Using a large sample of Main Sequence stars with 7-D measurements supplied by Gaia and SDSS, we study the kinematic properties of the local (within ∼10 kpc from the Sun) stellar halo. We demonstrate that the halo's velocity ellipsoid evolves strongly with metallicity. At the low [Fe/H] end, the orbital anisotropy (the amount of motion in the radial direction compared to the tangential one) is mildly radial with 0.2 < β < 0.4. However, for stars with [Fe/H]> −1.7 we measure extreme values of β ∼ 0.9. Across the metallicity range considered, i.e. −3 <[Fe/H]−1, the stellar halo's spin is minimal, at the level of 20 10 10 M ⊙ around the epoch of the Galactic disc formation, i.e. between 8 and 11 Gyr ago. The radical halo anisotropy is the result of the dramatic radialisation of the massive progenitor's orbit, amplified by the action of the growing disc.

We introduce a new maximum‐likelihood method to model the density profile of blue horizontal branch and blue straggler stars and apply it to the Sloan Digital Sky Survey Data Release 8 photometric catalogue. There are a large number (∼20 000) of these tracers available over an impressive 14 000 deg2 in both Northern and Southern Galactic hemispheres, and they provide a robust measurement of the shape of the Milky Way stellar halo. After masking out stars in the vicinity of the Virgo overdensity and the Sagittarius stream, the data are consistent with a smooth, oblate stellar halo with a density that follows a broken power law. The best‐fitting model has an inner slope αin∼ 2.3 and an outer slope αout∼ 4.6, together with a break radius occurring at ∼27 kpc and a constant halo flattening (i.e. ratio of minor axis to major axis) of q∼ 0.6. Although a broken power law describes the density fall‐off most adequately, it is also well fitted by an Einasto profile. There is no strong evidence for variations in flattening with radius, or for triaxiality of the stellar halo.

Using a large sample of bright nearby stars with accurate Gaia Data Release 2 astrometry and auxiliary spectroscopy we map out the properties of the principle Galactic components such as the "thin" and "thick" discs and the halo. We show that in the Solar neighborhood, there exists a large population of metal-rich ([Fe/H]> −0.7) stars on highly eccentric orbits. By studying the evolution of elemental abundances, kinematics and stellar ages in the plane of azimuthal velocity v φ and metallicity [Fe/H], we demonstrate that this metal-rich halo-like component, which we dub the Splash, is linked to the α-rich (or "thick") disc. Splash stars have little to no angular momentum and many are on retrograde orbits. They are predominantly old, but not as old as the stars deposited into the Milky Way in the last major merger. We argue, in agreement with several recent studies, that the Splash stars may have been born in the Milky Way's proto-disc prior to the massive ancient accretion event which drastically altered their orbits. We can not, however, rule out other (alternative) formation channels. Taking advantage of the causal connection between the merger and the Splash, we put constraints of the epoch of the last massive accretion event to have finished 9.5 Gyr ago. The link between the local metal-rich and metal-poor retrograde stars is confirmed using a large suite of cutting-edge numerical simulations of the Milky Way's formation.

We determine the Milky Way (MW) mass profile inferred from fitting physically motivated models to the Gaia DR2 Galactic rotation curve and other data. Using various hydrodynamical simulations of MW-mass haloes, we show that the presence of baryons induces a contraction of the dark matter (DM) distribution in the inner regions, r 20 kpc. We provide an analytic expression that relates the baryonic distribution to the change in the DM halo profile. For our galaxy, the contraction increases the enclosed DM halo mass by factors of roughly 1.3, 2 and 4 at radial distances of 20, 8 and 1 kpc, respectively compared to an uncontracted halo. Ignoring this contraction results in systematic biases in the inferred halo mass and concentration. We provide a best-fitting contracted NFW halo model to the MW rotation curve that matches the data very well. The best-fit has a DM halo mass, M DM 200 = 0.99 +0.18 −0.20 ×10 12 M , and concentration before baryon contraction of 8.2 +1.7 −1.5 , which lie close to the median halo mass-concentration relation predicted in ΛCDM. The inferred total mass, M total 200 = 1.12 +0.20 −0.22 × 10 12 M , is in good agreement with recent measurements. The model gives a MW stellar mass of 4.99 +0.34 −0.50 × 10 10 M , of which 60% is contained in the thin stellar disc, with a bulge-to-total ratio of 0.2. We infer that the DM density at the Solar position is ρ DM = 9.0 +0.5 −0.4 × 10 −3 M pc −3 ≡ 0.34 +0.02 −0.02 GeV cm −3 . The rotation curve data can also be fitted with an uncontracted NFW halo model, but with very different DM and stellar parameters. The observations prefer the physically motivated contracted NFW halo, but the measurement uncertainties are too large to rule out the uncontracted NFW halo.

