Observations indicate massive galaxies may complete star formation and chemical enrichment well before cosmic noon
One of the central questions in galaxy evolution is how massive galaxies can stop forming stars so early in cosmic time, when the Universe is still rich in cold gas, the raw fuel for new star formation. A study led by Massi Hamadouche, a postdoctoral scholar at UMass Amherst, examines James Webb Space Telescope (JWST) spectroscopic data of ten such massive galaxies that have shut down star formation, often referred to as quiescent galaxies. Her work shows that these galaxies can form, evolve, and quench so rapidly that they complete this lifecycle well before the cosmic peak of star formation and galaxy evolution – a result that stands in tension with theoretical predictions.
With the arrival of brand new observations from JWST, Hamadouche jumped at the chance to extend her earlier studies of quiescent galaxies – work she began during her PhD at the University of Edinburgh – back to much earlier cosmic times, allowing her to study massive galaxies in the early Universe in far greater detail than previously possible. Her sample consists of ten massive quiescent galaxies at high redshift, meaning we are observing them as they existed in the early Universe. By modeling photometric and spectroscopic data, Hamadouche derives key physical properties such as stellar ages, masses, chemical abundances, and star formation histories. These measurements provide critical insight into how such galaxies formed and shut down star formation so early.
For a galaxy to be quenched, it must have stopped forming appreciable amounts of new stars. While that definition is simple, understanding how this happens is not. When asked how galaxies like these quench, Hamadouche notes that “we don’t fully know yet,” pointing to possible mechanisms such as galaxy interactions that heat gas and prevent new stars from forming, or supermassive black holes that expel the remaining gas from the galaxy. The innate mystery behind what drives this process is what makes this study so compelling. Even more, these galaxies challenge our understanding of galaxy evolution timescales: for systems this massive to form, grow, and shut down star formation so early in the Universe pushes against our current understanding of galaxy evolution as a whole.
One of the key tools Hamadouche used to understand how quickly galaxies form is alpha-element enhancement, which acts as a kind of cosmic clock. Alpha elements, such as oxygen, silicone, and magnesium, are produced in short-lived, massive stars that explode as core-collapse supernovae, enriching galaxies on relatively short timescales. Iron, by contrast, is largely produced in Type Ia supernovae, which occur much later. Because of this, a high ratio of alpha elements to iron typically indicates that a galaxy formed its stars rapidly, before iron from longer-lived systems could build up. Given that the galaxies in this sample formed and quenched quickly, the expectation was that they would all show high alpha enhancement. Instead, Hamadouche found no clear trend between alpha enhancement and star formation history. This unexpected diversity suggests that galaxy formation in the early Universe may not follow a single pathway, though it may also reflect limitations in current models.
As Hamadouche noted, “things are much more complicated than we thought.” A central part of her work involves spectral energy distribution fitting, a process she describes as integral yet challenging. This method compares photometric and spectroscopic observations to theoretical models to extract physical properties such as stellar ages, star formation rates, and chemical abundances. While powerful, it requires navigating a large space of adjustable parameters, which can make the process time-consuming. At the same time, the complexity is part of what makes the work rewarding. Hamadouche recalls going on a “deep dive” to find a model capable of accurately capturing star formation histories, eventually uncovering one that fit her needs perfectly. Learning how to use it and seeing it produce meaningful results felt, in her words, like a “treasure hunt.”
The project also points toward clear directions for future work. Hamadouche is particularly interested in improving models for chemical abundances, especially those that allow for more flexible and realistic variations. She also highlights the challenges of interpreting some galaxies, where models suggest activity that does not align with expectations – underscoring the need for continued refinement in these approaches. Reflecting on her experience, she emphasizes the importance of patience and curiosity in research. Her advice to those entering the field is simple: work hard, take the time to understand the underlying concepts, and don’t forget to enjoy the process along the way.
About the Author:
Dan Kidwell is a senior undergraduate studying astronomy and physics at UMass Amherst. He currently works in high-energy astronomy, conducting simulated observations of stellar winds in the Galactic Center. Outside of his studies, he enjoys hiking, camping, and photographing the night sky.