The thick disk

A disk galaxy is described by a set of distinct stellar entities: a disk population, a bulge component, and a stellar halo. Deep surface photometry of external early-type galaxies (Burstein, 1979; Tsikoudi, 1979) and later elaborate measurements using star counts in our own Galaxy [Gilmore and Reid1983] revealed the need for an additional component of stars. This was called a 'thick disk' [Burstein1979], since it exhibited a disk-like distribution with larger scaleheight compared to the inner, dominating 'thin disk'. It was originally detected as an excess of light at high galactic latitudes.
The most studied and well known thick disk is that of our own Galaxy. The properties of the Milky Way's thick disk have revealed many differences from the thin disk. Structurally the Milky Way's thick disk has a scaleheight of 0.6-1 kpc, which is about 3 times larger than the thin disk scaleheight. The thick disk also may have a somewhat longer scalelength (3.7 kpc, to 2.8 kpc for the thin disk), a typical thick disk feature stated for external galaxies by De Grijs and Peletier (1997), although Abe et. al. (1999) find a thick disk with a shorter scalelength. For the Milky Way, the observed local (near the Sun) number density of thick disk stars is about 6%-13% of that of the thin disk. Thick disk stars are older ($\thickapprox$10 Gyr) and more metal-poor than stars in the thin disk, so thick disks are likely to trace the early stages of disk evolution. They have a wide range of metallicity, -2.2 $\leq$ [Fe/H] $\leq$ -0.5, although the metal-weak tail of the distribution contributes only $\sim$1 % of the thick disk and may be a different population than that in the canonical thick disk. Kinematically, Milky Way thick disk stars have both larger velocity dispersions and and slower net rotation than stars in the thick disk.
There is increasing evidence that chemical trends in the thick and thin disk stars are different, showing that the thick is a truly distinct component of the Milky Way. A major diagnostic coming from such different chemical trends between thin and thick disk is the different $\alpha$-element-to-iron abundance ratios, indicating different formation timescales (see for references Brook et al. 2004).
The measurements required to characterise thick disks are difficult to make outside the Milky Way. The Milky Way thick disk provides less than 10% of the local stellar density, and this faintness makes it hard to do a detailed study of comparable extragalactic thick disks. Studies on thick disk components for external galaxies analyze galaxies in the edge-on orientation, which allows clear delineation between regions where thin and thick disk stars dominate the flux. The edge-on orientation also provides line-of-sight integrations of faint stellar populations to reach detectable levels (see for references Yoachim & Dalcanton 2006).