Strange things are in the way in the Milky Way.
According to a new analysis of Gaia satellite data, the closest stellar cluster to our solar system is currently destroyed – disturbed not only by normal processes, but also by the gravitational pull of something massive that we cannot see.
This disruption, astronomers say, could be an indication that an invisible pile of dark matter is nearby, causing gravitational havoc on everything within its reach.
In fact, groups of stars that are removed by gravitational forces are inevitable. A group of stars is, as the name suggests, a close and dense concentration of stars. Even internally, gravitational interactions can become quite agitated.
Between those internal interactions and the external galactic tidal forces – the gravity exerted by the galaxy itself – groups of stars can end up separated into rivers of stars: what is known as a tidal flow.
These fluxes are difficult to see in the sky because it is often quite difficult to measure stellar distances and therefore group stars together. But the Gaia satellite worked to map the Milky Way galaxy in three dimensions with the most detailed and highest achievable accuracy, as well as the most accurate position and speed data on as many stars as possible.
Because stars drawn from a group of stars still have the same speed (more or less) as stars in the cluster, Gaia data helped astronomers identify many previously unknown tidal streams and groups of stars with tidal tails – strands of stars that began to detach from the cluster both in front and behind it.
In 2019, astronomers revealed that they found evidence in the second version of Gaia data of the tidal tails flowing from Hyades; 153 light-years away, it is the closest group of stars to Earth.
This caught the attention of astronomer Tereza Jerabkova and her colleagues at the European Space Agency and the European Southern Observatory. When Gaia Data Release 2.5 (DR2.5) and DR3 became available, they took refuge, extending the search parameters to catch the stars that previous detections had missed.
They found hundreds and hundreds of stars associated with Hyades. The central cluster has a radius of about 60 light-years; the tidal tails stretch for thousands of light-years.
Having such tails is quite normal for a group of stars interrupted by galactic tidal forces, but the team noticed something strange. They ran simulations of cluster disruption and found many more stars in the final queue of the simulation. In the real cluster, some stars are missing.
The team performed several simulations to find out what could cause these stars to wander – 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 really massive group, and Hyades has just been shattered,” Jerabkova said.
The big problem with this scenario is that right now we can’t see anything so massive anywhere nearby. However, the Universe is actually full of invisible things – dark matter, the name we give to the mysterious mass whose existence we can only deduce from its gravitational effects on the things we can see.
According to these gravitational effects, scientists have calculated that about 80% of all matter in the universe is dark matter. Dark matter is believed to be an essential part of galaxy formation – large groups of it in the early Universe collected and modeled normal matter in the galaxies we see today.
Schematic diagram of the dark matter halo of our galaxy. (Digital Universe / American Museum of Natural History)
Those piles of dark matter can still be found in “dark halos” spread around galaxies. The Milky Way is believed to be 1.9 million light-years long. Inside those halos, astronomers predict denser clusters, called subhalos of dark matter, just drifting.
Future searches may transform a structure that could have caused the strange disappearance of the stars in the tail of Hyades; otherwise, researchers believe the disruption could be the work of a subhalo of dark matter.
The discovery also suggests that tidal streams and tidal queues could be great places to look for sources of mysterious gravitational interactions.
“With Gaia, the way we see the Milky Way has changed completely,” Jerabkova said. “And with these discoveries, we will be able to map the substructures of the Milky Way much better than ever.”
The research was published in Astronomy and astrophysics.