For most of us, dust is just something we have to clean up. For astronomers, interstellar dust is a hindrance when they want to study distant objects. However, recent James Webb Space Telescope (JWST) observations of a distant galaxy are changing that. This infrared-sensitive observatory is letting them find a way to use dust to understand the evolution of early galaxies. In addition, it uncovered a special property of that galaxy's ice-covered dust, indicating it could be similar to the materials that formed our Solar System.
The galaxy is called SSTXFLS J172458.3+591545 and lies well outside our own "local" part of the Universe, at a distance of some 5 billion light-years. That means we're seeing it as it was about the same time that our own Solar System began forming.
SSTXFLS J172458.3+591545 contains molecules in solid form, such as carbon dioxide ice, carbon monoxide ice, and water ice on the dust grains it contains. They're almost exactly what we see in our galactic neighborhood, so that gives astronomers a better idea of conditions beyond our local neighborhood. “If the properties of the dust in distant galaxies are similar to those of our own Milky Way, then we expect the properties of their planets to be similar, too," said Tufts University astronomer Anna Sajina. “So, five billion years in the past, if planets are forming in these distant galaxies, they would have the same raw materials to start with.”
Understanding the Role of Dust
Sajina, who heads up a team that used JWST to study SSTXFLS J172458.3+591545, focuses on galaxy regions where dust blocks the view of star- and black hole formation. Both processes are an important part of galaxy evolution. Star formation populates a galaxy and uses up the clouds of gas and dust it contains. Black holes form at the hearts of galaxies, and astronomers are still figuring out the sequence of events that lead to their creation and evolution as galaxies merge and grow. The presence of dust hides that activity, but its characteristics and distribution also contain clues to the same events.
Our own Milky Way is a dusty place, and that dust keeps us from seeing the center of our galaxy. NASA's infrared-sensitive Spitzer Space Telescope observed the galactic center and shows us dust clouds being lit up by young stars. Now, JWST can focus its infrared spectrograph instrument (MIRI) on distant dust clouds to study their composition. Courtesy NASA/JPL-Caltech
The dust in galaxies consists of tiny particles of elements such as iron, silicon, and carbon. Starbirth crèches are filled with that material, along with clouds of gas molecules. While that rich mix of materials provides a lot of "seed material" for stars, it also blocks the view of the actual starbirth, as well as other interesting objects and events. The dust also absorbs starlight and then re-radiates it as infrared light. We can't see in the infrared, but JWST can. The data it gathers reveal information about starbirth and other activities and objects that emit in the infrared. That's why it's helpful in dust studies, according to Sajina. "Much of astronomers’ understanding of star formation relies on the ability to correct for dust obscuration,” she explained. “To correct for that, you have to make some assumptions about the properties of the dust.”
Detecting and Understanding Dust
Studying dust in distant galaxies is difficult and complex, and such observations beyond the Milky Way are pretty rare. There are several reasons for this. Dust absorbs visible light, which makes it difficult to see, even in nearby regions. Until the advent of infrared-sensitive instruments in space, telescopes on Earth had to contend with observations through our atmosphere, which absorbs and interferes with infrared light transmission. In addition, astronomers had fewer tools to observe the dust. That's why they had to assume that the properties of dust in other galaxies are at least similar to the dust we observe here, "at home." Finally, light from very distant objects is redshifted into the infrared part of the spectrum. So, that makes it more difficult to determine anything about the properties of dust clouds at cosmological distances.
Sajina's team is the first to push beyond the local Universe and so far into the past in a quest to understand interstellar dust in early galaxies. "The spectral detail is so much better that we can understand more about the chemistry that goes on the surfaces of these grains," she said.
This HST image shows how a cloud of gas and dust is obstructing the view of a "dense core" region in the constellation Serpens. This is a hidden stellar birthplace in the Milky Way Galaxy. ESA/Hubble, NASA & STScI, C. Britt, T. Huard, A. Pagan
Dust in SSTXFLS J172458.3+591545 and Beyond
The presence of the iced-over dust particles in SSTXFLS J172458.3+591545 shows that the galaxy region where they formed is populated by dense, gas-dust clumps. They belie the presence of a compact galaxy nucleus. There are several reasons why this might be so, but the most likely explanation is that dust clumps in a galaxy nucleus, and especially in an active one, provide an indirect method to determine the presence of a central black hole. Most galaxies have supermassive black holes at their hearts, and dust clumps can exist nearby or even move into orbit in the accretion disk of the black hole. Infrared studies have shown that dust in a disk or torus around the black hole isn't always a smooth distribution. It can and does accrete into clumps.
The JWST findings released by Sajina's team are the first combined detections of carbon dioxide, carbon monoxide, and other dust-rich ices in a very distant star-forming galaxy. Their data show that JWST's MIRI instrument will be a valuable tool as astronomers study the dust aggregations in other galaxies in both the near and early Universe. In particular, they should also reveal new details about their galaxies and the star and planet formation that takes place in their starbirth regions.
For More Information
Peeking Through Space Dust to See How the Ancient Universe Formed
Halfway to the Peak: ice absorption bands at z ˜ 0.5 with JWST MIRI/MRS
