Webb Telescope reveals stunning structures in nearby spiral galaxies
New images taken with the James Webb Space Telescope reveal intricate, never-before-seen structures and features hidden in visible light. The insights will help astronomers better understand the history of the Milky Way.

Daniel Stolte, University Communications and Claire Blome, STScI
Jan. 29, 2024

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A spiral-shaped galaxy in space
Spiral galaxy NGC 628, also known as the Phantom Galaxy, lies 32 million light-years from Earth. In this split image, the James Webb Space Telescope observations appear at top left, and the Hubble Space Telescope's on bottom right. Hubble showcases visible light, and dark regions are where starlight is absorbed by dust. This dust then re-emits absorbed ultraviolet and visible light as infrared, making it glow in the JWST image. NASA, ESA, CSA, STScI, Janice Lee (STScI), Thomas Williams (Oxford), PHANGS team.

An international team of astronomers, including researchers at the University of Arizona, has released a new trove of images taken with NASA's James Webb Space Telescope, or JWST. The images reveal never-before-seen details of 19 spiral galaxies.

Astronomers have observed these "nearby" spiral galaxies – located between 50 million to 65 million light-years from Earth – for decades. Both space- and ground-based telescopes have contributed to a cache of data in wavelengths from radio to ultraviolet light. Researchers had long planned to use the JWST to obtain the highest resolution infrared images ever taken of these galaxies. Made possible by the JWST's capability to detect light in the near- and mid-infrared wavelengths, the newly available images show off every facet of these spiral galaxies, all of which are visible face-on from Earth, making them ideal subjects to study their architecture.

Undoubtedly the most generally recognized and iconic type of galaxies, spiral galaxies include the Milky Way, which hosts the solar system. Studying its structure has proved difficult from Earth's vantage point, much like it is tricky to understand the structure of a building if the observer is confined to one particular room inside that building. Viewing the Milky Way from outside is impossible, which is why astronomers look to other, similar galaxies to learn as much as they can about our own.

The newly released images are part of a large, long-standing project known as the Physics at High Angular resolution in Nearby GalaxieS, or PHANGS, program, which is supported by more than 150 astronomers worldwide.

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This composite image of the 19 galaxies imaged in the PHANGS survey reveals previously unseen features, such as nascent stars encased in gas and dust that appear bright red.
These images of 19 face-on spiral galaxies are the first to show large, spherical shells in the gas and dust in such great detail. The holes may have been created by stars that exploded and carved out giant regions in the interstellar material. Images: NASA, ESA, CSA, STScI, Janice Lee (STScI), Thomas Williams (Oxford), PHANGS Team. Design: Elizabeth Wheatley (STScI)

The PHANGS program has played a critical role in cataloguing the bounty of new observing data and making it available to the greater astronomy community and the public, according to Jimena Rodriguez, a postdoctoral fellow at the UArizona's Steward Observatory who has contributed to the effort.

"In spite of the fact that we are such a large collaboration, I feel there is so much more science here than we can do," Rodriguez said. "It is important to open it up to others, so the entire community can explore the current thinking about what we think goes into building a galaxy. It advances our field and astronomy as a whole."

The PHANGS team chose these 19 galaxies because there already has been much data available from other telescopes such as NASA's Hubble Space Telescope, the Very Large Telescope's Multi-Unit Spectroscopic Explorer and the Atacama Large Millimeter/submillimeter Array, including observations in ultraviolet, visible and radio light. Now, Webb's near- and mid-infrared contributions have provided several new puzzle pieces.

Everyone can explore Webb's newest set of exquisite images, which show stars, gas and dust on small scales beyond our own galaxy. Teams of researchers are studying these images to uncover the origins of these intricate structures. The research community's collective analysis will ultimately inform theorists' simulations and advance our understanding of star formation and the evolution of spiral galaxies.

