Weblog
12 January 2022
ESA (artist’s impression and composition); Koppelman, Villalobos and Helmi (simulation)
A central side of galactic evolution is that they have to eat or be eaten. Darkish power strives to push galaxies aside, however gravity tries to drag them collectively. Consequently, galaxies are inclined to type into native teams. As these superclusters of galaxies develop into extra remoted attributable to cosmic growth, they gravitationally activate one another, and in time the most important galaxies of the group will devour the smaller ones. The Milky Method is among the bigger galaxies in our native group, and so it has consumed smaller galaxies up 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 latest research exhibits. The outcomes have been printed within the Astrophysical Journal and reconstructs when the final main galactic merger with the Milky Method 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 Method. It is a roughly spherical assortment of stars that surrounds our galaxy. The latest merger appears to have been with a small galaxy referred to as Gaia-Sausage-Enceladus (GSE), which occurred about 8 – 10 billion years in the past. However not many particulars have been identified about this merger. For instance, did GSE orbit the Milky Method 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 Method and mixed that information with a survey of the outer edges of the Milky Method referred to as the H3 Survey. This gave the crew understanding of the place and movement of halo stars. They then created numerical simulations of assorted hypothetical collisions between GSE and the Milky Method. They discovered that the mannequin which most closely fits is a galaxy of about half a billion stars that collided with the Milky Method 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 a lot of the Milky Method’s mass enhance over the previous 10 billion years. It was an enormous breakfast for our galaxy’s cosmic day. In a while, the menu are different meals, together with these of the small and huge Magellanic clouds, each of that are approaching the Milky Method. Then, after all, there may be the grand meal to return, when the Milky Method and Andromeda galaxies collide in about 5 billion years.
