UTSA researchers offer alternative to dark matter model in dwarf galaxies

JUNE 11, 2025 — Groundbreaking research led by UTSA graduate student Nathaniel Lujan and Karl Gebhardt, UT Austin professor and chair of astronomy, is challenging long-held assumptions about the relationships among dark matter, galaxy formation and black holes.

This research is the product of the new UTSA/UT Austin joint course Galactic Dynamics—a unique collaboration across institutions to combine specialties and classrooms to synergistic effect.

In collaboration with black hole astrophysicist Richard Anantua, UTSA assistant professor of physics and astronomy, Lujan and Gebhardt investigated a dwarf galaxy 75,000 light-years from the Milky Way named Segue 1.

“Our work may revolutionize the modeling of dwarf galaxies or star clusters to include supermassive black holes as opposed to just dark halos.”

Nathaniel Lujan

Known for its large mass despite emitting very little light, Segue 1 is traditionally viewed as a prime candidate for studying dark matter-dominated galaxies. However, the unique investigation performed by Lujan revealed that black holes are likely to play a greater role holding the Segue 1 cluster together than dark matter.

Dark matter is a mysterious substance that does not emit, absorb or reflect light, making it invisible to telescopes. However, scientists can infer the presence of dark matter from its gravitational effects on stars and galaxies. Adding supermassive black holes— which trap light behind their horizons— to the picture opens the door to a richer interplay of dark components than originally thought in the analysis of dwarf galaxies.

Lujan’s research originated from a spring 2024 course on Galactic and Gravitational Dynamics, co-taught by Anantua and Gebhardt. Designed to foster collaboration between UTSA and UT Austin, the course gave students hands-on experience with cutting-edge dynamical modeling techniques.

Richard Anantua

“Nate came into his first astronomy elective with a great work ethic and a penchant for learning new methodologies,” said Anantua. “Both were needed for this project, which combined a steep computational learning curve with a theoretical grasp of strong gravity in the vicinity of supermassive black holes.”

Using advanced computational models, Lujan made a breakthrough discovery suggesting that Segue 1 may not contain high levels of dark matter but is instead home to a supermassive black hole nearly half a million times the mass of the Sun.

This new understanding suggests that supermassive black holes are either more common than previously believed or that Segue 1 is the stripped-down core of a much older, more massive galaxy. Either scenario reshapes how astronomers and astrophysicists think about black hole formation and reduces the amount of dark matter needed to explain certain galaxies—bringing scientists another step closer to uncovering the true nature of dark matter.

Karl Gebhardt

“Our work may revolutionize the modeling of dwarf galaxies or star clusters to include supermassive black holes as opposed to just dark halos,” said Lujan. “It may also be a new method of systematically finding supermassive black holes outside the centers of galaxies.”

Working in partnership with mentors at UTSA and UT Austin, Lujan combined computational astrophysics with observational data to test his hypothesis. The research team used sophisticated models and high-performance computational tools developed by Gebhardt.

By simulating thousands of potential stellar trajectories, Lujan and his team found that models containing a black hole aligned more closely with observed stellar motions than those solely based on dark matter.

“The tools I developed during this work are the same ones I am now applying and expanding upon in graduate school, both through follow-up studies and new projects involving primordial black holes and dark matter,” explained Lujan.

An article on the research findings entitled, “The ‘Dark-Matter Dominated’ Galaxy Segue 1 Modeled with a Black Hole and no Dark Halo” is under consideration for publication in the Astrophysical Journal Letters, a peer-reviewed scientific journal of astrophysics and astronomy established in 1895.

The publication involves several physics and astronomy student co-authors from UTSA who had taken the Galactic Dynamics and Gravitational Dynamics course, including Loraine V. Gomez, Izabella Marron and Hayley West.

Lujan presented the research at the American Astronomical Society (AAS) 246th press conference in Anchorage, Alaska. The press conference was livestreamed on the AAS YouTube channel.

— Ryan Schoensee