Late-type disk galaxies could be described as a system with a variety of vertical stellar structure components. They contain a prominent dustlane in the midplane in which stars are currently formed, often around a young disk. Around this young disk lies the thin disk, the main visible structure component of the galaxy, consisting of mainly young metal-rich stars. When one goes to higher vertical heights, the situation becomes more uncertain. A possible other component could emerge: the thick disk, but whether all disk galaxies possess them is still uncertain. The thick disk is considered to consist of older and more metal-poor stars than the stars in the thin disk and has a larger scaleheight and scalelength than the thin disk. Around the disk we expect to find the stellar halo, which is part of the galaxy but not of the disk.
To be able to study vertical structures one has to observe the galaxies edge-on. The surface brightness profile has to be decomposed to find out the structural properties of its components and the distribution and properties of the stars. This is of fundamental importance to gain insight in the formation and evolution of galaxies. A still unanswered, but very important question is if all galaxies have thick disks, since it addresses the viability of different hypotheses for the creation of a thick disk and henceforth also the formation and evolution of disk galaxies. One of these hypotheses considers the thick disk as a separate entity produced in an early phase of enhanced star formation during the initial proto-galactic collapse.
Disk galaxy simulations on cosmological N-body+SPH galaxy formation models by Abadi et al. abadi2003 find a thick disk that is composed of tidal debris from disrupted satellites, while comparable simulations by Brook et. al. (2004) find that thick disks form during a period of chaotic mergers of gas-rich building blocks.
A family of models propagates the thick disk as an extension (by dynamical heating) of the thin disk, where it is assumed that after the initial collapse all gas settles down into the galactic plane and starts forming stars, experiencing different types of heating mechanisms. Suggestions include heating by spiral density waves, encounters with giant molecular clouds, scattering by massive black holes, energy input by accretion of satellite galaxies, or bar bending instabilities (see for references Pohlen et. al. 2004).
The formation of the thick disk holds the key to unravelling the evolution of disk galaxies. To do so determining the intrinsic components and properties of the vertical structures in disk galaxies is essential and sets strong constraints for galaxy formation and evolution models.