Observations with the Webb Space Telescope have identified galaxies in the most distant and early universe that glow with a multitude of hot, young stars.
These results are reported at the 242nd meeting of the American Astronomical Society, in Albuquerque, New Mexico. Ryan Endsley of the University of Texas at Austin led research on galaxies that existed between 500 million and 850 million years after the Big Bang. This was a pivotal moment known as the epoch of reionization. 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 lifted and the universe became transparent, a process known as reionization. Scientists have debated whether active supermassive black holes or galaxies filled with hot, young stars were the primary cause of reionization.
As part of the JADES (Advanced Deep Extragalactic Survey) program — a major one in Webb’s first year of science — Endsley and his colleagues studied these galaxies with Webb’s NIRSpec (Near-Infrared Spectrograph) instrument to look for signs of star formation, and found them in abundance. “Almost all of the galaxies we are finding show these unusually strong emission line signatures that indicate recent intense star formation. These early galaxies were very good at creating hot, massive stars,” Endsley said in a NASA statement. These massive, bright stars emitted torrents of ultraviolet light, which turned the surrounding gas from opaque to transparent by ionizing atoms, removing electrons from their nuclei. Since these early galaxies had such a large population of hot, massive stars, they may have been the main driver of the reionization process. The subsequent reunion of the electrons and nuclei produces the distinctively strong emission lines.
Endsley and his colleagues also found evidence that these young galaxies experienced periods of rapid star formation interspersed with quiet periods in which fewer stars formed. These fits and starts may have occurred when galaxies captured clumps of the gaseous raw materials needed to form stars. Alternatively, since massive stars explode quickly, they may have injected energy into the surrounding environment periodically, preventing gas from condensing to form new stars. Another element of the JADES program involves searching 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 first few 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 tell how far away it is, and therefore when it existed in the early universe. Before Webb, only a few dozen galaxies were 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 by looking at the spectrum of a galaxy, which measures its brightness at a myriad of closely spaced wavelengths. But a good approximation can be determined by taking pictures of a galaxy using filters that cover a narrow band of colors to get a handful of brightness measurements. In this way, researchers can determine estimates of the distances of many thousands of galaxies at a time.
Kevin Hainline of the University of Arizona in Tucson and his colleagues used Webb’s NIRCam (Near Infrared Camera) instrument to obtain these measurements, called photometric redshifts, and identified more than 700 candidate galaxies that existed when the universe was between 370 million and 650 million. years. The sheer number of these galaxies was far beyond the predictions of observations made before Webb’s launch. The observatory’s resolution and sensitivity are allowing astronomers to get a better view of these distant galaxies than ever before.