James Webb Space Telescope

JWST Sees Organic Molecules Ludicrously Far Away

When astronomers used the JWST to look at a galaxy more than 12 billion light years away, they were also looking back in time. And when they found organic molecules in that distant galaxy, they found them in the early Universe.

The organic molecules are usually found where stars are forming, but in this case, they’re not.

The organic molecules are polycyclic aromatic hydrocarbons (PAH) which scientists think are building blocks for life. They occur naturally here on Earth as by-products of combustion. They’re also prevalent in the interstellar medium (ISM), the matter and radiation in between stars. They indicate regions of cold gas from which stars form.

Finding PAHs in a distant, ancient galaxy required advanced technology, skill, and some fortuitous luck. The JWST provided the technology with its keen infrared observing capabilities, and a foreground galaxy only 3 billion light years away lined up with the distant galaxy provided the luck. It’s lined up just right and acts as a gravitational lens, amplifying the light from the distant galaxy.

“Discoveries like this are precisely what Webb was built to do: understand the earliest stages of the universe in new and exciting ways.”

Kedar Phadke, University of Illinois

The distant galaxy is called SPT0418-47. Not only is it 12 billion light years away, but astronomers are seeing the light it emitted when the Universe was only 10% of its current age, about 1.5 billion years after the Big Bang.

SPT0418-47’s ancient light forms a ring around the foreground galaxy called an Einstein ring. Einstein predicted these rings in his theory of general relativity. With the light amplified, the powerful JWST examined it with its Mid-Infrared Instrument (MIRI.)

The galaxy observed by Webb shows an Einstein ring caused by a phenomenon known as lensing, which occurs when two galaxies are almost perfectly aligned from our perspective on Earth. The gravity from the galaxy in the foreground causes the light from the background galaxy to be magnified, like looking through the stem of a wine glass. Because they are magnified, lensing allows astronomers to study very distant galaxies in more detail than otherwise possible. Image Credit: S. Doyle / J. Spilker

The researchers who found the PAHs come from different institutions in North and South America and Europe. They presented their findings in an article in Nature titled “Spatial variations in aromatic hydrocarbon emission in a dust-rich galaxy.” The lead author is Justin Spilker from the Department of Physics and Astronomy at Texas A&M University.

“By combining Webb’s amazing capabilities with a natural ‘cosmic magnifying glass,’ we were able to see even more detail than we otherwise could,” said lead author Spilker. “That level of magnification is actually what made us interested in looking at this galaxy with Webb in the first place because it really lets us see all the rich details of what makes up a galaxy in the early universe that we could never do otherwise.”

There are several types of PAHs, but one thing they have in common is size. Even the simplest one, naphthalene, has 10 carbon atoms and 8 hydrogen atoms. Larger ones can have 50 carbon atoms.

“These big molecules are actually pretty common in space,” Spilker explained. “Astronomers used to think they were a good sign that new stars were forming. Anywhere you saw these molecules, baby stars were also right there blazing away.”

PAHs have long been associated with star birth. Astronomers have found them in regions of active star birth in different parts of the Milky Way. They’ve also spotted the large molecules near bright young stars.

Orion is the closest star-forming region to Earth, and this image shows the Barnard 30 region in Orion. The stars in this region are young, only about 2 to 3 million years old. PAHs are abundant, seen as dark red at the lower left and bottom center. Image Credit: NASA/JPL-Caltech

But in this case, the ancient galaxy contains abundant PAHs absent star formation, and star formation absent PAHs.

“Thanks to the high-definition images from Webb, we found a lot of regions with smoke but no star formation and others with new stars forming but no smoke,” Spilker said.

Something’s going on in distant, ancient SPT0418-47 that needs a rethink. The link between PAHs and star formation is not as strong as once thought. Or not in the early Universe, anyway.

Astronomers can’t reach a conclusion based on observations of one galaxy. The mismatch between the presence of PAHs and star formation can only be understood with more observations. Astronomers can lean on the JWST to provide more.

“Discoveries like this are precisely what Webb was built to do: understand the earliest stages of the universe in new and exciting ways,” said the University of Illinois Urbana-Champaign graduate student Kedar Phadke, who led the technical development of the team’s Webb observations. “It’s amazing that we can identify molecules billions of light-years away that we’re familiar with here on Earth, even if they show up in ways we don’t like, like smog and smoke. It’s also a powerful statement about the amazing capabilities of Webb that we’ve never had before.”

