Thin Disk

The defining morphological component of disk galaxies in general and spiral galaxies in particular. The thin disk contains stars, star clusters, gas and dust confined to the galaxy’s plane of rotation. There is much more information on the disk of the Milky Way than for spiral galaxies, but the disk of the Milky Way is considered typical.

Thin disks contain the majority of the baryonic material in spiral galaxies (on the order of 80% of the baryonic material in the Milky Way is in the thin disk). The thin disk of the Milky Way has a scale height of ~400 light years and scale length of ~10,000 light years and rotates about the galactic center at ~220 km/sec. Its outer regions appear to be warped, a phenomenon observed in ~50% of spiral galaxies. Although the origin of warps is uncertain, it is thought that they are probably the result of galaxy interactions.

The thin disks of spiral galaxies contain a lot of gas and dust, and an active site for ongoing star formation, especially in the spiral arms. For this reason, stars in the thin disk tend to be relatively young (average age ~6 Ga), although individual ages range from 0–10 Ga. This is evidence for secular evolution in thin disks. Thin disk stars also tend to be metal-rich compared to thick disk and halo stars, and typically have similar metallicities to the Sun (a thin disk star).