Strange things are going on in the Milky Way.
According to a new analysis of Gaia satellite data, the nearest star cluster to our solar system is currently disintegrating – disrupted not only by normal processes, but also by the gravitational pull of something massive that we cannot see.
This disturbance, astronomers say, could be a hint that there is an invisible cluster of dark matter nearby that creates gravitational havoc on anything within his reach.
In fact, star clusters separated by gravitational forces are inevitable. The star cluster is, as the name suggests, a narrow, dense concentration of stars. Even from the inside, gravitational interactions can become quite inconsistent.
Between these internal interactions and the external galactic tidal forces – the gravity exerted by the galaxy itself – star clusters can eventually be separated into rivers of stars: what is known as the tidal current.
These streams are difficult to see in the sky because it is often quite inconvenient to measure stellar distances and therefore group stars together. But the Gaia satellite is working to map the Milky Way galaxy in three dimensions with the most detail and the highest possible precision, and the most accurate position and velocity data on as many stars as possible.
Because stars drawn from a star cluster still share the same speed (more or less) as stars u Egg, Gaia data have helped astronomers identify many previously unknown tidal currents and clusters of stars with tidal tails – nor stars that have begun to release from the cluster both in front of and behind it.
In 2019, astronomers discovered that they had found evidence in another publication of data on the tidal tails of Gaia flowing from the Hyades; at a distance of 153 light-years, it is the closest star cluster to Earth.
This attracted the attention of astronomer Teresa 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 became domesticated, expanding search parameters to catch stars that earlier discoveries missed.
They found hundreds and hundreds of stars associated with the Hyades. The width of the central flock is about 60 light-years; tidal tails exceed thousands of light years.
Having such tails is pretty normal for a cluster of stars disrupted by galactic tidal forces, but the team noticed something strange. They performed flock interference simulations and found significantly more stars in the back tail of the simulation. Some stars are missing in the actual set.
The team conducted several simulations to discover what could lead to the stray of these stars – and found that interacting 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 huge crowd, and the Hijades have just been broken up,” 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 names we give to a mysterious mass whose existence we can conclude only by its gravitational effects on things we can see.
According to these gravitational effects, scientists have calculated that approximately 80 percent of all matter in the Universe is dark matter. Dark matter is thought to be an essential part of galaxy formation – large clusters in the early universe collected and shaped normal matter into the galaxies we see today.
These clusters of dark matter can still be found today in the extended ‘dark halos’ around galaxies. The Milky Way has one thought that it is 1.9 million light years long. Within these halos, astronomers predict denser clusters, called subhalos of dark matter, that only permeate.
Future searches could reveal a structure that could have caused a strange disappearance of stars in the backward tail of the Hyades; if not, researchers think the disorder could be the work of a dark matter subhaloa.
The finding also suggests that tidal currents and tidal tails could be excellent places to search for sources of mysterious gravitational interactions.
“With Gay, the way we see the Milky Way has completely changed,” Jerabkova said. “And with these discoveries, we will be able to map the substructures of the Milky Way much better than ever before.”
The research was published in Astronomy and astrophysics.