The Milky Way’s last significant merger, the Gaia Enceladus/Sausage (GES), is thought to have taken place between 8-11 Gyr ago. Recent studies in the literature suggest that the bar of the Milky Way is rather old, indicating that it formed at a similar epoch to the GES merger

The Milky Way’s last major merger, the Gaia Enceladus/Sausage (GES), is thought to have happened between 8–11 billion years ago. The Milky Way absorbed a small dwarf galaxy, Gaia-Sausage-Enceladus, after the two collided. This absorption changed the structure of the Milky Way and its chemical makeup.

Recent studies suggest that the Milky Way’s bar is old, indicating that it formed at a similar time to the GES merger. While the study doesn’t prove that the Milky Way’s bar was directly caused by the Gaia Sausage collision, it does show that evidence for an old bar structure strongly implies that its formation was triggered by the collision

The Milky Way is thought to have formed around 14 billion years ago. There are two main theories for the formation of the Milky Way: 

• Top-down: A single gas cloud collapses under its own gravitational pull. 
• Bottom-up: Smaller clusters of matter clump together to form a larger galaxy. 

The Milky Way’s formation may have started with gas and dust clouds collapsing under gravity.  The first stars formed from these collapsed clouds.  As time passed, gravity attracted the star clusters together, forming galaxies. The Milky Way may have formed from a series of proto galaxies that settled into a spiral shape. It may have also absorbed many smaller galaxies in its local group. 

The Milky Way’s formation may have also involved a “top-down” process. The “ELS” model suggests that the Milky Way formed from the rapid collapse of a single gas cloud

Here are some characteristics of the Milky Way galaxy: 

• Size The Milky Way is a barred spiral galaxy with a diameter of 170,000–200,000 light years. It’s estimated to contain 100–400 billion stars and planets. 
• Spiral arms The Milky Way’s spiral arms are curved extensions that start at the bulge. They contain gas, dust, and young blue stars. 
• Bulge The Milky Way’s bulge is stretched out into a bar shape. It’s a circular to oval structure of old stars. The bulge is red in color and contains the galaxy’s black hole. 
• Halo The Milky Way’s halo surrounds the disk and contains the oldest stars. It’s limited in size by the orbits of the Large and Small Magellanic Clouds. 
• Dark matter The Milky Way’s dark matter halo may span as much as 2 million light years. 

The Milky Way is constantly rotating.

The Milky Way galaxy has four major structural features: 

• Disk A thin, circular distribution of gas, dust, and stars. The disk contains most of the galaxy’s gas and dust, as well as young stars and regions of star formation. 
• Nuclear bulge A spherical region with a radius of about 200 parsecs around the center of the galaxy. It contains stars ranging in age from a few million years to over a billion years. 
• Halo A nearly spherical volume of thinly scattered stars, globular clusters of stars, and tenuous gas. The Milky Way is made up of about 90% dark matter, which causes an invisible halo. 
• Spiral arms Two spiral arms, the Scutum–Centaurus arm and the Carina–Sagittarius arm, have tangent points inside the Sun’s orbit. 

Other structural features of the Milky Way galaxy include: Nucleus, Galactic corona. (Full article source google)

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