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Published online by Cambridge University Press: 05 March 2015
Thick discs are defined to be disc-like components with a scale height larger than that of the classical discs. They are ubiquitous (Yoachim & Dalcanton 2006; Comerón et al. 2011a), they are made of mostly old and metal-poor stars and are most easily detected in close to edge-on galaxies. Their origin has been considered mysterious and several formation theories have been proposed:
• The thick disc being formed secularly by thin disc stars heated by disc overdensities such as giant molecular clouds or spiral arms (Villumsen 1985, ApJ, 290, 75) and by stars moved outwards from their original orbits by radial migration mechanisms (Schönrich & Binney 2009).
• The thick disc being formed by the heating of the thin disc by satellites (Quinn et al. 1993) and the tidal stripping of them (Abadi et al. 2003).
• The thick disc being formed fast and already thick at high redshift in an highly unstable disc. Inside that thick disc, a thin disc would form afterwards as suggested by Elemgreen & Elmegreen (2006).
• The thick disc being formed originally thick at high redshift by the merger of gas-rich protogalactic fragments and a thin disc forming afterwards within it (Brook et al. 2007).
The first mechanism is a secular evolution mechanism. The time-scale of the second one is dependent on the merger history of the main galaxy. In the two last mechanisms, the thick disc forms already thick in a short time-scale at high redshift.
Recent Milky Way studies, (see, e.g., Bovy et al. 2012), have shown indications that there is no discontinuity between the thin and the thick disc chemical and kinematic properties. Instead, those studies indicate the presence of a monotonic distribution of disc thicknesses. This would suggest a secular origin for the Milky Way thick disc.
Studies in external galaxies (Yoachim & Dalcanton 2006; Comerón et al. 2011b), have shown that low-mass disc galaxies have thick disc relative masses much larger than those found in large-mass galaxies. Because low-mass galaxies are dynamically younger than their larger counterparts, it seems difficult for their thick discs to have a secular evolution origin, but simulations show that their thick disc masses are compatible with those of a thick disc formed at high redshift.
Thus, recent studies seem to indicate that large-mass galaxies have their thick discs formed mainly due to secular evolution and that low-mass galaxies have them formed at high redshift.
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