Could Sagittarius A* Be Fermionic Dark Matter Rather Than a Supermassive Black Hole?

There is little doubt that an enormous object lies at the centre of the Milky Way galaxy, yet a new study considers whether a supermassive black hole is necessarily the only answer.

Every measurement of the galactic centre so far agrees with the presence of an exceptionally dense object with a mass roughly 4 million times that of the Sun. The new paper argues, however, that the same evidence could also fit a huge, compact concentration of fermionic dark matter with no event horizon.

At present, observations are not precise enough to distinguish between these two possibilities. If dark matter makes up the galactic nucleus, though, astronomers could gain a fresh way to interpret the dark matter distribution across the whole galaxy.

"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," explains astrophysicist Carlos Argüelles of the Institute of Astrophysics La Plata in Argentina.

Dark matter at the Milky Way's centre

Dark matter remains one of the greatest puzzles in the Universe. Researchers can determine the quantity of ordinary matter in the Universe very accurately, but adding it all together still leaves far more gravity than that matter can explain.

Whatever creates this additional gravitational pull neither absorbs nor gives off light. Its existence is known only through its gravitational effects. This unseen material is dark matter, and it accounts for about 84 percent of the Universe's matter budget.

Gravity was also the means by which scientists established the existence and mass of the massive object at the centre of the Milky Way. They tracked the long, sweeping paths and varying speeds of fast-moving stars orbiting the galactic centre.

The simplest account of this mass, requiring the fewest assumptions, is a supermassive black hole called Sagittarius A* (Sgr A*). In 2022, an image captured by the Event Horizon Telescope (EHT) collaboration even seemed to reveal the black hole's 'shadow'.

Yet this is not the sole possible explanation. Earlier research, for instance, found that an accretion disc shining around a dense concentration of dark matter might create a shadow strikingly like the EHT image.

An international team led by astrophysicist Valentina Crespi of the Institute of Astrophysics La Plata sought to take the idea further. Could the observed orbits of stars around Sgr A* likewise be accounted for by a dark matter core?

Some dark matter models describe a thin, diffuse material, whereas one candidate permits dense clumps: fermionic dark matter. Its particles follow quantum rules that stop them being compressed without limit, much as electrons and neutrons cannot be forced together beyond a particular density threshold.

In theory, this would produce an ultradense, gravitationally stable clump that is, in principle, comparable to a white dwarf or neutron star. Instead of ordinary matter particles, however, it would consist of dark matter fermions.

S2 star orbits and fermionic dark matter

The researchers therefore asked whether an object of this kind at the galactic centre would alter the behaviour of stars in orbit around it.

Several so-called S stars follow elaborate paths around the galactic centre, tracing the gravitational potential created by the mass there. The key tracer is a star known as S2, whose relatively brief 16-year orbit has been observed and described in exceptional detail.

The team modelled S2's behaviour under both the standard black hole interpretation of Sgr A* and their fermionic dark matter clump model.

Each model recreated the star's movement with almost exactly the same degree of accuracy. This does not demonstrate that Sgr A* is dark matter; rather, it shows that it could be, while the available data remain too limited to decide.

Fermionic dark matter has another apparent advantage. The Gaia spacecraft's Milky Way map, the most comprehensive yet produced, indicates that the galaxy's rotation slows at larger distances from the galactic centre.

According to the researchers, this so-called Keplerian decline is more readily accounted for by an immense, extended halo of fermionic dark matter surrounding the Milky Way than by alternative dark matter models.

"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," Argüelles says.

Future Event Horizon Telescope observations

Upcoming observations could help answer the compelling question of Sgr A's true nature. Long-term monitoring might uncover subtle properties in stellar orbits that favour one explanation over the other. Stars orbiting even nearer to Sgr A than S2 could provide further evidence.

Future Event Horizon Telescope observations could also expose more detailed features in the region where light bends around Sgr A*. If the central object is a dark matter core without a horizon rather than a black hole, features linked to a black hole's extreme gravity, including a clearly defined photon ring, may be missing or changed.

The research has been published in the Monthly Notices of the Royal Astronomical Society.