Did the last great galactic merger create the Milky Way’s bar?

The Milky Way is a spiral galaxy. Extra particularly, it’s a barred spiral galaxy, which means that inside its central area, there’s a bar form off of which the spirals emanate. About two-thirds of spiral galaxies are barred spirals, and astronomers have thought this distinction is only a variance in how density waves cluster stars in a galaxy. However a brand new research means that the bar of the Milky Way could have been attributable to an historical collision with one other galaxy.

Though it’s our residence, the Milky Way is in some methods the least understood galaxy. It is troublesome for us to map its construction intimately. Luckily, giant sky surveys such because the one by the Gaia spacecraft are lastly giving us an in depth view. One of many issues Gaia has proven is that a number of galaxies have collided with the Milky Way prior to now.

One of many largest collisions is called the Gaia Sausage. It’s the stays of a dwarf galaxy that struck the Milky Way about 8 to 11 billion years in the past. On account of this collision, our galaxy gained about 50 billion solar masses, and at the very least eight globular clusters. Gaia observations present the remnants of this collision within the velocity distribution of stars in our galaxy.

Apparently, 8 to 11 billion years in the past was across the identical time that the Milky Way developed its barred spiral construction. So the group needed to search out out if there was a connection. Utilizing computer simulations, they checked out how galactic collisions might lead to a barred galaxy.

Earlier research have proven that galactic collisions can usually create a barred construction, so merely exhibiting a collision might have shaped by way of a collision is not a robust argument that it did. So the group targeted on the actual timing of how the bar shaped. Based mostly on their simulations, the group discovered that with out the Gaia Sausage collision, there may very well be a 2-billion-year delay within the formation of a bar. The paper is published on the arXiv preprint server.

The circumstances of the early Milky Way would in some methods act to delay bar formation. With the Gaia Sausage collision, a bar shaped a lot earlier within the simulations. So whereas the research does not show the Milky Way’s bar was instantly attributable to the Gaia Sausage collision, it does present that proof for an outdated bar construction strongly implies that its formation was triggered by the collision.

As extra sky surveys come on-line, and we proceed to make an in depth map of our galaxy, we’ll higher perceive the dynamics of our galactic construction. For now, nonetheless, it appears that evidently barring a collision the Milky Way could have remained a easy spiral galaxy for billions of years.