Confirmed: two galaxies near the Milky Way collided ‘recently’

Two satellite galaxies of the Milky Way – the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC) – collided as recently as a few hundred million years ago, US astronomers have revealed.

The unambiguous evidence, they say, is their discovery that all the stars in the south-east region or “wing” of the SMC are moving in a similar direction at a similar speed.

“This is really one of our exciting results,” says Sally Oey from the University of Michigan. “You can actually see that the wing is its own separate region that’s moving away from the rest of the SMC.”

The findings are reported in a paper published in The Astrophysical Journal Letters.{%recommended 4128%}

Oey and an international team used newly released data from Gaia, the European Space Agency’s orbiting telescope, to look for “runaway” stars that had been ejected from clusters within the SMC.

Astronomers from the University of Arizona predicted a few years ago that a direct collision would cause the SMC’s wing region to move toward the LMC, whereas if the two galaxies simply passed near each other the wing stars would be moving in a perpendicular direction. 

Instead, says Oey, the wing is moving away from the SMC towards the LMC, confirming that a direct collision occurred.

“We want as much information about these stars as possible to better constrain these ejection mechanisms,” says her Michigan colleague Johnny Dorigo Jones. 

“Everyone loves marvelling at images of galaxies and nebulae that are incredibly far away. The SMC is so close to us, however, that we can see its beauty in the night sky with just our naked eye.

“This fact, along with the data from Gaia, allows us to analyse the complex motions of stars within the SMC and even determine factors of its evolution.”