Newly Discovered Stellar Stream Near The Solar System Might Have Been Another Galaxy

Gaia Map of the full sky. ESA/Gaia/DPAC

A group of stars located a few thousand light-years away all seem to be moving at the same speed around the Milky Way but lagging behind the rest of the stuff in the galaxy. The team behind this new research believe the stellar stream used to be its own galaxy before it was ripped apart and cannibalized by the Milky Way. The study is published in Nature Astronomy.

The stellar stream (and its original galaxy) has been nicknamed Nyx, after the Greek goddess of the Night. To uncover the stream, the team applied sophisticated algorithms to stellar maps produced by the European Space Agency’s Gaia satellite, which contain information on the position and velocity of 1 billion stars. They then used the software to discover related groups of stars and found 232 that were lagging together.

Due to gravitational perturbations, it is possible for groups of stars in the Milky Way to be a few tens of kilometers per second slower than the average rotational speed of the galaxy. However, the stars in the Nyx stream were 90 kilometers (56 miles) per second slower – too much to be simply considered a slow group. The data suggests this was not an anomaly and that these objects are likely related to each other.

By combining these results with simulations of the phenomena, the team believe that Nyx might have been a dwarf galaxy absorbed by our own about 1 billion years ago. The data analysis also suggests that a second group called Nyx-2 might exist. The team plans to further investigate these stars and their chemical composition in order to find out if their makeup is consistent with stars that formed together in a dwarf galaxy. 

While there are still uncertainties about the nature of Nyx, this wouldn’t be the first galaxy our Milky Way has cannibalized. It also won’t be the last. Gaia alone has brought to light evidence of past mergers such as the Antlia-2 dwarf and the Gaia-Enceladus. There are also signs of upcoming mergers with the Milky Way.

If Nyx is confirmed, it will be in good company.   

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