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Strange things are happening in the Milky Way.
According to a new analysis of data from the Gaia satellite, the star cluster closest to our solar system is currently being torn apart – disturbed not only by normal processes, but also by the gravitational pull of something. massive that we can’t see.
This disturbance, say astronomers, could be a clue that an invisible cluster of dark matter is nearby, wreaking gravitational havoc on anything within its reach.
In fact, star clusters being separated by gravitational forces are inevitable. A star cluster is, as the name suggests, a dense and tight concentration of stars. Even internally, gravitational interactions can get quite rowdy.
Between these internal interactions and external galactic tidal forces – the gravity exerted by the galaxy itself – star clusters can eventually separate into rivers of stars: what is called a tidal current.
These streams are difficult to see in the sky, because it is often quite difficult to measure stellar distances, and therefore to group the stars. But the Gaia satellite worked to map the Milky Way galaxy in three dimensions in as much detail and accuracy as possible, and the most accurate position and speed data on as many stars as possible.
Because the stars taken from a star cluster always share the same speed (more or less) as the stars in cluster, Gaia’s data has helped astronomers identify many previously unknown tidal currents and star clusters with tidal tails – strands of stars that began to break away from the cluster at the both in front and behind him.
In 2019, astronomers revealed they found evidence in Gaia’s second data release of tide tails from the Hyades; 153 light years away, it is the closest star cluster to Earth.
This caught the attention of astronomer Tereza Jerabkova and his colleagues at the European Space Agency and the European Southern Observatory. When Gaia Data Release 2.5 (DR2.5) and DR3 became available, they settled in, expanding search parameters to catch stars missed by previous detections.
They found hundreds and hundreds of stars associated with the Hyades. The central cluster is about 60 light years in diameter; tidal tails stretch for thousands of light years.
Having such tails is pretty normal for a star cluster disturbed by galactic tidal forces, but the team noticed something odd. They performed cluster disturbance simulations and found many more stars in the back tail of the simulation. In the real cluster, some stars are missing.
The team ran more simulations to find out what could cause these stars to stray – and found that an interaction with something large, about 10 million times the mass of the Sun, could mimic the observed phenomenon.
“There must have been close interaction with this really massive clump, and the Hyades just got run over,” Jerabkova said.
The big problem with this scenario is that we currently can’t see anything so massive nearby. However, the Universe is actually full of unseen things – dark matter, the name we give to the mysterious mass that we can only infer to exist through its gravitational effects on things we can see.
Based on 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 clusters in the early Universe collected and shaped 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 have a diameter of 1.9 million light years. In these halos, astronomers predict denser clusters, called dark matter subhalos, which simply drift.
Future research may reveal a structure that could have caused the strange disappearance of stars in the Hyades’ trailing tail; if they don’t, the researchers think the disturbance could be the work of a dark matter subhalo.
The discovery also suggests that tidal currents and tidal tails could be great places to search for sources of mysterious gravitational interactions.
“With Gaia, the way we see the Milky Way has changed completely,” Jerabkova said. “And with these findings, we’ll be able to map the Milky Way’s substructures much better than ever.”
The research was published in Astronomy and astrophysics.
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