This is a summary of the paper The Milky Way Has No Distinct Thick Disk by (Bovy 2012).
Traditionally the stars within the disks of spiral galaxies are considered to form two distinct populations. One population, termed the “thin disk”, is generally comprised of young and metal-rich stars while the other population of older and more metal-poor stars make up the “thick disk”. The paper from Bovy et al. (2012) challenges this assumption of bi-modality within the Galactic disk and argue for a “continuous and monotonic scale-height distritbution”.
In general, stars in spiral disk galaxies orbit within a 2D plane. However disk stars also have a component of their orbits in the vertical direction. The population of disk stars that show a distribution of vertical heights can be characterized by a scale height, h. For any single scale height, the probability of finding a star at a given height z above the disk is proportional to exp(-z/h). If a galaxy is characterized by a bimodal distribution of scale heights, this implies that the galaxy is really composed of two different kinds of disks: a thick disk and a thin disk. It is this fact that Bovy et al. are arguing against.
The results of this paper rest on the use of stellar spectra from the SDSS SEGUE survey. Stellar spectra allow observers to calculate the spectral type, iron metallicity, and other chemical abundances of a population of stars. Two of the most common abundances to examine are iron metallicity [Fe/H] relative to the sun, and α-element abundance relative to iron [α/Fe].
The iron abundance [Fe/H] can give information about the age of the star, since older stars were born when the heavy elements in the universe had not yet been synthesised inside massive stars or supernovae. Therefore older stars will have lower metallicty and hence iron abundance.
Alpha elements such as C, N, O, and Ca, are produced in triple-alpha reactions in the cores of red giant stars. The α-element abundance can also give clues to the age and star formation history of a population. According to Wikipedia it is a characteristic of older Population II stars that despite their lower overall metallicity, they often have a higher ratio of alpha elements (O, Si, Ne, etc.) relative to Fe as compared to younger Population I stars.
Theory suggests “this is the result of Type II supernovae being more important contributors to the interstellar medium at the time of their formation, whereas Type Ia supernovae metal enrichment came later in the universe’s evolution”. (Wikipedia 2014)
In a previous paper, Bovy et al. plotted [Fe/H] against [α/Fe] and found that stars within each bin can be characterised by a single scale height.
The authors calculate the surface density of stars in the disk of the milky way at the solar circle Σ(R0), as a function of scale height, h. This result is plotted as a black solid line above. The individual data points are the average mass surface densities and scale heights for each subsample in ([Fe/H],[α,Fe]) space and are colored by the α-abundance relative to iron.
Stars that are α-enhanced tend to have large scale heights, while stars with lower α-abundance ([α,Fe] \(\sim\) 0) tend to have relatively small scale heights. Looking only at α-abundance, it appears there are two populations. However by using the SEGUE survey to infer the stellar mass distribution, the authors show that the stars have a smooth distribution of scale heights.