Hungry Galaxies? What triggers Galaxies to Fuel their Super Massive Black Holes
Examining how galaxy morphology impacts accretion
One of the central open questions in galaxy evolution is how their central super massive black holes (SMBH) begin accreting matter. Will Jarvis, a first-year PhD student at the University of Massachusetts Amherst, is taking a direct approach to solving this problem by testing whether galaxy mergers trigger black hole activity. The idea is intuitive: major mergers can strongly disturb a galaxy, redistributing stars, gas, and dust in ways that funnel material towards the center, driving black holes to begin to “eat”. Yet despite this expectation, Jarvis’s results suggest that mergers may not be the dominant mechanism fueling central black holes.
The work behind this result began during Will’s undergraduate studies at the University of Wisconsin-Madison, where he first became involved with research through an internship in Hawaii. During this time he worked on galactic centers, a project that later morphed into a deeper focus on active galactic nuclei (AGN) and the mechanisms that drive their accretion. This project ultimately guided him towards graduate school. Building on this foundation, Will applied for and was awarded an NSF Graduate Research Fellowship for a proposal centered on AGN fueling. He describes the application process as “standardized,” emphasizing the importance of developing a clear personal voice. After taking a year to continue this work, he began his PhD at UMass Amherst, where he completed the final stages of this AGN study while beginning to explore new research directions, supported by the flexibility of his NSF fellowship.
Will’s research investigates how AGN begin to accrete matter from their host galaxies. As material falls inward under gravity, it forms a hot, luminous accretion disk that can inject energy back into the surrounding galaxy through a process known as feedback . This feedback is a key ingredient in cosmological simulations of the universe, helping them to reproduce observed galaxy properties. However, as Will explains, “when we actually look at feeding supermassive black holes, it’s difficult to really measure that feedback process and its impact on galaxy properties like star formation.” Without a good understanding of what triggers black hole accretion, it becomes even more difficult to understand how they recycle energy into their host galaxies. Thus, Will’s work focuses on answering the question: what initially triggers SMBHs to begin accreting.
One leading hypothesis is that galaxy mergers drive this process. To illustrate the idea, Will draws an analogy to our own solar system: planets orbit the Sun in stable configurations, but a sufficiently massive external perturbation could disrupt those orbits and send material inward. On galactic scales, mergers between galaxies can similarly disturb the distribution of gas, stars, and dust, potentially funneling galactic material towards the central black hole. This infalling matter powers the accretion disk and, in turn, the feedback process thought to shape galaxy evolution.
To test whether mergers are responsible for triggering AGN activity, Will analyzed a variety of observational datasets. A key challenge was identifying which galaxies were undergoing mergers. He approached this using both qualitative and quantitative methods: training undergraduate and high school students to recognize the hallmarks of a galactic merger event from imaging, and applying galaxy modeling techniques to classify galaxies more systematically. His datasets were drawn from multiple archives, including the RAINBOW cosmological survey database and MAST (Mikulski Archive for Space Telescopes), incorporating observations from missions such as Chandra, Spitzer, Hubble, and JWST.
Using a Python-based modeling tool GaLight, Will fit the galaxy light profiles to imaging data while accounting for the bright emission from the central AGN. This step is critical, as the accretion disk can dominate the observed light and obscure underlying galaxy structure. By modeling and subtracting the central component he was able to assess whether features such as spiral arms or tidal distortions remained – key indicators of merger activity. This careful separation of nuclear and host-galaxy light allowed for a more reliable classification of each system.
Contrary to expectations, Will’s study found that only about one-third of AGN-hosting galaxies in his sample showed evidence of mergers. This result suggests that mergers are not the dominant mechanism for triggering black hole accretion. Instead, other internal, or intragalactic, processes must also play a significant role. As is often the case with scientific research, the absence of expected evidence opens new directions for inquiry, pointing toward a more complex picture of how galaxies fuel their central black holes.
While this study lays the groundwork for future studies of AGN fueling, Will has since moved on to new questions, now focusing on distant massive star clusters. Reflecting on his earlier research, he emphasizes both the scientific growth it enabled and the opportunity to mentor younger students. His advice to aspiring scientists is straightforward: “Physics is really important: don’t just focus on the arithmetic…make sure you’re developing the intuition for the way these physical systems work.” In this way, his work continues to shape not only our understanding of galaxies, but also the next generation of researchers.
About the Authors:
Bradley Mills is a sophomore undergraduate at the College of Natural Sciences at UMass Amherst, studying Astronomy. Outside of their studies, they enjoy reading, writing, and playing Dungeons and Dragons.
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.
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