We develop a Bayesian Machine Learning framework called BINGO (Bayesian INference for Galactic archaeOlogy) centred around a Bayesian neural network. After being trained on the APOGEE and emph{Kepler} asteroseismic age data, BINGO is used to obtain p
recise relative stellar age estimates with uncertainties for the APOGEE stars. We carefully construct a training set to minimise bias and apply BINGO to a stellar population that is similar to our training set. We then select the 17,305 stars with ages from BINGO and reliable kinematic properties obtained from textit{Gaia} DR2. By combining the age and chemo-kinematical information, we dissect the Galactic disc stars into three components, namely, the thick disc (old, high-[$alpha$/Fe], [$alpha$/Fe] $gtrsim$ 0.12), the thin disc (young, low-[$alpha$/Fe]) and the Bridge, which is a region between the thick and thin discs. Our results indicate that the thick disc formed at an early epoch only in the inner region, and the inner disc smoothly transforms to the thin disc. We found that the outer disc follows a different chemical evolution pathway from the inner disc. The outer metal-poor stars only start forming after the compact thick disc phase has completed and the star-forming gas disc extended outwardly with metal-poor gas accretion. We found that in the Bridge region the range of [Fe/H] becomes wider with decreasing age, which suggests that the Bridge region corresponds to the transition phase from the smaller chemically well-mixed thick to a larger thin disc with a metallicity gradient.
We make a first attempt to find dwarf galaxies in eight Fermi-LAT extended, unassociated, source fields using Gaia DR2. We probe previously unexplored heliocentric distances of $d<20$~kpc with an extreme-deconvolution (XD) technique. We find no signa
ture of a dwarf galaxy in any of these fields despite Gaias excellent astrometric accuracy. We estimate our detection limits by applying the XD method to mock data, obtaining a conservative limit on the stellar mass of $M_* < 10^4$~M$_{sun}$ for $d < 20$, kpc. Such a low stellar mass implies either a low-mass subhalo, or a massive stripped-down subhalo. We use an analytic model for stripped subhalos to argue that, given the sizes and fluxes of the Fermi-LAT sources, we can reject the hypothesis that they owe to dark matter annihilation.
By taking advantage of the superb measurements of position and velocity for an unprecedented large number of stars provided in Gaia DR2, we have generated the first maps of the rotation velocity, $V_{rm rot}$, and vertical velocity, $V_{rm z}$, distr
ibutions as a function of the Galactocentric radius, $R_{rm gal}$, across a radial range of $5<R_{rm gal}<12$~kpc. In the $R-V_{rm rot}$ map, we have identified many diagonal ridge features, which are compared with the location of the spiral arms and the expected outer Lindblad resonance of the Galactic bar. We have detected also radial wave-like oscillations of the peak of the vertical velocity distribution.
We investigate the vertical metallicity gradients of five mono-age stellar populations between 0 and 11 Gyr for a sample of 18 435 dwarf stars selected from the cross-matched Tycho-Gaia Astrometric Solution (TGAS) and RAdial Velocity Experiment (RAVE
) Data Release 5. We find a correlation between the vertical metallicity gradients and age, with no vertical metallicity gradient in the youngest population and an increasingly steeper negative vertical metallicity gradient for the older stellar populations. The metallicity at disc plane remains almost constant between 2 and 8 Gyr, and it becomes significantly lower for the $8 < tau leqslant 11$ Gyr population. The current analysis also reveals that the intrinsic dispersion in metallicity increases steadily with age. We discuss that our results are consistent with a scenario that (thin) disc stars formed from a flaring (thin) star-forming disc.
