We present an analysis of the large-scale structure of the halo of the Andromeda galaxy, based on the Pan-Andromeda Archeological Survey (PAndAS), currently the most complete map of resolved stellar populations in any galactic halo. Despite the presence of copious substructures, the global halo populations follow closely powerlaw profiles that become steeper with increasing metallicity. We divide the sample into stream-like populations and a smooth halo component (defined as the population that cannot be resolved into spatially distinct substructures with PAndAS). Fitting a three-dimensional halo model reveals that the most metal-poor populations ([Fe/H] < −1.7) are distributed approximately spherically (slightly prolate with ellipticity c/a = 1.09 ± 0.03), with only a relatively small fraction residing in discernible stream-like structures (f stream = 42%). The sphericity of the ancient smooth component strongly hints that the dark matter halo is also approximately spherical. More metal-rich populations contain higher fractions of stars in streams, with f stream becoming as high as 86% for [Fe/H] > −0.6. The space density of the smooth metal-poor component has a global power-law slope of γ = −3.08 ± 0.07, and a nonparametric fit shows that the slope remains nearly constant from 30 kpc to ∼300 kpc. The total stellar mass in the halo at distances beyond 2• is ∼1.1 × 10 10 M , while that of the smooth component is ∼3 × 10 9 M . Extrapolating into the inner galaxy, the total stellar mass of the smooth halo is plausibly ∼8 × 10 9 M . We detect a substantial metallicity gradient, which declines from [Fe/H] = −0.7 at R = 30 kpc to [Fe/H] = −1.5 at R = 150 kpc for the full sample, with the smooth halo being ∼0.2 dex more metal poor than the full sample at each radius. While qualitatively in line with expectations from cosmological simulations, these observations are of great importance as they provide a prototype template that such simulations must now be able to reproduce in quantitative detail.

RR Lyrae stars may be the best practical tracers of Galactic halo (sub-)structure and kinematics. The PanSTARRS1 (PS1) 3π survey offers multi-band, multi-epoch, precise photometry across much of the sky, but a robust identification of RR Lyrae stars in this data set poses a challenge, given PS1's sparse, asynchronous multi-band light curves ( 12 epochs in each of five bands, taken over a 4.5-year period). We present a novel template fitting technique that uses well-defined and physically motivated multi-band light curves of RR Lyrae stars, and demonstrate that we get accurate period estimates, precise to 2 sec in > 80% of cases. We augment these light curve fits with other features from photometric time-series and provide them to progressively more detailed machine-learned classification models. From these models we are able to select the widest (3/4 of the sky) and deepest (reaching 120 kpc) sample of RR Lyrae stars to date. The PS1 sample of ∼ 45, 000 RRab stars is pure (90%), and complete (80% at 80 kpc) at high galactic latitudes. It also provides distances precise to 3%, measured with newly derived period-luminosity relations for optical/near-infrared PS1 bands. With the addition of proper motions from Gaia and radial velocity measurements from multi-object spectroscopic surveys, we expect the PS1 sample of RR Lyrae stars to become the premier source for studying the structure, kinematics, and the gravitational potential of the Galactic halo. The techniques presented in this study should translate well to other sparse, multi-band data sets, such as those produced by the Dark Energy Survey and the upcoming Large Synoptic Survey Telescope Galactic plane sub-survey.

Knowledge of ages for stars formed over a galaxy's lifetime is fundamental to understand its formation and evolution. However, stellar ages are difficult to obtain since they cannot be measured from observations, being comparison with stellar models 1 required. Alternatively, age distributions can be derived applying the robust technique of colour-magnitude diagram fitting 2 , till now mainly employed to study nearby galaxies. The new distances to individual Milky Way stars from the Gaia mission 3 have allowed us to use this technique to derive ages from a thick disk colour-magnitude diagram, and from the enigmatic, two-sequenced colour-magnitude diagram of the kinematically hot local halo 4 , which blue-sequence has been linked to a major accretion event 5, 6 . Because accurate ages were lacking, the time of the merger and its role on our Galaxy's early evolution remained unclear. We show that the stars in both halo sequences share identical age distributions, and are older than the bulk of thick disc stars. The sharp halo age cut 10 Gyr ago can be identified with the accretion of Gaia-Enceladus. Along with state-of-the-art cosmological simulations of galaxy formation 7 , these robust ages allow us to order the early sequence of events that shaped our Galaxy, identifying the red-sequence as the first stars formed within the Milky Way progenitor which, because of their kinematics, can be described as its long sought in-situ halo.The new accurate parallaxes and luminosities provided in the second data release (DR2) of the Gaia mission 3 have allowed us to construct, for the first time, colour-magnitude diagrams (CMD) in the absolute plane for stars located in a large volume of the Milky Way, encompasing different Galactic structural components. These CMDs, in units of absolute magnitudes and colours, are what is required to derive star formation histories 2 and stellar age distributions, by comparing them with theoretical CMDs derived from stellar evolution models 8 .The top panels of Figure 1 show the CMD of two sub-populations of Milky Way stars taken from a parent population that lies within a sphere of 2 Kpc around the Sun, as observed by Gaia. In this volume, accurate distances and absolute magnitudes can be derived directly from parallaxes. The CMD in the top left panel contains about sixty thousand stars from this spherical region with large tangential velocities relative to the Sun (greater than 200 km/s). Stars with such high velocities are classified in this study as belonging to a kinematically defined stellar halo 4 . The CMD in the top right panel is of some arXiv:1901.02900v2 [astro-ph.GA] 29 Jul 2019 half million stars from the same spherical region but selected to be at least 1.1 Kpc above or below the Galactic plane. At this distance from the plane, the majority of stars are expected to belong to the thick disc 9 , rather than to the young thin disc component. We have excluded from this sample the stars with high velocity (greater than 200 km/s) that have been included in the halo CMD. Note that o...