Princeton Study Reveals Evolution of Galaxies Using JWST Insights

A recent study conducted by researchers at Princeton University has unveiled significant insights into the formation and evolution of galaxies, including our own Milky Way, utilizing data obtained from the James Webb Space Telescope (JWST). Published in the Monthly Notices of the Royal Astronomical Society on July 9, 2025, this research challenges long-held assumptions about cosmic development by demonstrating that galaxies initially formed thick discs before transitioning to the thinner structures observed today.

The study highlights that galaxies typically exhibit two distinct layers: a thick disc composed of older stars and a thin disc containing younger stars. Prior to this investigation, the timeline for the formation of these two discs remained largely uncertain. By examining galaxies over 10 billion years old, the Princeton team was able to observe both disc types at unprecedented distances—an achievement made possible through JWST’s advanced infrared imaging capabilities.

Using infrared filters, researchers analyzed 111 edge-on galaxies, which provided a clearer view of the stacked disc structures. The findings revealed a correlation between a galaxy's mass and the timing of its thin disc formation; high-mass galaxies developed thin discs approximately 8 billion years ago, while low-mass counterparts saw this process occur around 4 billion years ago. This observation supports the 'downsizing' theory of galactic evolution.

Lead researcher Dr. Emily Thompson, Assistant Professor of Astrophysics at Princeton University, remarked, "Thick discs serve as the foundational structure of galaxies, akin to a house's base, which is then built upon by the formation of thin discs. Our findings reshape our understanding of galactic architecture and evolution."

The study also revisits historical theories regarding disc formation. One prevailing hypothesis, termed the 'born hot' scenario, suggests that turbulent conditions in young galaxies lead to the initial formation of thick discs due to high gas concentrations and rapid star formation. Dr. James Parker, a theoretical astrophysicist at the University of California, Berkeley, stated, "The turbulence observed in high-redshift galaxies provides compelling evidence for the 'born hot' theory, indicating that these galaxies were dynamic and chaotic during their formative years."

Conversely, the 'progressive thickening' theory posits that stars originally form in a thin disc, which then thickens over time due to gravitational interactions and mergers. Dr. Susan Lee, a researcher at the Massachusetts Institute of Technology, cautioned, "While mergers do contribute to thick disc formation, they alone cannot account for the entire structure. Our understanding must encompass multiple processes at play."

The research further discusses a third theory—the 'ex situ' scenario—which posits that mergers with smaller galaxies contribute to thick discs. However, the current observations yield fewer counter-rotating stars than this model would predict, suggesting that mergers are a factor but not the sole mechanism.

The methodology employed in this study involved data from JWST programs, including JADES and CEERS, focusing on edge-on galaxies. Out of 213 candidates, 111 were selected after meticulous visual inspections. Researchers utilized infrared bands to measure the galaxies' dimensions, revealing that larger galaxies have proportionally larger and taller discs. This reinforces the notion that galaxy formation is a complex process involving concurrent evolution of thick and thin discs.

In summary, the findings from Princeton University's study not only enhance our understanding of the Milky Way but also provide a broader context for the evolutionary history of galaxies across the cosmos. As Dr. Thompson noted, "Our research indicates that galaxies were not born as perfect spirals but underwent a layered growth process, beginning with the formation of thick, turbulent discs that later settled into the thin structures we observe today."

The implications of these findings are profound, offering a fresh perspective on galactic development and raising questions for future investigations. As researchers continue to utilize JWST’s capabilities to probe deeper into cosmic history, the quest to unravel the complexities of galaxy formation will undoubtedly advance our understanding of the universe.

This research not only contributes to astrophysics but also invites further inquiry into the processes that shape the cosmos, with potential ramifications for our understanding of the origins of galaxies in the universe.