Thick disk

The thick disk is a source of early kinematic and chemical evidence for a galaxy's composition and thus is regarded as a very significant component for understanding galaxy formation.

With the availability of observations at larger distances away from the Sun, it has become apparent that the Milky Way thick disk does not have the same chemical and age composition at all distances from the galactic centre. Instead, it is metal-poor inside the solar radius, but becomes more metal-rich outside it. Additionally, observations have revealed that the average stellar age of thick disk stars quickly decreases as one moves from the inner to the outer disk.

Origin

Various scenarios for the formation of this structure have been proposed, including:

• Thick disks come from the heating of the thin disk
• It is a result of a merger event between the Milky Way and a dwarf galaxy
• More energetic stars migrate outwards from the inner galaxy to form a thick disk at larger radii
• The disk forms thick at high redshift with the thin disk forming later

• Disk flaring combined with inside-out disk formation
• Scattering by massive clumps:

Stars born in massive gas clumps tend to be scattered into a thick disc, and to be enriched in alpha process elements

Stars formed outside of such clumps tend to settle into a thin disc, and are alpha-element poor