Sausage Breakfast | by Brian Koberlein

A central side of galactic evolution is that they have to eat or be eaten. Darkish vitality strives to push galaxies aside, however gravity tries to tug them collectively. In consequence, galaxies are likely to kind into native teams. As these superclusters of galaxies change into extra remoted attributable to cosmic enlargement, they gravitationally activate one another, and in time the biggest galaxies of the group will devour the smaller ones. The Milky Manner is among the bigger galaxies in our native group, and so it has consumed smaller galaxies prior to now. However piecing collectively the historical past of those galactic meals is an actual problem.

However because of the Gaia spacecraft we’re studying fairly a bit, as a current examine reveals. ^{The outcomes had been printed within the Astrophysical Journal and reconstructs when the final main galactic merger with the Milky Manner occurred.}

NASA, ESA, A. Sarajedini, and G. Piotto

Based mostly on earlier observations, we all know that most of the remnant stars from galactic collisions find yourself within the halo of the Milky Manner. This can be a roughly spherical assortment of stars that surrounds our galaxy. The newest merger appears to have been with a small galaxy often known as Gaia-Sausage-Enceladus (GSE), which occurred about 8 – 10 billion years in the past. However not many particulars have been recognized about this merger. For instance, did GSE orbit the Milky Manner earlier than colliding, solely to be ripped aside by our galaxy’s tidal forces, or was the collision extra direct and head-on?

To reply this query, the crew used Gaia observations of stars within the halo of the Milky Manner and mixed that knowledge with a survey of the outer edges of the Milky Manner often known as the H3 Survey. This gave the crew a superb understanding of the place and movement of halo stars. They then created numerical simulations of assorted hypothetical collisions between GSE and the Milky Manner. They discovered that the mannequin which most closely fits is a galaxy of about half a billion stars that collided with the Milky Manner reverse to our galaxy’s rotation. That’s, a extra direct collision. Based mostly upon this mannequin, about half of our galaxy’s halo stars, and about 20% of the darkish matter halo got here from this one collision.

The merger with Gaia-Sausage-Enceladus accounts for many of the Milky Manner’s mass improve over the previous 10 billion years. It was a giant breakfast for our galaxy’s cosmic day. Afterward, the menu are different meals, together with these of the small and huge Magellanic clouds, each of that are approaching the Milky Manner. Then, in fact, there may be the grand meal to return, when the Milky Manner and Andromeda galaxies collide in about 5 billion years.