Warping of the Milky Way disc due to massive infall
The merger with a massive satellite with tidal mass of about 6e10 solar mass can be responsible for causing a warp-like structure in the Milky Way's disc outer part. Also we observe a feature representative of the Monoceros ring at about 20 kpc from the Galactic centre, reaching heights of about 3-4 kpc above/below the plane.
To read the article: https://www.aanda.org/articles/aa/abs/2017/08/aa30024-16/aa30024-16.html