloes around modern galaxies. Instead, it was mixed in with regular matter. What role did it play?

The early Universe consisted of hydrogen, helium, and light. Somehow, these three things combined to create the complexity we see around us in our times. We have a long list of questions about what happened and how things became what they are today.

Astronomers don’t have answers to all these questions, and for a long time, observational evidence was scant, and it was left mostly to theory to figure it all out. The venerable Hubble Space Telescope helped, but now we have the JWST, our most powerful instrument yet for studying the early Universe. Whether JADES can generate answers to these questions is not certain yet. But don’t bet against it.

The JADES team is publishing 14 new papers, and those papers are helping astronomers find answers.

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Evan Gough

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