Click For Photo: https://www.sciencedaily.com/images/2018/09/180919133035_1_540x360.jpg
The close encounter likely took place sometime in the past 300-900 million years. It was discovered because of the pattern of movement it has given to stars in the Milky Way disc -- one of the major components of our Galaxy.
The pattern was revealed because Gaia not only accurately measures the positions of more than a billion stars but also precisely measures their velocities on the plane of the sky. For a subset of a few million stars, Gaia provided an estimate of the full three-dimensional velocities, allowing a study of stellar motion using the combination of position and velocity, which is known as 'phase space'.
Phase - Space - Motions - Interesting - Pattern
In phase space, the stellar motions revealed an interesting and totally unexpected pattern when the star's positions were plotted against their velocities. Teresa Antoja from Universitat de Barcelona, Spain, who led the research couldn't quite believe her eyes when she first saw it on her computer screen.
One shape in particular caught her attention. It was a snail shell-like pattern in the graph that plotted the stars' altitude above or below the plane of the Galaxy against their velocity in the same direction. It had never been seen before.
Features - Teresa - Bit - Problem - Data
"At the beginning the features were very weird to us," says Teresa. "I was a bit shocked and I thought there could be a problem with the data because the shapes are so clear."
But the Gaia data had undergone multiple validation tests by the Gaia Data Processing and Analysis Consortium teams all over Europe before release. Also, together with collaborators, Teresa had performed many tests on the data to look for errors that could be forcing such shapes on the data. Yet no matter what they checked, the only conclusion they could draw was that these features do indeed exist in reality.
The reason they had not been seen...
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