The galaxy cluster MACS-J0417.5-1154 is so massive it is warping the fabric of space-time and distorting the appearance of galaxies behind it, an effect known as gravitational lensing. This natural phenomenon magnifies distant galaxies and can also make them appear in an image multiple times, as NASA’s James Webb Space Telescope saw here. Two distant, interacting galaxies — a face-on spiral and a dusty red galaxy seen from the side — appear multiple times, tracing a familiar shape across the sky. Active star formation, and the face-on galaxy’s remarkably intact spiral shape, indicate that these galaxies’ interaction is just beginning. (Credit:
NASA, ESA, CSA, STScI, V. Estrada-Carpenter, Saint Mary’s University.)
In a nutshell
- A rare cosmic alignment has given astronomers an unprecedented view of two galaxies from 7 billion years ago, when the universe was at its peak of star formation. This discovery shows us what our own Milky Way galaxy might have looked like in its “teenage” years.
- The James Webb Space Telescope revealed a previously hidden dusty red galaxy that the Hubble Space Telescope couldn’t see, forming part of what astronomers call the “Question Mark Pair.” This is one of only three or four known examples of this type of gravitational lensing in the observable universe.
- Using a revolutionary new analysis technique, scientists mapped 20 distinct star-forming regions across the galaxies: 7 showed intense bursts of star formation, 10 were running out of fuel and shutting down, and 3 maintained steady star formation rates. This gives us new insights into how galaxies grow and evolve over time.
HALIFAX, Nova Scotia — When the James Webb Space Telescope (JWST) peered into a distant galaxy cluster, it found something previous telescopes had missed: a question mark written in starlight. This rare cosmic alignment, featuring two galaxies caught in a gravitational dance, opened a window into one of the most important periods of the universe’s history when star formation was just beginning to slow from its peak.
In research published in the Monthly Notices of the Royal Astronomical Society, a team of international scientists found that these galaxies, known as the “Question Mark Pair,” represent one of only three or four known examples of their kind in the observable universe. The larger galaxy appears as a beautiful face-on spiral dotted with brilliant clumps of star formation, while its companion, a dusty, edge-on galaxy glowing red, was only revealed by JWST’s superior infrared vision. Previous observations by the Hubble Space Telescope missed this dusty galaxy entirely, as its light was trapped by cosmic dust that JWST’s infrared instruments can peer through.
This rare alignment occurs within the galaxy cluster MACS-J0417.5-1154, which acts like a giant magnifying glass in space. The cluster’s immense mass warps the fabric of space-time itself, creating what astronomers call gravitational lensing. This effect not only magnifies distant galaxies but also distorts them, creating multiple images that appear smeared across the sky in arcs. The particular alignment here produces what scientists call a hyperbolic umbilic gravitational lens, resulting in five separate images of the galaxy pair scattered across JWST’s field of view.
The ability to observe this galaxy pair in such detail represents a major leap forward in astronomical capabilities. While Hubble has been studying distant galaxies for decades, JWST’s larger mirror and specialized infrared instruments allow it to detect light that has been stretched and reddened by the expansion of the universe. This makes JWST particularly adept at studying galaxies from this crucial period in cosmic history when the universe was experiencing its peak of star formation.
Scientists led by Vicente Estrada-Carpenter from Saint Mary’s University in Canada used JWST‘s Near Infrared Imager and Slitless Spectrograph (NIRISS) to study these galaxies in exquisite detail.
“Knowing when, where, and how star formation occurs within galaxies is crucial to understanding how galaxies have evolved over the history of the universe,” says Estrada-Carpenter, in a statement.
The research team developed a revolutionary new technique for analyzing JWST’s data. Rather than treating each galaxy as a uniform entity, they divided it into hundreds of small regions based on color and brightness. This allowed them to study how stellar populations, groups of stars of different ages and types, vary across different parts of each galaxy. The result is the most detailed map ever created of star formation in galaxies at this distance.
By measuring two different types of light, hydrogen-alpha emission from very young stars and ultraviolet light that persists longer, the team could determine the history of star formation across different regions of the galaxies. Hydrogen-alpha emission comes from massive, hot stars that live only about 10 million years, while ultraviolet light continues shining for about 100 million years after stars form. This difference in timescales allows astronomers to track how star formation rates change over time.
