Constraining the thick disc formation scenario of the Milky Way
  We study the shape of the thick disc using photometric data at high and
intermediate latitudes from SDSS and 2MASS surveys. We use the population
synthesis approach using an ABC-MCMC method to characterize the thick disc
shape, scale height, scale length, local density and flare, and we investigate
the extend of the thick disc formation period by simulating several formation
episodes. We find that the vertical variation in density is not exponential,
but much closer to an hyperbolic secant squared. Assuming a single formation
epoch, the thick disc is better fitted with a sech2 scale height of 470 pc and
a scale length of 2.3 kpc. However if one simulates two successive formation
episodes, mimicking an extended formation period, the older episode has a
higher scale height and a larger scale length than the younger episode,
indicating a contraction during the collapse phase. The scale height decreases
from 800 pc to 340 pc, and the scale length from 3.2 kpc to 2 kpc. The star
formation increases from the old episode to the young one. During the fitting
process, the halo parameters are also determined. The constraint on the halo
shows that a transition between a inner and outer halo, if exists, cannot be at
a distance of less than about 30 kpc, which is the limit of our investigation
using turnoff halo stars. Finally, we show that extrapolating the thick disc
towards the bulge region explains well the stellar populations observed there,
that there is no longer need to invoke a classical bulge. To explain these
results, the most probable scenario for the thick disc is that it formed while
the Galaxy was gravitationally collapsing from well mixed gas-rich giant clumps
sustained by high turbulence which awhile prevent a thin disc to form, as
proposed by Bournaud et al. (2009). This scenario explains the observations in
the thick disc region as well as in the bulge region. (abridged)
