Strange things are happening in the Milky Way.
According to a new analysis of Gaia satellite data, the star cluster closest to our solar system is currently being torn apart – disrupted not only by normal processes, but also by the gravity of something massive that we cannot see.
This disturbance, astronomers say, could be an indication that there is an invisible lump of dark matter nearby, causing gravitational damage to everything within its reach.
Actually, it is inevitable that star clusters will be pulled apart by gravity. A star cluster, as its name suggests, is a dense, dense concentration of stars. Even internally, the gravitational interactions can get quite noisy.
Between those internal interactions and external galactic tidal forces – the gravitational force exerted by the galaxy itself – star clusters can be pulled apart into rivers of stars: what is known as a tidal current.
These currents are difficult to see in the sky, because it is often quite difficult to measure star distances and therefore group stars together. But the Gaia satellite has worked to map the Milky Way Galaxy in three dimensions with the most detailed and attainable precision, and with the most accurate position and velocity data for as many stars as possible.
Because stars from a cluster still share the same speed (more or less) as the stars in the cluster, the Gaia data has helped astronomers identify many previously unknown tidal currents and star clusters with tidal tails – threads of stars that began to separate from the cluster both in front and behind it.
In 2019, astronomers revealed that they had found evidence in the second Gaia data release of tide tails pouring out of the Hyades; 153 light-years away, this is the star cluster closest to Earth.
This caught the attention of astronomer Tereza Jerabkova and her colleagues from the European Space Agency and the European Southern Observatory. When Gaia Data Release 2.5 (DR2.5) and DR3 became available, they went in and expanded the search parameters to catch the stars that the previous detections missed.
They found hundreds and hundreds of stars associated with the Hyades. The central cluster is about 60 light-years across; the tide tails span thousands of light years.
Having such tails is fairly normal for a star cluster disrupted by galactic tidal forces, but the team noted something odd. They ran simulations of the disruption of the cluster and found significantly more stars in the back tail of the simulation. Some stars are missing in the real cluster.
The team ran more simulations to find out what could lead these stars astray – and found that an interaction with something large, about 10 million times the mass of the sun, could reproduce the observed phenomenon.
“There must have been a close interaction with this truly enormous mass, and the Hyades have just been destroyed,” said Jerabkova.
The big problem with that scenario is that right now we can’t see anything so massive anywhere near it. However, the universe is actually full of invisible things – dark matter, the name we give to the mysterious mass whose existence we can deduce only through the gravitational effects on the things we can see.
According to these gravitational effects, scientists have calculated that about 80 percent of all matter in the Universe is dark matter. Dark matter is thought to be an essential part of galaxy formation – large clumps of it collected in the early Universe and formed the normal matter into the galaxies we see today.
Schematic diagram of the halo of the dark matter of our galaxy. (Digital Universe / American Museum of Natural History)
Those clumps of dark matter can still be found in sprawling “dark halos” around galaxies. The Milky Way is 1.9 million light years across. Within those halos, astronomers predict denser clumps called dark matter subhalos that just float around.
Future quests may yield a structure that could have caused the strange disappearance of stars in Hyades’ rear tail; if they don’t, the researchers think the perturbation could be the work of a dark matter subhalo.
The finding also suggests that tidal currents and tide tails could be excellent places to look for sources of mysterious gravitational interactions.
“With Gaia, the way we see the Milky Way has completely changed,” said Jerabkova. “And with these discoveries, we will be able to map the Milky Way’s sub-structures much better than ever before.”
The research is published in Astronomy and Astrophysics