We use the suite of simulations to study the density profiles of L * -type galaxy stellar haloes. Observations of the Milky Way and M31 stellar haloes show contrasting results: the Milky Way has a 'broken' profile, where the density falls off more rapidly beyond ∼ 25 kpc, while M31 has a smooth profile out to 100 kpc with no obvious break. Simulated stellar haloes, built solely by the accretion of dwarf galaxies, also exhibit this behavior: some haloes have breaks, while others don't. The presence or absence of a break in the stellar halo profile can be related to the accretion history of the galaxy. We find that a break radius is strongly related to the build up of stars at apocentres. We relate these findings to observations, and find that the 'break' in the Milky Way density profile is likely associated with a relatively early (∼ 6 − 9 Gyr ago) and massive accretion event. In contrast, the absence of a break in the M31 stellar halo profile suggests that its accreted satellites have a wide range of apocentres. Hence, it is likely that M31 has had a much more prolonged accretion history than the Milky Way.

We present and apply a method to infer the mass of the Milky Way (MW) by comparing the dynamics of MW satellites to those of model satellites in the EAGLE cosmological hydrodynamics simulations. A distribution function (DF) for galactic satellites is constructed from EAGLE using specific angular momentum and specific energy, which are scaled so as to be independent of host halo mass. In this 2-dimensional space, the orbital properties of satellite galaxies vary according to the host halo mass. The halo mass can be inferred by calculating the likelihood that the observed satellite population is drawn from this DF. Our method is robustly calibrated on mock EAGLE systems. We validate it by applying it to the completely independent suite of 30 AURIGA high-resolution simulations of MW-like galaxies: the method accurately recovers their true mass and associated uncertainties. We then apply it to ten classical satellites of the MW with 6D phase-space measurements, including updated proper motions from the Gaia satellite. The mass of the MW is estimated to be M MW 200 = 1.17 +0.21 −0.15 × 10 12 M (68% confidence limits). We combine our total mass estimate with recent mass estimates in the inner regions of the Galaxy to infer an inner dark matter (DM) mass fraction M DM (< 20 kpc)/M DM 200 = 0.12 which is typical of ∼10 12 M ΛCDM haloes in hydrodynamical galaxy formation simulations. Assuming an NFW profile, this is equivalent to a halo concentration of c MW 200 = 10.9 +2.6 −2.0 .

We present the discovery of stellar tidal tails around the Large and the Small Magellanic Clouds in the Gaia DR1 data. In between the Clouds, their tidal arms are stretched towards each other to form an almost continuous stellar bridge. Our analysis relies on the exquisite quality of the Gaia's photometric catalogue to build detailed star-count maps of the Clouds. We demonstrate that the Gaia DR1 data can be used to detect variable stars across the whole sky, and in particular, RR Lyrae stars in and around the LMC and the SMC. Additionally, we use a combination of Gaia and Galex to follow the distribution of Young Main Sequence stars in the Magellanic System. Viewed by Gaia, the Clouds show unmistakable signs of interaction. Around the LMC, clumps of RR Lyrae are observable as far as ∼ 20 • , in agreement with the most recent map of Mira-like stars reported in Deason et al. (2016). The SMC's outer stellar density contours show a characteristic S-shape, symptomatic of the on-set of tidal stripping. Beyond several degrees from the center of the dwarf, the Gaia RR Lyrae stars trace the Cloud's trailing arm, extending towards the LMC. This stellar tidal tail mapped with RR Lyrae is not aligned with the gaseous Magellanic Bridge, and is shifted by some ∼ 5 • from the Young Main Sequence bridge. We use the offset between the bridges to put constraints on the density of the hot gaseous corona of the Milky Way.

We use distant blue horizontal branch stars with Galactocentric distances 16 < r < 48 kpc as kinematic tracers of the Milky Way dark halo. We model the tracer density as an oblate, power law embedded within a spherical power-law potential. Using a distribution function method, we estimate the overall power-law potential and the velocity anisotropy of the halo tracers. We measure the slope of the potential to be γ ∼ 0.4, and the overall mass within 50 kpc is ∼4 × 10 11 M . The tracer velocity anisotropy is radially biased with β ∼ 0.5, which is in good agreement with local solar neighbourhood studies. Our results provide an accurate outer circular velocity profile for the Milky Way and suggest a relatively high-concentration dark matter halo (c vir ∼ 20).