A new study led by Dr. Vivian Tan, who recently completed her Ph.D. at York University under the supervision of Prof. Adam Muzzin, provides the most detailed reconstruction yet of how the Milky Way may have evolved from its earliest phases to the structured spiral we see today. Tan and her colleagues examined 877 "Milky Way twins" — galaxies whose masses and properties closely match what astronomers expect the Milky Way would have looked like at different ages across cosmic time. By observing more distant, and therefore progressively younger examples of these galactic look-alikes, the team effectively charted a timeline of our galaxy's life, with surprising results. Our Milky Way's history started from a remarkably tumultuous youth before its more settled adulthood.
The galaxies in the sample span a remarkable range of cosmic time, from when the Universe was just 1.5 billion years old (12.3 billion years ago) to 10 billion years old (3.5 billion years ago). This period covers as far back as when the Universe was only 10 per cent its current age, a crucial epoch when galaxies transformed from small, irregular systems into the stable disk galaxies familiar today.
To carry out this work, the team combined high-resolution imaging from JWST and the Hubble Space Telescope (HST). The JWST observations come from the Canadian NIRISS Unbiased Cluster Survey (CANUCS), a major Canadian observing program that uses five massive galaxy clusters as natural gravitational lenses. These clusters magnify background galaxies, revealing faint structures that would otherwise be too distant and too dim to study in detail.
CANUCS takes advantage of Canada's hardware contributions to the JWST mission through the Near-Infrared Imager and Slitless Spectrograph (NIRISS) instrument, built for the mission by the Canadian Space Agency in partnership with the Université de Montréal, the National Research Council Herzberg Centre for Astronomy and Astrophysics, and Honeywell. In return, Canadian astronomers received valuable guaranteed observing time on JWST, including the data that enabled this study.
Building galaxies from the inside out
JWST's exceptional spatial resolution allowed the researchers to create detailed maps of the stellar mass and star formation activity across each galaxy. These maps show where stars were already in place and where new stars were forming at different phases in a galaxy's life.
Across the entire sample, the results point to a clear pattern: galaxies like our Milky Way grow from the inside out. The earliest Milky Way twins are dominated by dense, compact central regions. Over time, their outer parts — the regions that will later become the disk — rapidly gain mass and become the primary sites of star formation. This gradual expansion outward creates the extended spiral structures we see in present-day galaxies.
"Astronomers have been modeling the formation of the Milky Way and other spiral galaxies for decades," says lead author Tan. "It's amazing that with the JWST, we can test their models and map out how Milky Way progenitors grow with the Universe itself."
Turbulent teenage years
The most exciting results of the study also reveal that young Milky Way-like galaxies lived through far more chaotic conditions than their older, more evolved counterparts. The youngest, most distant systems show highly disturbed shapes, asymmetric features, and evidence of frequent galaxy–galaxy interactions and mergers. These disturbances are signatures of a dynamic environment where galaxies were constantly colliding, accreting material, and triggering intense bursts of star formation.
By contrast, the Milky Way twins at later cosmic times appear much more stable and orderly. Their structures are smoother, their star formation is more evenly distributed, and signs of major interactions become far less common. Overall, they point to a more chaotic past for our Galaxy than we had expected.
Comparing observations and simulations
Tan and her collaborators compared their observations to state-of-the-art computer simulations that track the evolution of Milky Way–like galaxies. The simulations broadly agree with the observed inside-out growth and early clumpy, merger-driven activity. However, they sometimes fail to reproduce the high central compactness seen in the earliest galaxies, and they underestimate how quickly mass accumulates in the outer regions between 8 and 11 billion years ago.
These differences provide important constraints on feedback, merger rates, and disk formation models, and highlight the need to refine theoretical predictions in the era of JWST.
Building on Webb's early insights
This study marks a significant milestone for Canada's growing leadership in JWST galaxy research. With NIRISS and CANUCS continuing to deliver exceptionally deep, high-resolution data, astronomers will be turning to even larger samples of Milky Way–like systems and extending their analysis to include gas content, dust, and kinematic structure.
"This study is a significant step forward in understanding the earliest stages of the formation of our Galaxy," says Muzzin, co-author of the study. "However, this is not the deepest we have pushed the telescope yet. In the coming years, with the combination of JWST and gravitational lensing we can move from observing Milky Way twins at 10 per cent their current age to when they are a mere 3 per cent of their current age, truly the embryonic stages of their formation."
Other co-authors from York University are Ghassan Sarrouh, Visal Sok, Naadiyah Jagga, and Westley Brown. Other co-authors include researchers from the University of Toronto, the University of Ljubljana, Saint Mary's University, Kyoto University, the University of Groningen, Columbia University, Wellesley College, the Space Telescope Science Institute, and the National Research Council Herzberg Astronomy & Astrophysics Research Centre.
This team and several international teams already have future JWST observations scheduled to do this. Combined with updated simulations, they will help determine precisely when galaxies like our Milky Way settle into stable disks, how long turbulent phases last, and what physical processes drive the transition between them. By expanding this work, the team aims to build an increasingly complete picture of how galaxies like our own assembled their stars and evolved from the early Universe to the present day.
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