“Both galaxies in the Question Mark Pair show active star formation in several compact regions, likely a result of gas from the two galaxies colliding,” says Estrada-Carpenter. “However, neither galaxy’s shape appears too disrupted, so we are probably seeing the beginning of their interaction with each other.”
Within the face-on galaxy, researchers identified 20 distinct star-forming clumps. Seven of these regions showed signs of intense bursts of star formation, while 10 were undergoing a process called quenching, gradually shutting down their stellar nurseries as they exhausted their supply of gas needed to form new stars. The remaining three maintained a steady state of star formation.
This variation in star formation rates provides important clues about how galaxies evolve. When galaxies interact, their mutual gravitational forces can trigger new waves of star formation by compressing gas clouds. However, these same forces can also disrupt existing stellar nurseries or strip away the gas needed to form new stars. The Question Mark Pair shows both effects in action, while some regions experience enhanced star formation, others show signs of decline.
“These galaxies, seen billions of years ago when star formation was at its peak, are similar to the mass that the Milky Way galaxy would have been at that time. Webb is allowing us to study what the teenage years of our own galaxy would have been like,” says lead researcher Marcin Sawicki from Saint Mary’s University.
NASA, ESA, CSA, STScI, V. Estrada-Carpenter, Saint Mary’s University.)
The research team’s innovative analysis techniques also revealed that star formation rates in the face-on galaxy’s outer regions had dropped by nearly a factor of five, while its dusty companion showed signs of increased activity. This contrast highlights how galaxy interactions can produce different effects in different environments, contributing to the complex diversity of galaxies we observe today.
These observations mark just the beginning of JWST’s contributions to our understanding of galaxy evolution. The telescope’s unprecedented sensitivity and resolution, combined with innovative analysis techniques, are allowing astronomers to study the detailed processes of star formation across cosmic time and space with clarity never before achieved. Future observations of similar galaxy pairs will help build a more complete picture of how galaxies like our Milky Way grew and evolved throughout cosmic history.
Paper Summary
Methodology
The researchers employed JWST’s NIRISS instrument to capture detailed spectroscopic data of the galaxy pair. They developed a new multi-region modeling approach that divided each galaxy into hundreds of small segments based on color and brightness. This allowed them to analyze the stellar populations and star formation rates in each segment independently, rather than treating each galaxy as a single uniform object. They specifically measured hydrogen-alpha emission (indicating very recent star formation) and ultraviolet light (showing longer-term star formation) across both galaxies.
Results
The study found that the face-on galaxy is experiencing overall decline in star formation, with rates dropping by a factor of 4.7 in its outer regions. Of 20 identified star-forming clumps, 7 showed active bursts of star formation, 10 were quenching, and 3 maintained steady formation rates. The edge-on galaxy displayed signs of increased star formation, suggesting it may be undergoing a starburst phase triggered by interaction with its companion.
Limitations
The researchers note that dust obscuration in the edge-on galaxy may have affected their ability to detect all star formation activity in that galaxy. Additionally, they couldn’t fully account for possible contributions to hydrogen-alpha emission from active galactic nuclei or shock waves, as they lacked the necessary data to distinguish these sources from star formation.
Discussion and Takeaways
This work demonstrates both the power of JWST for studying distant galaxies and the importance of considering spatial variations in stellar populations when analyzing galaxy evolution. The findings suggest that galaxy interactions can simultaneously trigger and suppress star formation in different regions, contributing to our understanding of how galaxies evolve through cosmic time.
Funding and Disclosures
The research was supported by grants from the Canadian Space Agency and the Natural Sciences and Engineering Research Council of Canada, among other sources. The team utilized multiple computing facilities including the Texas A&M University Brazos HPC cluster.
Publication Information
This study was published in the Monthly Notices of the Royal Astronomical Society (MNRAS 532, 577-591) in 2024, with the title “When, where, and how star formation happens in a galaxy pair at cosmic noon using CANUCS JWST/NIRISS grism spectroscopy.”
