The more astronomers look at the early universe, the more discoveries they make. Some of these finds are changing what they thought they knew about the beginnings of the cosmos. For example, the James Webb Space Telescope (JWST) recently found evidence that carbon-based molecules and dust existed just a billion years after the Big Bang. It looks a little different than the dust observed later in the universe.
The discovery of JWST comes from an early galaxy survey called JADES (JWST Advanced Deep Extragalactic Survey). The survey spent 32 days of telescope time observing and characterizing faint early galaxies. The observed dust was in at least one of the hundreds of galaxies studied. It appears to be composed of graphite or diamond-like grains, of which we see plenty in later stages of cosmic history.
Their chemical signatures are remarkably similar to carbon-based molecules called “polycyclic aromatic hydrocarbons” (PAHs). These molecules are plentiful later in the universe, but are unlikely to have existed when the universe was only a billion years old. So how could the lookalikes of PAH molecules exist so early in history?
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Locate a source of these carbon-based molecules
Astronomer Joris Witsok, lead author of a paper describing the discovery, speculated that the diamond-like grains came from debris ejected in supernova explosions. “This could also potentially be produced by Wolf-Rayet stars or supernova ejections in a short period of time,” he explained.
Wolf-Rayet stars are known to be efficient dust producers, especially carbon-based molecules. Courtesy: Mid-Infrared Instrument (MIRI) at the James Webb Space Telescope.
These old, super-hot stars could be precursors to some types of supernova explosions. They would be the perfect cradles for making nanodiamonds and other carbon-based dusts. In fact, some models show that carbon-rich grains come from certain types of Wolf-Rayet stars. In addition, these grains can survive when the stars explode as supernovae. The next question is whether such stars existed in the early Universe to produce the dust observed by JWST. The dust certainly provides tantalizing clues about the earliest stellar populations in the Universe. That’s because those first stars were massive and exploded as supernovae.
The earliest formed when the universe was still quite young – perhaps a hundred million years after the Big Bang. Certainly there are the first galaxies about 400 million years after the Big Bang. The first stars were huge, crazy creatures made of hydrogen and helium. They lived short, fast lives and exploded as supernovae. These explosions may have been the earliest examples of dust in the universe. With more episodes of star formation in the earliest galaxies, dust accumulated, and that’s exactly what the JWST found. Carbon-based molecules and nanodiamonds require specific hot, energetic conditions that may have been provided by the earliest stars.
Artist’s rendering of the first generation of massive, luminous stars in the Universe. As they died, their supernova explosions created dust. Photo credit: NAOC
Carbon-based molecules and dust
Dust exists throughout the cosmos. Since it’s a product of stellar evolution, it’s no surprise to find the stuff in the early universe. It provides insights into stellar processes, but also hides a lot. For example, dust obscures our view of the core of the Milky Way and objects in the young Universe. Luckily, there are ways to “see through” it, and that’s exactly what JWST does.
The chemical analysis of the dust provides detailed information about its composition. Certain dust molecules interact with certain types of light. Astronomers use this property to find out what the dust is made of. That’s what Witstok’s team did with their JWST observations. “Carbon-rich dust grains are particularly efficient at absorbing ultraviolet light with a wavelength around 217.5 nanometers, which we observed for the first time directly in the spectra of very early galaxies,” he said of their observation.
The 217.50 nanometer absorbance function is a great tool for dust observation and plays an important role in observing PAH molecules across the universe. It identifies both PAH molecules and nanoscale graphite grains. It would be cool if PAHs existed early in cosmic history. However, their formation process is more associated with the formation of newborn stars and exoplanets. They were not observed until about two billion years after the Big Bang. Interestingly, PAHs are also among the basic chemical building blocks of life.
If not PAH, then what’s out there?
Interestingly, the features detected by the JWST actually peaked at 226.3 nanometers. This is not significantly different from the 217.5 nm measurement and could be a measurement error. However, it is also very possible that this tiny difference in wavelength indicates that the composition of early cosmic dust is only slightly different from the dust we see in later epochs. And that’s kind of exciting, according to Witstok. “This slight wavelength shift where the absorption is strongest suggests that we may be seeing a different mix of grains, say graphite-like or diamond-like grains,” he said. “This could possibly also be produced by Wolf-Rayet stars or supernova ejections in a short time.”
This image highlights the position of the galaxy JADES-GS-z6 in part of an area of sky known as GOODS-South observed by the JWST Advanced Deep Extragalactic Survey (JADES). These observations provided evidence for carbon-based molecules.
All this negatively affects further exploration of early galaxies. Before JWST, astronomers had to image several galaxies in the early Universe. Repeated observations provided enough information about these early stars and how dust absorption affects their light. However, this limited the observations to galaxies that have long been forming stars and dust. There hasn’t been much opportunity to study younger galaxies and stars to determine their dust production. JWST enabled observations of individual dwarf galaxies that existed in the first billion years of cosmic time. That gives them a window of opportunity to study the origins of cosmic dust when the universe was still in its infancy.
Of course, according to team member Irene Shivaei, we still have a lot of work to do. “We plan to continue collaborating with theorists who model dust production and growth in galaxies,” Shivaei said. “This will shed light on the origin of dust and heavy elements in the early universe.”
For more informations
Webb sees carbon-rich dust grains in the first billion years of cosmic time
Carbonaceous dust grains seen in the first billion years of cosmic time
Carbonaceous dust grains in galaxies seen in the first billion years of cosmic time (by arXiv)