"Webb's new images are extraordinary," said Janice Lee, project scientist at the Space Telescope Science Institute in Baltimore, affiliate astronomer at Steward Observatory, and principal investigator of the program. "They're mind-blowing even for researchers who have studied these same galaxies for decades. Bubbles and filaments are resolved down to the smallest scales ever observed and tell a story about the star formation cycle."

Following their clearly defined arms, which are brimming with stars, to the galaxies' centers, their intricate details come into view, including old star clusters and sometimes active supermassive black holes.

Rodriguez is particularly interested in how stars are born, a field of study fraught with technical difficulties because stellar nurseries are often shrouded in gas and dust, making them elusive to visible light, which is what the Hubble Space Telescope detects. Unencumbered by the dust, infrared light allows astronomers to peer right through these optical barriers, revealing unprecedented insights into the birthplaces of stars.

"With JWST, we are seeing all this stuff from star-forming regions in these 19 galaxies that we have surveyed," she said. "This allows us to compare stellar nurseries not only in different regions within the same galaxy, for example the central bar structure or in the spiral arms, but also in other galaxies that have a different morphologies and properties. That helps us better understand how a star forms in our Milky Way."

A galactic skeleton

Webb's primary camera, NIRCam (Near-Infrared Camera), captured millions of stars in these images, which sparkle in blue tones. Some stars are spread throughout the spiral arms, but others are clumped tightly together in star clusters. The data release includes the largest catalog to date of roughly 100,000 star clusters, a huge effort in which Daniel Maschmann, a postdoctoral research associate at Steward, played a crucial role. Maschmann also assembled the data structure of the released dataset.

The telescope's MIRI (Mid-Infrared Instrument) data highlights glowing dust, showing us where it exists around and between stars. It also spotlights stars that haven't yet fully formed – they are still encased in the gas and dust that feed their growth, like bright red seeds at the tips of dusty peaks.

"These are where we can find the newest, most massive stars in the galaxies," said Erik Rosolowsky, a professor of physics at the University of Alberta in Edmonton, Canada.

University of Arizona Regents Professor of astronomy Marcia Rieke led the development of NIRCam and serves as the instrument's principal investigator. George Rieke, also a Regents Professor of astronomy and Marcia Rieke's husband, is the science team lead for MIRI.

While images taken with Hubble primarily show stars themselves, JWST reveals structures that previously remained obscured by dust, Rodriguez explained.

The JWST images reveal all the gas and the dust distributed throughout the galaxy, Rodriguez explained. Dust absorbs ultraviolet and visible light, and then re-emits it in the infrared.

"The dust appears bright because there are stars behind it, lighting it up," she said. "If you compare that with, for example, the Hubble image that we have for the same galaxy, you'll see that they don't look exactly the same. In a way, you could say, we are seeing the skeleton of the galaxy, with all the structure of gas and filaments that we missed before."

Something else that amazed astronomers? Webb's images show large, spherical shells in the gas and dust. "These holes may have been created by one or more stars that exploded, carving out giant holes in the interstellar material," explained Adam Leroy, a professor of astronomy at Ohio State in Columbus.

Now, trace the spiral arms to find extended regions of gas that appear red and orange.

"These structures tend to follow the same pattern in certain parts of the galaxies," Rosolowsky added. "We think of these like waves, and their spacing tells us a lot about how a galaxy distributes its gas and dust."

Study of these structures will provide key insights about how galaxies build, maintain and shut off star formation.

The new images help confirm evidence suggesting that galaxies grow from inside out – star formation begins at galaxies' cores and spreads along their arms, spiraling away from the center. The farther a star is from the galaxy's core, the more likely it is to be younger. In contrast, the areas near the cores that look lit by a blue spotlight are populations of older stars.

What about galaxy cores that are awash in pink-and-red diffraction spikes?

"That's a clear sign that there may be an active supermassive black hole," said Eva Schinnerer, a staff scientist at the Max Planck Institute for Astronomy in Heidelberg, Germany. "Or, the star clusters toward the center are so bright that they have saturated that area of the image."

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Daniel Stolte

Science Writer, University Communications