Astronomers are challenging the long-held belief that a supermassive black hole resides at the center of the Milky Way galaxy. New research suggests that an enormous concentration of dark matter could be responsible for the powerful gravitational effects previously attributed to Sagittarius A* (Sgr A*), the galaxy’s central object.
This potential shift in understanding stems from the ability of this unseen dark matter to explain two distinct sets of observations. Stars in the immediate vicinity of the galactic center exhibit rapid, chaotic movements, while those in the outer regions of the Milky Way rotate more smoothly. The new model, detailed in Monthly Notices of the Royal Astronomical Society, proposes a unified explanation for both phenomena.
Challenging the Black Hole Paradigm
For decades, the prevailing theory posited that Sgr A*, a supermassive black hole, governs the extreme orbits of stars known as S-stars. These stars race around the galactic center at speeds reaching thousands of kilometers per second. However, the recent study questions this interpretation, proposing that a specific type of dark matter – composed of fermions, lightweight subatomic particles – could create a cosmic structure that accurately replicates observed stellar motions.
A Dark Matter Core and Halo
The proposed model envisions a dense, compact core of fermionic dark matter surrounded by a larger, more diffuse halo. This core and halo would function as a single, interconnected system. The inner core, possessing sufficient mass and concentration, could mimic the gravitational pull of a black hole, accounting for the observed paths of the S-stars and the movements of nearby dust-shrouded objects called G-sources.
Evidence from the Galaxy’s Outer Reaches
Crucially, evidence supporting this model comes from recent observations made by the European Space Agency’s GAIA DR3 mission. This survey meticulously mapped the movement of stars and gas in the Milky Way’s outer halo, revealing the galaxy’s rotation curve with unprecedented precision. The data revealed a slowing of orbital speeds at greater distances from the center – a pattern known as the Keplerian decline.
Researchers argue that this behavior aligns with predictions generated by the dark matter halo within their model, when considered alongside the known mass of the Milky Way’s disk and central bulge. This alignment strengthens the case for the fermionic dark matter explanation. Traditional Cold Dark Matter models predict halos that extend outward with a long power law tail, whereas the fermionic model suggests a more compact halo with sharper outer boundaries.
An International Collaborative Effort
The research was a collaborative effort involving scientists from institutions across Argentina, Italy, Colombia, and Germany, including the Institute of Astrophysics La Plata, the International Centre for Relativistic Astrophysics Network, the National Institute for Astrophysics, the Relativity and Gravitation Research Group, and the Institute of Physics University of Cologne.
“This is the first time a dark matter model has successfully bridged these vastly different scales and various object orbits, including modern rotation curve and central stars data,” said Dr. Carlos Argüelles of the Institute of Astrophysics La Plata. “We are not just replacing the black hole with a dark object; we are proposing that the supermassive central object and the galaxy’s dark matter halo are two manifestations of the same, continuous substance.”
Mimicking the Black Hole Shadow
The model has already demonstrated a remarkable consistency with existing observations. A prior study, published alongside the current research in Monthly Notices of the Royal Astronomical Society, showed that when light interacts with an accretion disk surrounding these dense dark matter cores, it creates a shadow-like feature. Remarkably, this shadow closely resembles the image captured by the Event Horizon Telescope (EHT) of Sgr A*.
“This is a pivotal point,” said lead author Valentina Crespi of the Institute of Astrophysics La Plata. “Our model not only explains the orbits of stars and the galaxy’s rotation but is also consistent with the famous ‘black hole shadow’ image. The dense dark matter core can mimic the shadow because it bends light so strongly, creating a central darkness surrounded by a bright ring.”
Future Observations to Refine the Theory
The research team directly compared their fermionic dark matter model with the traditional black hole explanation using statistical methods. While current data from stars near the galactic center do not definitively favor one scenario over the other, the dark matter model provides a single, cohesive framework that explains both the galactic center’s dynamics and the broader structure of the galaxy.
Future observations hold the key to resolving this debate. More precise measurements from instruments like the GRAVITY interferometer on the Very Large Telescope in Chile, along with searches for photon rings (which are expected around true black holes but would not appear in the dark matter core model), could provide conclusive evidence. If confirmed, these findings could fundamentally alter our understanding of the massive object at the heart of the Milky Way.
