It’s undeniable that something exceptionally massive resides at the core of the Milky Way galaxy, yet a recent study raises the question of whether a supermassive black hole is the sole plausible explanation. The findings of this investigation are detailed in the journal Monthly Notices of the Royal Astronomical Society.
All measurements conducted to date concerning the galactic center are consistent with the concept of an incredibly dense object possessing a mass roughly four million times that of the Sun. However, according to this new paper, a closer look suggests these data points could equally describe a gigantic, compact clump of fermionic dark matter, one that lacks an event horizon.
Presently, our observational precision is insufficient to definitively distinguish between these two models. Nonetheless, understanding the composition of the dark matter in the galactic nucleus would equip astronomers with a novel apparatus for interpreting the dark matter structure of the entire galaxy.
“We aren’t simply swapping the black hole for a dark object; we are proposing that the supermassive central object and the galaxy’s dark matter halo represent two manifestations of the same continuous substance,” explains Carlos Argüelles from the Institute of Astrophysics of La Plata in Argentina.
Dark matter remains one of the Cosmos’ greatest enigmas. Scientists can calculate the quantity of ordinary matter in the universe with high accuracy. Yet, when all this is tallied, the resulting gravitational influence far exceeds what this known matter can account for.
Whatever is responsible for all this excess gravity neither absorbs nor emits light; we only perceive its existence through its gravitational effects. This is dark matter, and it generates so much gravitational pull that it constitutes approximately 84 percent of the universe’s total material content.
The methodology used by scientists to confirm the presence and determine the mass of the massive object at the Milky Way’s heart also relied on gravity—specifically, tracing the long, looping trajectories and changing velocities of high-speed stars orbiting the galactic center.
The most straightforward explanation for this mass, one requiring the fewest assumptions, posits a supermassive black hole designated Sagittarius A* (Sgr A*). In 2022, an image captured by the Event Horizon Telescope (EHT) collaboration even appeared to show the ‘shadow’ of this black hole.
But this is not the only interpretation. For instance, prior research indicated that an accretion disk swirling around a concentrated dark matter clump might potentially generate a shadow surprisingly similar to the one recorded by the EHT.
An international team of researchers, led by astrophysicist Valentina Crespí from the Institute of Astrophysics of La Plata, chose to develop this concept further: could the observed stellar orbits around Sgr A* be explained by the presence of a dark matter core instead?
While some dark matter models are thin and diffuse, one specific framework permits the existence of dense aggregations—fermionic dark matter, whose particles adhere to quantum rules that prevent them from collapsing infinitely, much like how electrons and neutrons resist further compaction below a certain density threshold.
The theoretical outcome is a super-dense, gravitationally stable configuration, analogous in principle to a white dwarf or a neutron star, but composed of dark matter fermions rather than normal matter particles.
This naturally leads to the question: If such an object occupied the galactic center, would the behavior of stars orbiting it change?
There is a collection of stars known as S-stars whose intricate movements around the galactic center reflect the gravitational potential exerted by the central mass. The most crucial of these indicators is a star named S2, due to its relatively short, 16-year orbit, which has been studied and characterized with exceptional precision.
The researchers simulated the behavior of S2 under both the traditional black hole interpretation of Sgr A* and their proposed fermionic dark matter clump.
Both models reproduced the star’s motion with nearly identical accuracy. Therefore, this finding doesn’t prove that Sgr A* is dark matter; it demonstrates that it could be, but at this time, we lack enough data to draw a definitive conclusion.
However, there is an additional argument leaning toward fermionic dark matter. The Milky Way map generated by the Gaia spacecraft—the most comprehensive to date—reveals that the galaxy’s rotation slows down at greater distances from the galactic center.
The researchers assert that this so-called Keplerian decline is more easily accounted for by the presence of an extensive, sprawling halo of fermionic dark matter enveloping the Milky Way than by other dark matter models.
“This is the first time a dark matter model has successfully reconciled these vastly different scales and the diverse orbits of objects, encompassing contemporary rotation curve data and central stars,” states Argüelles.
Future observations may help resolve the intriguing question regarding the true nature of Sgr A. For example, long-term monitoring could expose subtle irregularities in stellar orbits that would tip the balance of the explanation one way or the other. Stars orbiting closer to Sgr A than S2 might also hold clues.
Furthermore, upcoming observations from the Event Horizon Telescope may reveal finer details in the light-bending region surrounding Sgr A*. Certain features associated with the extreme gravity of a black hole, such as a well-defined photon ring, might be absent or altered if the central object is, in fact, a horizonless dark matter core.
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