It started with a homework assignment: A University of Texas astronomy class run jointly by its Austin and San Antonio campuses has led to students making a discovery that has landed in the pages of a scientific journal.
Researchers have believed that Segue 1, a puny galaxy orbiting the Milky Way just 75,000 light-years away, was packed with dark matter, a substance in space that doesn’t shine or interact with light. Some scientists have recommended it as a key place in the local universe to study this mysterious material.
But a new study indicates that the dim dwarf galaxy gets most of its mass from a previously unknown supermassive black hole, which is also invisible, not dark matter. The black hole is estimated to weigh upward of 450,000 suns. Nathaniel Lujan, a graduate student in San Antonio, used advanced computer modeling techniques he learned in his Galactic and Gravitational Dynamics course to help discover this cosmic giant lurking in Segue 1’s shadows.
The finding raises the possibility that gigantic black holes are more ubiquitous than previously thought — even existing in the tiniest of galaxies — and that astronomers’ understanding of what holds together dwarf galaxies may not be all that it once seemed.
« It’s amazing, » said Lujan during a presentation at the American Astronomical Society meeting in Anchorage, Alaska, « because Segue 1 is a galaxy we can hardly see, but I’m suggesting that it hosts a half-million solar-mass supermassive black hole in its center. »
Segue 1, first spotted in 2006 with the Sloan Digital Sky Survey telescope in New Mexico, has too few stars to have the gravity needed to hold itself together in space. Previously, scientists have speculated that a dark matter halo — essentially a ball of the invisible material surrounding the galaxy — must be keeping it from scattering.
The students began exploring Segue 1 as homework, said Karl Gebhardt, a UT Austin astronomy professor who taught the class with UT San Antonio’s assistant professor Richard Anantua. He wanted to show them how to run computer simulations to infer what can’t be seen.
Because Segue 1 was known to have a lot of dark matter, the class used it for the exercise. The professors broke the students up into three groups: one to focus on dark matter, another to include a hypothetical black hole, and another to look at the amount of stars. The goal was to find which scenario provided the best match to the actual behavior of the stars in the system.
First they had to eliminate from their dataset the stars on the outskirts of the galaxy pulled by the Milky Way. The purpose was to isolate the stars that were more clearly under Segue 1’s gravitational effects. Then, the students studied the speed and direction of the remaining stars.
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They found that the stars toward the center were traveling in quick, tight orbits, a clue they were circling a black hole. The models that included the black hole were by far the best fit for Segue 1’s actual star movements.
« I did not expect that, » Gebhardt told Mashable. « I said, ‘OK, well, we got to write this up.' »
The results recently appeared in The Astrophysical Journal Letters, with several of the students from the spring 2024 class credited as coauthors.
Black holes are mysterious regions in space where gravity is so strong that nothing — not even light — can escape. Just decades ago, their existence was uncertain, but in 2019, the Event Horizon Telescope captured the first image of one in the Messier 87 galaxy, about 53 million light-years away. Instead of a surface, black holes have something called an « event horizon » — a point beyond which anything that crosses is lost forever. Scientists are only able to photograph black holes by capturing the glowing gas swirling around them and the shadow their gravity casts on that light.
The extra surprise for the class was just how massive the central black hole seems to be. Their models reveal it could be about 10 times greater than all of the stars in the dwarf galaxy. In most cases, the stars of a galaxy outweigh the black hole.
« This could be because Segue 1 is actually kind of lying to us, » Lujan said. « It could have started as a much larger galaxy, and due to its close interactions with the Milky Way, the Milky Way has siphoned off gas and stunted star formation in the galaxy. »
For his doctoral thesis, Lujan plans to use advanced computers and artificial intelligence to run more simulations on other dwarf galaxies previously believed to be dominated by dark matter.
Another possible explanation for Segue 1 is that it might be similar to a new type of galaxy discovered by NASA‘s James Webb Space Telescope. Scientists are referring to these objects found in the early universe as « little red dots. » They appear to have developed with enormous black holes and few stars. One research team has proposed that they might be giant spheres of gas wrapped around black holes, dubbing them « black hole stars. »
The study is an important reminder that new things can be learned just by looking at old data in a different way, Gebhardt said.
« What gets me really excited is that these galaxies that we’re finding here like Segue 1 may be analogs to the early universe, where the black holes are really massive, » he said. « And so, irregardless of whether we’ve thought hard enough about how to build these things, nature has found a way. »
