The Milky Way Might Have a Core of Dark Matter Instead of a Black Hole

The second of three images of ESO’s GigaGalaxy Zoom project is a new and wonderful 340-million-pixel vista of the central parts of our galactic home, a 34 by 20-degree wide image that provides us with a view as experienced by amateur astronomers around the world. Taken by Stéphane Guisard, an ESO engineer and world-renowned astrophotographer, from Cerro Paranal, home of ESO’s Very Large Telescope, this second image directly benefits from the quality of Paranal’s sky, one of the best on the planet. The image shows the region spanning the sky from the constellation of Sagittarius (the Archer) to Scorpius (the Scorpion). The very colourful Rho Ophiuchi and Antares region features prominently to the right, as well as much darker areas, such as the Pipe and Snake Nebulae. The dusty lane of our Milky Way runs obliquely through the image, dotted with remarkable bright, reddish nebulae, such as the Lagoon and the Trifid Nebulae, as well as NGC 6357 and NGC 6334. This dark lane also hosts the very centre of our Galaxy, where a supermassive black hole is lurking. The image was obtained by observing with a 10-cm Takahashi FSQ106Ed f/3.6 telescope and a SBIG STL CCD camera, using a NJP160 mount. Images were collected through three different filters (B, V and R) and then stitched together. This mosaic was assembled from 52 different sky fields made from about 1200 individual images totalling 200 hours exposure time, with the final image having a size of 24 403 x 13 973 pixels. Note that the final, full resolution image is only available through Stéphane Guisard. #L

The current scientific consensus is that a supermassive black hole lurks at the center of our galaxy. We know that’s true in other galaxies — there’s even photographic evidence of a black hole in M87. However, a new study suggests that the Milky Way might not have a black hole. The object, known as Sagittarius A*, may actually be a blob of dark matter, based on the properties of several objects spotted zipping around it. If true, this would have major implications for our understanding of the universe. 

The problem is we can’t see Sagittarius A* (pronounced “Sagittarius A star”) directly, which is what you’d expect from a black hole. We can only infer its presence from the motion of objects around it, which do indeed appear to be under the influence of a very massive object. In the past, scientists have used the motion of objects like the star S2 to validate general relativity as they swung past the supposed black hole. But then there’s G2; this object, which may or may not be a cloud of hydrogen gas, flew past Sagittarius A* a while back, and it didn’t get torn asunder as expected. 

Last year, a group of Italian researchers showed that the movement of S2 and G2 was also consistent with a different model, one in which the center of the Milky Way is inhabited by dark matter fermions. These particles are ultra-light, so they would not collapse into a black hole until there were about 100 times more of them. At the same time, they could cluster together in a dark blob and affect nearby objects with their gravity. 

In the new study, which was accepted for publication in MNRAS Letters, the team expanded that model to the 17 best-characterized S-group stars orbiting the galactic center. And sure enough, it’s a fit — these orbits are compatible with a bubble of dark matter in Sagittarius A* instead of a black hole. 

The M87 supermassive black hole imaged in 2019.

Scientists currently estimate the visible part of the universe only makes up about 20 percent of its total mass. The remainder is dark matter, a substance we have yet to properly characterize. Like black holes, we can only infer dark matter’s presence from gravitational effects. This study could mean that the two phenomena are related. The team proposes that these clumps of dark matter can reach critical mass and collapse into black holes. That could help to explain how these enormous supermassive objects come to be in the first place, something that we have only been able to speculate on. At the same time, this could account for a lot of the universe’s missing mass.

However, this is all far from certain. There could be other explanations, or the analysis carried out by this group could be wrong. We won’t know until scientists have spent a lot more time staring at this corner of space.

Now read:

• Scientists will try to directly image Sagittarius A*, the black hole at the center of the Milky Way
• The giant black hole at the center of our galaxy has exploded before and will explode again
• General Relativity Still Holds Up in New Analysis of a Supermassive Black Hole