There are strange things in the Milky Way.
According to a new analysis of data from Gaia satellites, currently the star cluster closest to our solar system is being broken, disturbed not only by normal processes, but also by the gravitational pull of something massive that does not. we can see.
This disruption, astronomers say, could be an indication that an invisible group of dark matter is nearby, causing gravitational damage to anything within its reach.
In reality, star clusters that are separated by gravitational forces are inevitable. A star cluster is, as its name suggests, a narrow, dense concentration of stars. Even internally, gravitational interactions can be quite annoying.
Between these internal interactions and the external galactic tidal forces (the gravity exerted by the galaxy itself), star clusters can end up separated in star rivers: what is known as a tidal current.
These streams are difficult to see in the sky, because it is often quite difficult to measure stellar distances and therefore group stars. But the Gaia satellite has been working to map the Milky Way galaxy in three dimensions with the maximum detail and maximum accuracy achievable and the most accurate position and speed data in the maximum number of stars possible.
Because stars extracted from a star cluster still share the same speed (more or less) as stars inside the cluster, Gaia data have helped astronomers identify many hitherto unknown tidal currents, and star clusters with tidal tails, threads of stars that have begun to detach from the cluster both in front and behind.
In 2019, astronomers revealed that they had found evidence in Gaia’s second release of tidal tail data flowing from the Hyades; 153 light-years away, it is the closest star cluster to Earth.
This caught the attention of astronomer Tereza Jerabkova and colleagues at the European Space Agency and the Southern European Observatory. When Gaia Data Release 2.5 (DR2.5) and DR3 were available, they went home, expanding search parameters to capture stars that previous detections had lost.
They found hundreds and hundreds of stars associated with the Hyades. The central cluster is about 60 light-years in diameter; tidal tails span thousands of light-years.
Having these tails is quite normal for a star cluster interrupted by galactic tidal forces, but the team noticed something strange. They performed cluster disruption simulations and found significantly more stars in the final queue of the simulation. In the real cluster, some stars are missing.
The team conducted further simulations to find out what could cause these stars to deviate 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 the Hyades are over,” Jerabkova said.
The big problem with this scenario is that we currently can’t see anything so massive anywhere nearby. However, the Universe is really 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 approximately 80 percent 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 formed 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)
These clusters of dark matter can still be found today in ‘dark halos’ spread around galaxies. The Milky Way is believed to be 1.9 million light-years in diameter. Within these halos, astronomers predict denser groups, called subhalos of dark matter, that only move around.
Future research may lead to a structure that could have caused the strange disappearance of stars in Hyades’ tail; if they don’t, researchers think the disruption could be the work of a subhalo of dark matter.
The finding also suggests that tidal currents and tidal queues could be excellent places to look for sources of mysterious gravitational interactions.
“With Gaia, our way of seeing 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.”
The research has been published in Astronomy and astrophysics.