Data from ESA’s Gaia star mapping satellite has revealed tantalizing evidence that the closest star cluster to the Sun is being disrupted by the gravitational influence of a massive but invisible structure in our galaxy.
If true, this could provide evidence of a suspicious population of ‘dark matter subhalos’. These invisible clouds of particles are believed to be relics of the formation of the Milky Way, and now they spread out across the galaxy, forming an invisible substructure that exerts a remarkable gravitational influence on anything that gets too close.
ESA researcher Tereza Jerabkova and her colleagues at ESA and the European Southern Observatory made the discovery while studying how a nearby star cluster merges with the general background of stars in our galaxy. This discovery was based on the first version of data from the third version (EDR3) of Gaia and on the data from the second version.
The team chose the Hyades as their target because it is the closest star cluster to the Sun. It is located just over 153 light years away and is easily visible to sky watchers in the northern and southern hemispheres as a striking shape. of ‘V’ of bright stars marking the head of the bull in the constellation of Taurus. Beyond the easily visible bright stars, telescopes reveal a hundred fainter stars contained in a spherical region of space, approximately 60 light-years across.
A star cluster will naturally lose stars because as those stars move within the cluster, they pull gravitationally at each other. This constant tug slightly changes the speeds of the stars, moving some towards the edges of the cluster. From there, the stars can be swept away by the galaxy’s gravitational force, forming two long tails.
One tail follows the star cluster, the other leads ahead. They are known as tidal tails and have been studied extensively in colliding galaxies, but no one had ever seen them from a nearby open star cluster, until very recently.
The key to detecting tidal tails is detecting which stars in the sky are moving similarly to the star cluster. Gaia makes it easy because it accurately measures the distance and motion of more than a billion stars in our galaxy. “These are the two most important quantities we need to search for tidal tails of star clusters in the Milky Way,” says Tereza.
Previous attempts by other teams had had limited success because the researchers had only looked for stars that closely matched the motion of the star cluster. This excluded members who left earlier in their 600 to 700 million year history and are therefore now traveling in different orbits.
To understand the range of orbits to look for, Tereza built a computer model that would simulate the various disturbances that escaping stars in the cluster could feel during their hundreds of millions of years in space. It was after running this code and then comparing the simulations with the actual data that the true extent of the Hyades tidal tails was revealed. Tereza and her colleagues found thousands of former members in the Gaia data. These stars now stretch for thousands of light years across the galaxy in two huge tidal tails.
But the real surprise was that the tail of the creeping tide seemed starless. This indicates that something much more brutal is going on than the gently ‘dissolving’ star cluster.
By running the simulations again, Tereza showed that the data could be reproduced if that tail had collided with a cloud of matter containing roughly 10 million solar masses. “There must have been a close interaction with this really massive group, and the Hyades were just crushed,” she says.
But what could that group be? There are no observations of such a massive gas cloud or star cluster in the vicinity. If no visible structure is detected even in future specific searches, Tereza suggests that the object could be a subhalo of dark matter. These are natural clumps of dark matter that are believed to help shape the galaxy during its formation. This new work shows how Gaia is helping astronomers map out this galaxy’s invisible dark matter framework.
“With Gaia, the way we view the Milky Way has completely changed. And with these discoveries, we will be able to map the substructures of the Milky Way much better than ever ”, says Tereza. And having tested the technique with the Hyades, Tereza and her colleagues are now expanding the work by looking for tidal tails from other more distant star clusters.
Reference: “The 800 pc long tidal tails of the Hyades star cluster: possible discovery of candidate epicyclic overdensities of an open star cluster” by Tereza Jerabkova, Henri MJ Boffin, Giacomo Beccari, Guido de Marchi, Jos HJ de Bruijne and Timo Prusti, March 24, 2021, Astronomy and Astrophysics.
DOI: 10.1051 / 0004-6361 / 202039949