Like most galaxies, the Milky Way is thought to host a supermassive black hole in its center – but perhaps its dark heart is made of different stuff. A new study proposes that it could instead be a dense core of dark matter, made up of hypothetical particles called “darkinos.”
The Milky Way is held together by a huge mass in the middle, equivalent to about 4 million Suns. Known as Sagittarius A* (Sgr A*), this massive object can’t be directly seen, but its existence can be inferred from the motions of stars around it. A supermassive black hole is the most logical candidate – but maybe it’s not the only explanation.
The doubts were seeded seven years ago. A gas cloud named G2 was found to be orbiting Sgr A*, due to pass perilously close to the object in early 2014. Astronomers watched with anticipation – if Sgr A* was a supermassive black hole as expected, then G2 should be torn to shreds before their eyes.
But surprisingly, G2 survived the sweep without issue. That’s led some scientists to speculate that perhaps it isn’t a gas cloud but a bloated dusty star, with enough gravity to keep its shape. In the new study however, the researchers questioned not the identity of G2, but Sgr A* itself.
Scientists at the International Center for Relativistic Astrophysics (ICRA) in Italy simulated what would happen if they replaced the supermassive black hole with a clump of dark matter. This mysterious stuff is already thought to be concentrated in the center of the galaxy, holding the whole thing together with its gravity.
The ICRA team found that if dark matter had certain properties, it could accurately account for a range of observations, in some cases better than the black hole model. This dark matter would consist of darkinos, neutral ultralight particles belonging to a group called fermions. These darkinos would clump together in the center of the galaxy, and spread into a more diffuse cloud further out.
A key characteristic of fermions is that only one of them can occupy a particular quantum state at a time in a given space, which limits how densely they can be packed together. As such, the core of this ball is a far less extreme environment than a supermassive black hole, which would allow G2 to pass by unscathed.
But that’s not the only observation that the model fits. The team found that if darkinos had a mass of around 56 keV, the simulation accurately predicted the motions of a cluster of nearby stars called the S-stars, as well as the rotation curve of the Milky Way’s outer halo.
As intriguing as the darkino hypothesis is, the case is far from settled. The supermassive black hole idea remains the most likely story, because it explains well-observed physics in a relatively simple way – and besides, we see black holes at the center of most other galaxies.
Still, it pays to keep an open mind, and the team says that further data releases could shed light on the idea, one way or the other.
The research was published in the journal Monthly Notices of the Royal Astronomical Society: Letters, with an earlier paper on the subject appearing in Astronomy & Astrophysics.