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.

Tidal streams in the Milky Way are sensitive probes of the population of low-mass dark-matter subhalos predicted in cold-dark-matter (CDM) simulations. We present a new calculus for computing the effect of subhalo fly-bys on cold streams based on the action-angle representation of streams. The heart of this calculus is a lineof-parallel-angle approach that calculates the perturbed distribution function of a stream segment by undoing the effect of all relevant impacts. This approach allows one to compute the perturbed stream density and track in any coordinate system in minutes for realizations of the subhalo distribution down to 10 5 M , accounting for the stream's internal dispersion and overlapping impacts. We study the statistical properties of density and track fluctuations with large suites of simulations of the effect of subhalo fly-bys. The one-dimensional density and track power spectra along the stream trace the subhalo mass function, with higher-mass subhalos producing power only on large scales, while lower mass subhalos cause structure on smaller scales. We also find significant density and track bispectra that are observationally accessible. We further demonstrate that different projections of the track all reflect the same pattern of perturbations, facilitating their observational measurement. We apply this formalism to data for the Pal 5 stream and make a first rigorous determination of 10 +11 −6 dark-matter subhalos with masses between 10 6.5 M and 10 9 M within 20 kpc from the Galactic center (corresponding to 1.4 +1.6 −0.9 times the number predicted by CDMonly simulations or to f sub (r < 20 kpc) ≈ 0.2 %) assuming that the Pal 5 stream is 5 Gyr old. Improved data will allow measurements of the subhalo mass function down to 10 5 M , thus definitively testing whether dark matter is clumpy on the smallest scales relevant for galaxy formation.

We present a catalogue of distances, masses and ages for ∼ 3 million stars in the second Gaia data release with spectroscopic parameters available from the large spectroscopic surveys: APOGEE, Gaia-ESO, GALAH, LAMOST, RAVE and SEGUE. We use a Bayesian framework to characterise the probability density functions of distance, mass and age using photometric, spectroscopic and astrometric information, supplemented with spectroscopic masses where available for giant stars. Furthermore, we provide posterior extinction estimates (A V ) to every star using published extinction maps as a prior input. We provide an appendix with extinction coefficients for Gaia photometry derived from stellar models, which account for variation with intrinsic colour and total extinction. Our pipeline provides output estimates of the spectroscopic parameters, which can be used to inform improved spectroscopic analysis. We complement our catalogues with Galactocentric coordinates and actions with associated uncertainties. As a demonstration of the power of our catalogue, we produce velocity dispersion profiles of the disc separated by age and Galactocentric radius (between 3 and 15 kpc from the Galactic centre). This suggests that the velocity dispersion profiles flatten with radius in the outer Galaxy (> 8 kpc) and that at all radii the velocity dispersion follows the smooth power law with age observed in the solar neighbourhood.

The Gaia Sausage is an elongated structure in velocity space discovered by Belokurov et al. (2018) using the kinematics of metal-rich halo stars. It was created by a massive dwarf galaxy (∼ 5×10 10 M ) on a strongly radial orbit that merged with the Milky Way at a redshift z 3. We search for the associated Sausage Globular Clusters by analysing the structure of 91 Milky Way globular clusters (GCs) in action space using the Gaia Data Release 2 catalogue, complemented with Hubble Space Telescope proper motions. There is a characteristic energy E crit which separates the in situ objects, such as the bulge/disc clusters, from the accreted objects, such as the young halo clusters. There are 15 old halo GCs that have E > E crit . Eight of the high energy, old halo GCs are strongly clumped in azimuthal and vertical action, yet strung out like beads on a chain at extreme radial action. They are very radially anisotropic (β ∼ 0.95) and move on orbits that are all highly eccentric (e 0.80). They also form a track in the age-metallicity plane distinct from the bulk of the Milky Way GCs and compatible with a dwarf spheroidal origin. These properties are consistent with GCs associated with the merger event that gave rise to the Gaia Sausage.

