Thick disk

A thick disk is a major structural component found in many galaxies. Scientists believe it exists in about two-thirds of all disk galaxies. This includes our own Milky Way galaxy. The thick disk is important because it helps us understand how galaxies form. By studying its stars, we can learn about the early history of a galaxy.

The thick disk has specific physical properties that set it apart. It is defined by its scale height, which is its thickness. In the Milky Way, this height is 3 to 4 times larger than the thin disk. It also has a scale length, which is its measurement along the horizontal axis. The thick disk is also different in its stellar kinematics. This term refers to the specific ways the stars move. These stars also have a unique chemical composition.

Chemistry plays a huge role in identifying thick disk stars. These stars have lower levels of metals compared to the thin disk. In astronomy, metals are any elements that are not hydrogen or helium. The thick disk is also composed almost entirely of older stars. In our solar neighborhood, the thick disk dominates the stellar number density. This means there are many thick disk stars in the space above the galactic plane.

Observations show that the thick disk is not the same everywhere. Its composition changes depending on the distance from the galactic center. Inside the solar radius, the stars are metal-poor. However, the stars become more metal-rich as you move outside that radius. The average age of the stars also changes with distance. The stellar age decreases quickly as you move from the inner disk to the outer disk.

Scientists have proposed several different theories for how thick disks form. One scenario suggests the thin disk was heated over time. Another idea is that a merger event occurred. This would happen if a dwarf galaxy collided with the Milky Way. Some stars might also undergo radial migration. This is when energetic stars move from the inner galaxy to larger radii. This movement creates the thick disk structure at those outer distances.

Other theories involve the movement of gas and stars. One idea is that the disk formed thick at a high redshift. This means it formed very early in the universe's history. Another theory involves scattering by massive clumps. Stars born in large gas clumps may be scattered into a thick disk. These stars are often enriched in alpha process elements. Stars that do not form in these clumps settle into a thin disk instead.

There is an ongoing scientific dispute regarding the nature of this structure. Many studies list the thick disk as a bona fide galactic structure. However, some researchers question if it is a single, separate component. They suggest the galactic disk might actually be a continuous spectrum. In this view, the disk simply has many different thicknesses. This debate helps scientists refine their models of how galaxies evolve.