杏吧原创

Our two nearest large neighbours, the Andromeda and Triangulum galaxies, appear to have grazed each other about 2.5 billion years ago 鈥� and a worse collision is on the horizon. The findings lend weight to the theory that big galaxies grow by gobbling smaller ones.

Remnants of galactic cannibalism have been found strewn about our own galaxy in the form of stellar streams, spaghetti-like strands of stars that were ripped from orbiting dwarf galaxies when they came too close to the Milky Way.

These observations support the 鈥渉ierarchical鈥� model of galaxy formation, in which big galaxies form when smaller ones merge.

But as a testbed for the theory, the Milky Way has a significant drawback: We can鈥檛 see all of it at once. 鈥淭he problem with the Milky Way is that we are inside the Milky Way,鈥� says of the NRC Herzberg Institute for Astrophysics in Victoria, Canada, lead author of the new work.

So astronomers turned their gaze outwards, to our near-twin, the Andromeda galaxy. They made the largest-ever map of the galaxy鈥檚 stars using the in Hawaii. The map extends 500,000 light years from the centre of Andromeda, more than 10 times the distance of our sun from the centre of the Milky Way.

Stellar immigrants

Even at the farthest edges of the galaxy, bright stars abounded. Because most of the star-forming gas is concentrated at the centre of the galaxy, these stars probably weren鈥檛 born there 鈥� they鈥檙e likely immigrants from dwarf galaxies that fell into and became part of Andromeda.

The map also showed stars in bright streams and clumps that were also likely ripped from dwarf galaxies that once orbited Andromeda. Some of these were already known, but several of them are new. These observations lend weight to the hierarchical model. 鈥淚t鈥檚 a nice vindication of a lot of the ideas of how you form galaxies in general,鈥� McConnachie says.

鈥淭his is a very nice paper. It will be very valuable for models of galaxy formation,鈥� says of the University of Chicago. 鈥淚t may be for a long while the best picture we have for evidence of hierarchical buildup of galaxies.鈥�

鈥楶ulled apart鈥�

The study also shows that Andromeda鈥檚 reach extends even farther than expected. The Triangulum galaxy is the next-largest local galaxy after the Milky Way, and it sits about 1 million light years from Andromeda. It鈥檚 large enough and far enough away that it is not considered a mere satellite of Andromeda 鈥� but that won鈥檛 save it from the larger galaxy鈥檚 voracious appetite.

鈥淭he Triangulum galaxy appears to be eaten by Andromeda,鈥� McConnachie says. 鈥淭hat was completely unexpected.鈥�

The map showed a 130,000-light-year-long tentacle extending from Triangulum towards Andromeda 鈥� 鈥渆xactly the sort of feature we expect to see for a galaxy being pulled apart by the gravitational effects of a bigger galaxy,鈥� McConnachie told New 杏吧原创.

It also revealed that both galaxies鈥� stellar discs were warped by their close encounter. Previous observations had found that Triangulum looked like a perfect spiral if you only looked at its stars, but its disc of hydrogen gas was extremely distorted. The discrepancy between the warped gas and the pristine stars is one of Triangulum鈥檚 biggest mysteries.

Warped disc

The new observations show the stellar disc is warped if you look at very faint stars.

鈥淚t鈥檚 not until you look at really, really faint light levels that you can actually see the mess that has also happened to the stars,鈥� McConnachie says. 鈥淲hatever caused the gas warp also warped the stars, and we suggest that could well have been this interaction with the Andromeda galaxy.鈥�

To figure out when and how the galaxies met, McConnachie and his colleagues ran computer simulations of their possible trajectories. The one that best fits the galaxies鈥� current positions shows them grazing each other at a distance of 130,000 light years around 2.5 billion years ago.

It was a close call, but Triangulum emerged mostly unscathed. 鈥淚t鈥檚 not gentle, but it鈥檚 not a violent process at the moment,鈥� McConnachie says.

But the models predict that they will meet again in another 2 billion years, and this time Triangulum won鈥檛 be so lucky. 鈥淭he next time around is going to be a much more violent process. It鈥檚 going to fall much closer into Andromeda, feel much stronger gravitational pull,鈥� McConnachie says. 鈥淲hat鈥檚 happened to it now is kind of a warning.鈥�

Journal reference: (vol 461, p 66)