Artistic representation of the Milky Way, where the innermost stars move at near relativistic speeds (defined as velocities that constitute a significant fraction of the speed of light, typically considered to be 10% or more) around a dense core of dark matter, with no black hole at the centre. At greater distances, the halo part of the same invisible dark matter distribution continues to shape the motions of stars in the outskirts of our galaxy, tracing the characteristic rotation curve. Credit: Valentina Crespi et al. (Attribution (CC BY 4.0))
A new study has suggested it is dark matter, and not a black hole, that could be powering the Milky Way’s heart.
Our Milky Way galaxy may not have a supermassive black hole at its centre but rather an enormous clump of mysterious dark matter exerting the same gravitational influence, astronomers say.
They believe this invisible substance – which makes up most of the Universe’s mass – can explain both the violent dance of stars just light-hours (often used to measure distances within our own solar system) away from the galactic centre and the gentle, large-scale rotation of the entire matter in the outskirts of the Milky Way.
It challenges the leading theory that Sagittarius A* (Sgr A*), a proposed black hole at the heart of our galaxy, is responsible for the observed orbits of a group of stars, known as the S-stars, which whip around at tremendous speeds of up to a few thousand kilometres per second.
The international team of researchers have instead put forward an alternative idea – that a specific type of dark matter made up of fermions, or light subatomic particles, can create a unique cosmic structure that also fits with what we know about the Milky Way’s core.
It would in theory produce a super-dense, compact core surrounded by a vast, diffuse halo, which together would act as a single, unified entity.
The inner core would be so compact and massive that it could mimic the gravitational pull of a black hole and explain the orbits of S-stars that have been observed in previous studies, as well as the orbits of the dust-shrouded objects known as G-sources which also exist nearby.
GAIA DR3
Of particular importance to the new research is the latest data from the European Space Agency’s GAIA DR3 mission, which has meticulously mapped the rotation curve of the Milky Way’s outer halo, showing how stars and gas orbit far from the centre.
It observed a slowdown of our galaxy’s rotation curve, known as the Keplerian decline, which the researchers say can be explained by their dark matter model’s outer halo when combined with the traditional disc and bulge mass components of ordinary matter.
This, they add, strengthens the ‘fermionic’ model by highlighting a key structural difference: while traditional Cold Dark Matter halos spread out following an extended ‘power law’ tail, the fermionic model predicts a tighter structure, leading to more compact halo tails.
The research has been carried out by an international collaboration involving the Institute of Astrophysics La Plata in Argentina, International Centre for Relativistic Astrophysics Network (ICRANet) and National Institute for Astrophysics (INAF) in Italy, Relativity and Gravitation Research Group in Colombia and Institute of Physics University of Cologne in Germany.
Study co-author Dr Carlos Argüelles, of the Institute of Astrophysics La Plata, said: “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.
“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.”
Pivotal point
Crucially, this fermionic dark matter model had already passed a significant test.
A previous study by Pelle et al. (2024) showed that when an accretion disk illuminates these dense dark matter cores, they cast a shadow-like feature strikingly similar to the one imaged by the Event Horizon Telescope (EHT) collaboration for Sgr A*.
Lead author Valentina Crespi, of the Institute of Astrophysics La Plata, said: “This is a pivotal point. 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.”
The researchers statistically compared their fermionic dark matter model to the traditional black hole model.
They found that while current data for the inner stars cannot yet decisively distinguish between the two scenarios, the dark matter model provides a unified framework that explains the galactic centre (central stars and shadow), and the galaxy at large.
The new study paves the way for future observations. More precise data from instruments such as the GRAVITY interferometer, on the Very Large Telescope in Chile, and the search for the unique signature of photon rings – a key feature of black holes and absent in the dark matter core scenario – will be crucial to test the predictions of this new model, the authors say.
The outcome of these findings could potentially reshape our understanding of the fundamental nature of the cosmic behemoth at the heart of the Milky Way.
The new study is published in Monthly Notices of the Royal Astronomical Society (MNRAS).
