Thick disk

[1] Soon after, it was proposed as a distinct galactic structure in the Milky Way, different from the thin disk and the halo in the 1983 article by Gilmore & Reid.

[2] It is supposed to dominate the stellar number density between 1 and 5 kiloparsecs (3.3 and 16.3 kly) above the galactic plane[2] and, in the solar neighborhood, is composed almost exclusively of older stars.

With the availability of observations at larger distances away from the Sun, more recently it has become apparent that the Milky Way thick disk does not have the same chemical and age composition at all galactic radii.

[8] In general, various scenarios for the formation of this structure have been proposed, including: Although the thick disk is mentioned as a bona fide galactic structure in numerous scientific studies and it's even thought to be a common component of disk galaxies in general,[20] its nature is still under dispute.

The view of the thick disk as a single separate component has been questioned by a series of papers that describe the galactic disk with a continuous spectrum of components with different thicknesses.