Population studies - The nature of the thick disk

It is shown that the stellar number counts as a function of distance from the Galactic plane, which led Gilmore and Reid to postulate in 1983 the existence of a distinct thick disk as a third major component of the Galaxy, can be well reproduced by the observed properties of a disk with continuously varying properties in the ranges [Fe/H] ⪆ -0.6, σz ⪉ 35 km s-1, asymmetric drift ⪆-10 to -30 km s-1, together with a halo component. It is suggested that the thick disk phenomenon is a manifestation of the (dispersive?) mechanisms which have established the well-established age - metal abundance - velocity dispersion relation for disk stars.