On the Flaring of Thick Disc of Galaxies: Insights from Simulations
  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.
