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Among the most fundamental questions in astronomy is: How did the first stars and galaxies form? NASA’s James Webb Space Telescope is already providing new insights into this question. One of the largest programs in Webb’s first year science is the JWST Advanced Deep Extragalactic Survey, or JADES, which will devote about 32 days of telescope time to discovering and characterizing faint and distant galaxies. While the data is still coming in, JADES has already discovered hundreds of galaxies that existed when the universe was less than 600 million years old. The team also identified glittering galaxies with a multitude of hot, young stars.
These results were reported at the 242nd meeting of the American Astronomical Society in Albuquerque, New Mexico.
“With JADES, we want to answer many questions, such as: How did the first galaxies assemble? How fast did they form stars? Why do some galaxies stop forming stars?” said Marcia Rieke of the University of Arizona at Tucson, co-lead of the JADES program.
Ryan Endsley of the University of Texas at Austin conducted a survey of galaxies that existed 500 to 850 million years after the Big Bang. This was a pivotal period known as the Reionization Era. For hundreds of millions of years after the Big Bang, the universe was filled with a gaseous fog that made it opaque to energetic light. A billion years after the Big Bang, the fog had cleared and the universe had become transparent, a process known as reionization. Scientists debated whether active, supermassive black holes or galaxies filled with hot, young stars were the main cause of reionization.
As part of the JADES program, Endsley and his colleagues studied these galaxies with Webb’s near-infrared spectrograph (NIRSpec) instrument to look for signatures of star formation and found them in abundance. “Nearly every single galaxy we’re finding shows these unusually strong emission line signatures that indicate intense recent star formation. These early galaxies were very good at creating hot, massive stars,” Endsley said.
These bright, massive stars emitted torrents of ultraviolet light, which turned the surrounding gas from opaque to transparent by ionizing atoms, stripping electrons from their nuclei. Because these early galaxies had such a large population of hot, massive stars, they may have been the primary driver of the reionization process. The subsequent reunion of the electrons and nuclei produces the distinctly strong emission lines.
Endsley and his colleagues also found evidence that these young galaxies went through periods of rapid star formation interspersed with quiet periods of fewer stars forming. These bouts and initiations may have occurred as galaxies captured clusters of the gaseous raw materials needed to form stars. Alternatively, because massive stars explode rapidly, they may have periodically injected energy into their surroundings, preventing the gas from condensing to form new stars.
The early universe revealed
Another element of the JADES program involves the search for the first galaxies that existed when the universe was less than 400 million years old. By studying these galaxies, astronomers can explore how star formation in the early years after the Big Bang was different from what is seen today. Light from distant galaxies is stretched to longer wavelengths and redder colors by the expansion of the universe, a phenomenon called redshift. By measuring a galaxy’s redshift, astronomers can find out how far away it is and, therefore, when it existed in the early universe. Before Webb, there were only a few dozen galaxies observed above a redshift of 8 when the universe was less than 650 million years old, but JADES has now discovered nearly a thousand of these extremely distant galaxies.
The gold standard for determining redshift is to look at a galaxy’s spectrum, which measures its brightness at a myriad of close wavelengths. But a good approximation can be determined by taking photos of a galaxy using filters that each cover a narrow band of colors to get a handful of brightness measurements. In this way, researchers can determine estimates for the distances of many thousands of galaxies simultaneously.
Kevin Hainline of the University of Arizona in Tucson and his colleagues used Webb’s near-infrared camera (NIRCam) instrument to obtain these measurements, called photometric redshifts, and identified more than 700 candidate galaxies that existed when the he universe was between 370 million and 650 million years old. The sheer number of these galaxies was far beyond the predictions of observations made before Webb’s launch. The observatory’s extraordinary resolution and sensitivity are enabling astronomers to get a better view of these distant galaxies than ever before.
“Previously, the first galaxies we could see looked like little blobs. Yet those blobs represent millions or even billions of stars at the beginning of the universe,” Hainline said. “Now we can see that some of them are actually extended objects with visible structure. We can see clusters of stars being born just a few hundred million years after the beginning of time.”
“We’re finding that star formation in the early universe is much more complicated than we thought,” added Rieke.
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