The universe was making carbon dust earlier than once thought

Carbon dust perfused young galaxies earlier than scientists once thought, according to new observations by the James Webb Space Telescope (JWST) (Nature 2023, DOI: 10.1038/s41586-023-06413-w).

Cosmic dust grains are the building blocks of galaxies, and studying their formation is an important step toward understanding how the universe evolved, says University of Cambridge astrophysicist Joris Witstok. There’s evidence from the Hubble Space Telescope and instruments that dust reservoirs were present just 600 million years after the Big Bang.

Though scientists have not made direct measurements of the composition of these first dust clouds, they mostly ruled out carbon as a constituent. That’s because carbon-rich dust in the Milky Way and nearby galaxies is primarily associated with dying stars that emit polycyclic aromatic hydrocarbons (PAHs) and graphite particles like a sputtering candle releasing puffs of smoke. It was unclear how the infant universe could have generated carbonaceous dust so soon after the Universe began, he says.

Witstok and his colleagues hoped that powerful spectroscopy instruments on JWST could help unpick the chemical complexity of this early Universe dust. When the team trained the telescope’s Near Infrared Spectrograph (NIRSpec) on a galaxy appearing as it was less than 1 billion years after the Big Bang, they were surprised to find a broad absorption signal in the ultraviolet (UV) range around 2175 Å. This feature, called the UV attenuation “bump”, is caused by carbonaceous dust. Witstok and his colleagues went on to find the same bump in spectra from around 50 galaxies of a similar age.

Justin Spilker, an astrophysicist at Texas A&M University who was not involved in the study, says it is interesting to see this feature broadly across multiple galaxies. “That tells us that forming dust grains very rapidly must not be a very difficult task, because it happens very often in these early-universe galaxies,” he writes in an email. “We’ve seen spectral features of ionized carbon in even earlier galaxies than this work—but the impressive part is that the carbon is able to cool and collect enough to form into complex carbonaceous molecules and dust grains,” he writes.

The finding adds to growing evidence from JWST that the universe must have other ways to produce carbon-based materials than previously thought, such as ejections from supernovae or fast-growing stars, says Witstok.