It took some sleuthing to differentiate between the infrared light from PAHs and from larger dust grains. Dust grains absorb about half of the radiation from stars throughout the Universe’s history and emit it as infrared light. All that infrared light from simple dust can cloud the picture of early galaxies.

Joaquin Vieira is an astronomy and physics professor at the University of Illinois Urbana-Champaign and was part of the research team. “This project started when I was in graduate school studying hard-to-detect, very distant galaxies obscured by dust,” Vieira said. “Dust grains absorb and re-emit about half of the stellar radiation produced in the universe, making infrared light from distant objects extremely faint or undetectable through ground-based telescopes.”

Prior to the JWST’s launch, there was no way to really observe these ancient galaxies. Instead, astronomers made do with what are known as bright compact dwarf (BCD) galaxies. These small galaxies are similar to the small galaxies that astronomers thought were common in the early Universe. Many researchers think that our galaxy and others like it grew so large via mergers involving BCDs. Some BCDs allowed for the creation of PAHs, but young stars shining brightly in UV could also destroy PAHs.

BCDs were used as stand-ins for ancient galaxies, and while observations were tantalizing, there were always questions. However, those early galaxies were out of reach.

This artist’s conception symbolically represents complex organic molecules, known as polycyclic aromatic hydrocarbons, seen in the early universe. These large molecules, comprised of carbon and hydrogen, are considered among the building blocks of life. NASA’s Spitzer Space Telescope detected these molecules in galaxies when our universe was about 3.5 billion years old. Now the JWST has found them even further back in time. Image Credit: NASA/JPL-Caltech/T. Pyle (SSC)

But the JWST’s formidable infrared observing power has changed that. When combined with gravitational lensing, the picture of distant objects becomes clearer.

“We didn’t expect this,” Vieira said. “Detecting these complex organic molecules at such a vast distance is game-changing regarding future observations. This work is just the first step, and we’re just now learning how to use it and learn its capabilities. We are very excited to see how this plays out.”

Since PAHs contain carbon, astronomers think they can only exist once generations of stars have lived and died. Elements heavier than hydrogen and helium weren’t created by the Big Bang. Only stellar nucleosynthesis can create them. Once they’re created in stars, they’re spread out into the Universe when the star “dies.”

“What this research is telling us right now – and we are still learning – is that we can see all of the regions where these smaller dust grains are located – regions that we could never see before the JWST,” Phadke said. “The new spectroscopic data lets us observe the galaxy’s atomic and molecular composition, providing very important insights into the formation of galaxies, their lifecycle and how they evolve.”

There’s no explanation yet for why PAHs are seen absent of star formation and vice versa. If observations of other ancient galaxies display the same thing, then astronomers are onto something.

These new JWST observations come from TEMPLATES, an early-release science program. TEMPLATES stands for Targeting Extremely Magnified Panchromatic Lensed Arcs and Their Extended Star formation. “Lensing magnification pushes JWST to the highest spatial resolutions possible at these redshifts,” the TEMPLATES team wrote, “to map the key spectral diagnostics of star formation and dust extinction: H-alpha, Pa-alpha, and 3.3um PAH within individual distant galaxies.”

TEMPLATES features observations of four galaxies, all through gravitational lensing. Two of the four galaxies are extremely dusty, and telescopes like the Hubble can’t see into them. But the JWST can pierce the dusty veil and map the dust. By doing that, it found PAHs and possibly new aspects of star formation prevalent in the early Universe.

“These are early days for the Webb Telescope, so astronomers are excited to see all the new things it can do for us,” Spilker said. “Detecting smoke in a galaxy early in the history of the universe? Webb makes this look easy. Now that we’ve shown this is possible for the first time, we’re looking forward to trying to understand whether it’s really true that where there’s smoke, there’s fire. Maybe we’ll even be able to find galaxies that are so young that complex molecules like these haven’t had time to form in the vacuum of space yet, so galaxies are all fire and no smoke. The only way to know for sure is to look at more galaxies, hopefully even further away than this one.”


Evan Gough

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