When astronomers look at distant objects—far away in both space and time—all too often, dust gets in their eyes. Well, not eyes, exactly—in their telescopes’ line of vision. It’s hard to take a close look at a galaxy formed, say, some five billion years ago, because there’s a lot of particulate matter floating in the galaxy itself that blocks a clear view.

Now, using the James Webb Space Telescope (JWST), a Tufts astronomer and her colleagues have found that the ice-covered dust in a far-away galaxy is much like dust that is closer to us. Knowing that will allow astronomers to more accurately calibrate their calculations for measuring things like star and black hole formation in the earlier universe. Their research was published recently in the Astrophysical Journal.

The universe is thought to have formed some 14 billion years ago, and Anna Sajina, a professor of astronomy and astrophysics at Tufts, studies galaxy and black hole formation. 

The difficulty in studying those far distant time periods is that star and black hole formation is very heavily obscured by dust. In her research, Sajina focuses on dust-obscured systems and on “understanding how to correct for the effects of dust,” she says.

Dust absorbs starlight and re-emits it in the infrared spectrum, invisible to the human eye. Much of astronomers’ understanding of star formation “relies on the ability to correct for dust obscuration,” says Sajina. “To correct for that, you have to make some assumptions about the properties of the dust.” 

Up until now, there were very limited means to study the properties of the dust in distant galaxies. Instead, “you had to assume that the dust is the same as locally,” says Sajina. “But we didn’t know this for sure.”

The dust in between stars is made up of little grains of things like carbon, silicon, and iron. “Places where stars form are filled with cold, dense gas, and are rich in dust—that’s why star formation is very obscured by dust in general.” The interstellar dust provides a lot of the raw materials for the eventual planet formation, too. “Therefore 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.”

The researchers used the JWST’s mid-infrared range viewing instrument, which can show “a spectral region that is very rich in information about the properties of the galaxies, a lot of dust and gas features,” she says. Time on the JWST is highly competitive—many more astronomers want to use it than there are hours of time available. 

Far Beyond the Milky Way

Sajina’s work focused on a galaxy about five billion light years away, meaning that the light that reached the JWST left home some five billion years ago. The galaxy, known as SSTXFLS J172458.3+591545, has a deeply obscured black hole that is actively growing by consuming interstellar gas, known as an active galactic nucleus.

With the high sensitivity of the JWST—much greater than other telescopes, even the Hubble Space Telescope—the researchers were able to detect molecules in solid form such as ices  of carbon dioxide, carbon monoxide, and water on the dust grains—the first time all of these together were detected in ice form outside of our “local” universe, meaning within the last billion years or so. 

“We’ve detected molecules and even very complex molecules in space in the form of gases for a long time,” says Sajina, but this is the first time “looking directly at the solid ice mantles on the dust grains—those are difficult to observe.” 

Observations beyond the Milky Way of these features “are very, very rare—these are the first that are really pushing beyond the local universe, going several billion years in the past,” she says. “The spectra detail is so much better that we can understand more about the chemistry that goes on the surfaces of these grains.”

The researchers found that the composition of the ice-covered dust particles “is basically the same as what we see in objects inside our own galaxy and very nearby objects,” says 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.” 

The presence of the ice feature also “tells us something about how dust is distributed in these obscured systems in the past—that there is a compact dense core, which is what we found for this galaxy.”