Ample observational capabilities exist today to detect the small density perturbations that low-mass dark matter subhaloes impart on stellar streams from disrupting Galactic satellites. In anticipation of these observations, we investigate the expected number and size of gaps by combining an analytic prescription for gap evolution on circular orbits with the flux of subhaloes near the stream. We explore the distribution of gap sizes and depths for a typical cold stream around the Milky Way and find that for a given stream age and gap depth, each subhalo mass produces a characteristic gap size. For a stream with an age of a few Gyr, orbiting at a distance of 10-20 kpc from the Galactic center, even modest subhaloes with a mass of 10 6 − 10 7 M produce gaps with sizes that are on the order of several degrees. We consider the number and distribution of gap sizes created by subhaloes with masses 10 5 − 10 9 M , accounting for the expected depletion of subhaloes by the Milky Way disk, and present predictions for six cold streams around the Milky Way. For Pal 5, we forecast 0.7 gaps with a density depletion of at least 25% and a typical gap size of 8 • . Thus, there appears to be no tension between the recent non-detection of density depletions in the Pal 5 tidal tails and ΛCDM expectations. These predictions can be used to guide the scale of future gap searches.

We extend models of our Galaxy based on distribution functions (DFs) that are analytic functions of the action integrals to extended distribution functions (EDFs), which have an analytic dependence on metallicity as well. We use a simple, but physically-motivated, functional forms for the metallicity of the interstellar medium as a function of radius and time and for the star-formation rate, and a model for the diffusion of stars through phase space to suggest the required functional form of an EDF. We introduce a simple prescription for radial migration that preserves the overall profile of the disc while allowing individual stars to migrate throughout the disc. Our models explicitly consider the thin and thick discs as two distinct components separated in age.We show how an EDF can be used to incorporate realistic selection functions in models, and to construct mock catalogues of observed samples. We show that the selection function of the Geneva-Copenhagen Survey (GCS) biases in favour of young stars, which have atypically small random velocities. With the selection function taken into account our models produce good fits of the GCS data in chemo-dynamical space and the Gilmore and Reid (1983) density data.From our EDF, we predict the structure of the SEGUE G-dwarf sample. The kinematics are successfully predicted. The predicted metallicity distribution has too few stars with [Fe/H] −0.5 dex and too many metal-rich stars. A significant problem may be the lack of any chemical-kinematic correlations in our thick disc. We argue that EDFs will prove essential tools for the analysis of both observational data and sophisticated models of Galaxy formation and evolution.

We report the discovery of a Milky Way satellite in the constellation of Antlia. The Antlia 2 dwarf galaxy is located behind the Galactic disc at a latitude of b ∼ 11 • and spans 1.26 degrees, which corresponds to ∼ 2.9 kpc at its distance of 130 kpc. While similar in spatial extent to the Large Magellanic Cloud, Antlia 2 is orders of magnitude fainter at M V = −9 mag, making it by far the lowest surface brightness system known (at ∼ 31.9 mag/arcsec 2 ), ∼ 100 times more diffuse than the so-called ultra diffuse galaxies. The satellite was identified using a combination of astrometry, photometry and variability data from Gaia Data Release 2, and its nature confirmed with deep archival DECam imaging, which revealed a conspicuous BHB signal. We have also obtained follow-up spectroscopy using AAOmega on the AAT, identifying 159 member stars, and we used them to measure the dwarf's systemic velocity, 290.9 ± 0.5km/s, its velocity dispersion, 5.7 ± 1.1 km/s, and mean metallicity, [Fe/H]= −1.4. From these properties we conclude that Antlia 2 inhabits one of the least dense Dark Matter (DM) halos probed to date. Dynamical modelling and tidal-disruption simulations suggest that a combination of a cored DM profile and strong tidal stripping may explain the observed properties of this satellite. The origin of this core may be consistent with aggressive feedback, or may even require alternatives to cold dark matter (such as ultra-light bosons).

We use the SDSS-Gaia catalogue to search for substructure in the stellar halo. The sample comprises 62 133 halo stars with full phase space coordinates and extends out to heliocentric distances of ∼ 10 kpc. As actions are conserved under slow changes of the potential, they permit identification of groups of stars with a common accretion history. We devise a method to identify halo substructures based on their clustering in action space, using metallicity as a secondary check. This is validated against smooth models and numerical constructed stellar halos from the Aquarius simulations. We identify 21 substructures in the SDSS-Gaia catalogue, including 7 high significance, high energy and retrograde ones.We investigate whether the retrograde substructures may be material stripped off the atypical globular cluster ω Centauri. Using a simple model of the accretion of the progenitor of the ω Centauri, we tentatively argue for the possible association of up to 5 of our new substructures (labelled Rg1, Rg3, Rg4, Rg6 and Rg7) with this event. This sets a minimum mass of 5 × 10 8 M for the progenitor, so as to bring ω Centauri to its current location in actionenergy space. Our proposal can be tested by high resolution spectroscopy of the candidates to look for the unusual abundance patterns possessed by ω Centauri stars.