The James Webb Space Telescope first observed the chemical structure of carbon-rich dust at redshift ~7, roughly equivalent to one billion years after the birth of the Universe.
Similar observational signatures have been observed in the much later Universe, associated with complex carbon molecules known as polycyclic aromatic hydrocarbons (PAHs). However, it is unlikely that PAHs originated within the first billion years of cosmic time.
Thus, this observation suggests the exciting possibility that Webb could have been observing a different kind of carbon-based molecule: perhaps small grains similar to graphite or diamond created by the earliest stars or supernovae.
The formation of cosmic dust
This observation offers exciting avenues for exploring both the formation of cosmic dust and the earliest populations of stars in our Universe, and was made possible by Webb’s unprecedented sensitivity.
The seemingly empty spaces in our Universe are often not empty at all, but are occupied by clouds of gas and cosmic dust. This dust consists of grains of various sizes and compositions that are formed and ejected into space in a variety of ways, including during supernovae. This material is crucial to the evolution of the Universe, as dust clouds eventually form the birthplaces of new stars and planets.
However, it can also be an obstacle for astronomers: dust absorbs starlight at certain wavelengths, making some regions of space very difficult to observe. However, the advantage is that certain molecules very consistently absorb or otherwise interact with a particular wavelength of light.
This means that astronomers can get information about the composition of cosmic dust by observing the wavelengths of light it blocks.
The appearance of early carbon
An international team of astronomers used this technique, combined with the Webb’s extraordinary sensitivity, to detect the presence of carbon-rich dust particles only a billion years after the birth of the Universe.
Joris Wheatstock of the University of Cambridge, lead author of this paper, explains:
“CARBON-RICH DUST GRAINS MAY BE PARTICULARLY EFFICIENT AT ABSORBING ULTRAVIOLET LIGHT AT WAVELENGTHS AROUND 217.5 NANOMETERS, WHICH WE FIRST DIRECTLY OBSERVED IN THE SPECTRA OF VERY EARLY GALAXIES.”
This prominent feature of 217.5 nanometers has been previously observed in the much newer and localized Universe, both in our own Milky Way galaxy and in galaxies up to redshift ~ 3. It has been attributed to two different types of carbon-based substances: polycyclic aromatic hydrocarbons (PAHs) or nanoscale graphite grains.
Surfactants
Surfactants are complex molecules, and current models predict that several hundred million years must pass before they form. Therefore, it would be surprising if the team observed a chemical signature of a mixture of dust particles that included species that were unlikely to have formed yet. However, according to the science team, this result is the oldest and most distant direct signature for this particular type of carbon-rich dust.
The answer may lie in the details of what was observed. As already mentioned, the property associated with cosmic dust composed of surfactants and tiny graphite grains is 217.5 nanometers. However, the feature observed by the team actually peaked at 226.3 nanometers. A nanometer is a millionth of a millimeter, and this discrepancy of less than ten nanometers can be attributed to measurement error. In addition, it may also indicate a difference in the composition of the cosmic dust mixture of the early Universe that the team found.
“THIS SMALL SHIFT IN THE WAVELENGTH WHERE THE ABSORPTION IS STRONGEST SUGGESTS THAT WE MAY BE OBSERVING A DIFFERENT MIXTURE OF GRAINS, SUCH AS THOSE SIMILAR TO GRAPHITE OR DIAMOND,” ADDS WITSTOCK.
In his opinion, this could also potentially be created in a short period of time by Wolf-Rayet stars or a supernova explosion.
Diamond grains
The discovery of this feature in the early Universe is unexpected and allows astronomers to speculate on the mechanisms that could have created such a mixture of dust particles. This involves using existing knowledge from observations and models.
Witstock suggests that the diamond grains are formed as a result of a supernova ejection, as models have previously suggested that nanodiamonds can form in this way. Wolf-Rayet stars are proposed because they are extremely hot for the rest of their lives, and very hot stars tend to live and die young quickly; giving enough time for generations of stars to be born, live and die to distribute carbon-rich grains into the surrounding cosmic dust in less than a billion years.
The models also showed that carbon-rich grains can be produced by certain types of Wolf-Rayet stars, and just as importantly, that these grains can survive the violent death of these stars. However, fully explaining these results with the existing understanding of the early formation of cosmic dust is still a challenge. Thus, these results will be used to develop improved models and future observations.
Detailing the observations
Prior to Webb, observations of multiple galaxies had to be combined to get strong enough signals to infer the star population of galaxies and learn how their light is affected by dust absorption. Importantly, astronomers have been limited to studying relatively old and mature galaxies that have had a long time to form stars and dust. This limited their ability to truly identify the key sources of cosmic dust.
With the advent of Webb, astronomers can now make very detailed observations of the light from individual dwarf galaxies that have been observed over the first billion years of cosmic time. The Webb finally allows us to study the origin of cosmic dust and its role in the crucial first stages of galaxy evolution.
This discovery was made possible by the incomparable increase in the sensitivity of Webb’s near-infrared spectroscopy, and in particular its Near Infrared Spectrograph (NIRSpec). This is emphasized by team member Roberto Maiolino of the University of Cambridge and University College London.
NIRSpec
NIRSpec was created for the European Space Agency by a consortium of European companies led by Airbus Defense and Space (ADS), with NASA’s Goddard Space Flight Center providing its detector and microshutter subsystems.
The main goal of NIRSpec is to enable large-scale spectroscopic surveys of astronomical objects such as stars or distant galaxies. This is made possible by a powerful multi-object spectroscopy mode that uses microshuttering. This mode is capable of acquiring spectra of nearly 200 objects simultaneously in a field of view of 3.6 × 3.4 angular minutes – the first time this has been provided from space. This mode makes very efficient use of Webb’s precious observing time.
The team also plans to further study the data and results. The scientists also plan to continue their collaboration with theorists who model the formation and growth of dust in galaxies. Team member Irene Shivai of the University of Arizona/Center for Astrobiology (CAB) said:
“THIS WILL SHED LIGHT ON THE ORIGIN OF DUST AND HEAVY ELEMENTS IN THE EARLY UNIVERSE.”
Observations
The observations were made as part of the JWST Advanced Deep Extragalactic Survey, or JADES, which dedicated about 32 days of telescope operation to discovering and characterizing faint, distant galaxies. This program has contributed to the discovery of hundreds of galaxies that existed when the Universe was less than 600 million years old, including some of the most distant galaxies known today.
The sheer number and maturity of these galaxies far exceeded predictions derived from observations made before Webb‘s launch. This new result from the dust grains of the early Universe contributes to our growing and evolutionary understanding of the evolution of stellar populations and galaxies during the first billion years of cosmic time.
“This discovery suggests that newborn galaxies in the early Universe are evolving much faster than we ever expected,” adds team member Renske Smith of Liverpool John Moores University in the U.K. “Webb shows us the complexity of the earliest birthplaces of stars (and planets) that models have not yet explained.
