Abstract
ABSTRACT Using simulated galaxies in their cosmological context, we analyse how the flaring of mono-age populations (MAPs) influences the flaring and the age structure of geometrically defined thick discs. We also explore under which circumstances the geometric thin and thick discs are meaningfully distinct components, or are part of a single continuous structure as in the Milky Way. We find that flat thick discs are created when MAPs barely flare or have low surface density at the radius where they start flaring. When looking at the vertical distribution of MAPs, these galaxies show a continuous thin/thick structure. They also have radial age gradients and tend to have quiescent merger histories. Those characteristics are consistent with what is observed in the Milky Way. Flared thick discs, on the other hand, are created when the MAPs that flare have a high surface density at the radius where they start flaring. The thick discs’ scale heights can either be dominated by multiple MAPs or just a few, depending on the mass and scale height distribution of the MAPs. In a large fraction of these galaxies, thin and thick discs are clearly distinct structures. Finally, flared thick discs have diverse radial age gradients and merger histories, with galaxies that are more massive or that have undergone massive mergers showing flatter age radial gradients in their thick disc.
Highlights
Thick discs were first discovered in external galaxies by Tsikoudi (1979) and Burstein (1979), and later on in the Milky Way by Gilmore & Reid (1983)
If we look at the upper right-hand panel of Fig. 9, we see that out of the mono-age populations (MAPs) contributing to the thick disc, MAPs between 6 and 7.5 Gyr old dominate by far the surface density at all radii
Because of the different configurations of flaring MAPs and surface density changing in the radial direction, in some galaxies the thin and the thick discs will be formed by different groups of stellar populations, being two distinct components, whereas in some others the stellar populations forming the thin and the thick discs will be much more mixed – forming a thin–thick disc continuum
Summary
Thick discs were first discovered in external galaxies by Tsikoudi (1979) and Burstein (1979), and later on in the Milky Way by Gilmore & Reid (1983). In the Milky Way, the geometrically thick disc is made of old stars in the solar neighbourhood and the inner disc, but shows a strong radial age gradient: between galactocentric radii of 6 and 12 kpc, the mean age of red clump stars in the thick disc drops from ∼9 to ∼5 Gyr (Martig et al 2016, see Ness et al 2016; Xiang et al 2017; Feuillet et al 2019). Minchev et al (2015) proposed a solution for this conundrum, showing that a flat thick disc can be recovered by superposing several stellar mono-age populations (MAPs), with different levels of flaring and radial density profiles. We look into the merger histories of our galaxies, and show the connection between mergers and different thick disc properties
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