Did the Final Nice Galactic Merger Create the Milky Manner’s Bar?

The Milky Manner is a spiral galaxy. Extra particularly, it’s a barred spiral galaxy, that 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 examine means that the bar of the Milky Manner could have been attributable to an historical collision with one other galaxy.

Though it’s our residence, the Milky Manner is in some methods the least understood galaxy. It’s tough for us to map its construction intimately. Happily, massive 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 Manner up to now.

One of many largest collisions is named the Gaia Sausage. It’s the stays of a dwarf galaxy that struck the Milky Manner about 8 – 11 billion years in the past. On account of this collision, our galaxy gained about 50 billion photo voltaic lots, and no less than 8 globular clusters. Gaia observations present the remnants of this collision within the velocity distribution of stars in our galaxy.

Curiously, 8 – 11 billion years in the past was across the similar time that the Milky Manner developed its barred spiral construction. So the workforce wished to seek out out if there was a connection. Utilizing laptop simulations, they checked out how galactic collisions may lead to a barred galaxy. Earlier research have proven that galactic collisions can typically create a barred construction, so merely exhibiting a collision may have shaped through a collision isn’t a robust argument that it did. So the workforce centered on the actual timing of how the bar shaped. Primarily based on their simulations, the workforce discovered that with out the Gaia Sausage collision, there could possibly be a 2 billion-year delay within the formation of a bar. The circumstances of the early Milky Manner 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 examine doesn’t show the Milky Manner’s bar was instantly attributable to the Gaia Sausage collision, it does present that proof for an previous 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, nevertheless, evidently barring a collision the Milky Manner could have remained a easy spiral galaxy for billions of years.

Reference: Merrow, Alex, et al. “Did the Gaia Enceladus/Sausage merger form the Milky Way’s bar?” arXiv preprint arXiv:2312.02318 (2023).