The GALAH Survey: Separating the thin and thick disks

Almost all spiral galaxies have a second disk component, the thick disk, in addition to the thin disk which defines their disk structure. Thick disks are believed to be ancient structures that predates the formation of the thin disks, but how they fit in to the overall picture of galaxy formation remains unknown. Although our Galaxy has a thick disk, the properties of this ancient component are not yet well determined. The relative dynamics and chemistry of the thin and thick disks are also poorly known: the velocity dispersion and rotational lag of the thick disk is uncertain, and we do not know if the thick disk has a metallicity gradient in R and z.

The Galactic thick disk is a structural component which spatially overlaps the thin disk, and part of the difficulty in determining the properties of the thick disk is determining which of these two components a particular star belongs to. Accurate abundance measurements in the past few years show that thick disk stars are enhanced in alpha-elements relative to thin disk stars of similar [Fe/H]. This can be used to sep arate thin and thick disk stars, but the separation in the alpha-abundances is small so accurate high resolution abundances of alpha-elements are needed for reliable separa- tion. Data from the GALAH survey (resolving power ∼ 30, 000, SNR ∼ 100) are ideal for this purpose.

The GALAH pilot survey has observed 10,000 disk stars at 5 different galactic latitudes. The thin/thick disk stars can be accurately separated from their measured [Fe/H] and [α/Fe] abundances. The survey extends over a few kpc in R and z and will give us a direct estimate of the metallicity gradients of the old thick disk component.