Something unknown and unseen — a suspected population of ‘dark matter sub-halos’ — has been destroying the stars in the enormous tidal tail of the nearest star cluster to our sun, reports the ESA. This thin, elongated region of stars and interstellar gas extends into space beyond the Milky Way and was created by tidal forces between interacting galaxies. Data from the ESA’s Gaia star-mapping satellite have revealed evidence that the Hyades star cluster is being disrupted by the gravitational influence of a massive but unseen structure in our galaxy.
“Gaia’s Forensic CSI”
The third early data release of the Gaia Space Observatory measured the distances of hundreds of millions of objects that are many thousands of light years away, at an accuracy equivalent to measuring the thickness of hair at a distance of more than 2,000 kilometers. The data allows astronomers to forensically analyze our Milky Way neighborhood, and tackle crucial questions about the origin and future of our Galaxy.
Among the oldest stars in the universe
Most stars in the universe, including our life-giving Population 1, main-sequence Sun, were formed in massive clusters created from the violent gravitational collapse of matter within a region of a large molecular cloud. Although these clusters are the building blocks of galaxies, their actual formation remains a mystery. Some of the oldest stars in the universe are found in ancient globular clusters that orbit around our home galaxy. Our Milky Way is circled by more than 150 globular clusters, each harboring hundreds of thousands, and sometimes millions of stars. Globular clusters formed very early in the vast halo surrounding the embryonic Milky Way, about 2.3 billion years after the Big Bang, before our galaxy flattened to form the spiral disc.
Dark matter sub-halos
The hypothetical dark matter sub-halos’ are regions that have been decoupled from cosmic expansion and contain gravitationally bound matter containing invisible clouds of particles thought to be relics from the formation of the Milky Way. They are now spread across the Milky Way galaxy, making up an invisible substructure that exerts a noticeable gravitational influence on anything that drifts too close. Their existence, explains the ESA, is detected through observations of their effects on the motions of stars and gas in galaxies and gravitational lensing.
ESA Research Fellow Tereza Jerabkova and colleagues from ESA and the European Southern Observatory made the discovery while studying the way a nearby star cluster is merging into the general background of stars in our galaxy. This discovery was based on Gaia’s Early third Data Release (EDR3) and data from the second release.
The team selected Hyades as their target because it is the nearest star cluster to the Sun, located just over 153 light years away. It is easily visible to skywatchers in both northern and southern hemispheres as a conspicuous ‘V’ shape of bright stars that marks the head of the bull in the constellation of Taurus. Beyond the easily visible bright stars, telescopes reveal a hundred or so fainter ones contained in a spherical region of space, roughly 60 light years across.
The Tidal Tails
A star cluster will naturally lose stars because as those stars move within the cluster they tug at each other gravitationally. This constant tugging slightly changes the stars’ velocities, moving some to the edges of the cluster. From there, the stars can be swept out by the gravitational pull of the galaxy, forming two long tails.
One tail trails the star cluster, the other pulls out ahead of it, explains the ESA, They are known as tidal tails, and have been widely studied in colliding galaxies but no one had ever seen them from a nearby open star cluster, until very recently.
Gaia makes detecting which stars in the sky are moving in a similar way to the star cluster easy because it is precisely measuring the distance and movement of more than a billion stars in our galaxy. “These are the two most important quantities that we need to search for tidal tails from star clusters in the Milky Way,” says Tereza.
Previous attempts by other teams, explains Tezera, had met with only limited success because the researchers had only looked for stars that closely matched the movement of the star cluster. This excluded members that left earlier in its 600–700 million year history and so are now travelling on different orbits.
True extent of the Hyades tidal tails
To understand the range of orbits to look for, Tereza constructed a computer model that would simulate the various perturbations that escaping stars in the cluster might feel during their hundreds of millions of years in space. It was after running this code, and then comparing the simulations to the real data that the true extent of the Hyades tidal tails were revealed. Tereza and colleagues found thousands of former members in the Gaia data. These stars now stretch for thousands of light years across the galaxy in two enormous tidal tails.
The Missing Stars
But the real surprise was that the trailing tidal tail seemed to be missing stars. This indicates that something much more brutal is taking place than the star cluster gently ‘dissolving’.
Evolution of Hyades star cluster
Running the simulations again, Tereza showed that the data could be reproduced if that tail had collided with a cloud of matter containing about 10 million solar masses. “There must have been a close interaction with this really massive clump, and the Hyades just got smashed,” she says.
But what could that clump be? There are no observations of a gas cloud or star cluster that massive nearby. If no visible structure is detected even in future targeted searches, Tereza suggests that the object could be a dark matter sub-halo. These are naturally occurring clumps of dark matter that are thought to help shape the galaxy during its formation. This new work shows how Gaia is helping astronomers map out this invisible dark matter framework of the galaxy.
“With Gaia, the way we see the Milky Way has completely changed. And with these discoveries, we will be able to map the Milky Way’s sub-structures much better than ever before,” says Tereza. And having proved the technique with the Hyades, Tereza and colleagues are now extending the work by looking for tidal tails from other, more distant star clusters.