ABSTRACT For stars with unresolved companions, motions of the centre of light and that of mass decouple, causing a single-source astrometric model to perform poorly. We show that such stars can be easily detected with the reduced χ2 statistic, or renormalized unit weight error (RUWE), provided as part of Gaia DR2. We convert RUWE into the amplitude of the image centroid wobble, which, if scaled by the source distance, is proportional to the physical separation between companions (for periods up to several years). We test this idea on a sample of known spectroscopic binaries and demonstrate that the amplitude of the centroid perturbation scales with the binary period and the mass ratio as expected. We apply this technique to the Gaia DR2 data and show how the binary fraction evolves across the Hertzsprung–Russell diagram. The observed incidence of unresolved companions is high for massive young stars and drops steadily with stellar mass, reaching its lowest levels for white dwarfs. We highlight the elevated binary fraction for the nearby blue stragglers and blue horizontal branch stars. We also illustrate how unresolved hierarchical triples inflate the relative velocity signal in wide binaries. Finally, we point out a hint of evidence for the existence of additional companions to the hosts of extrasolar hot Jupiters.

Using the Illustris simulation, we follow thousands of elliptical galaxies back in time to identify how the dichotomy between fast and slow rotating ellipticals (FRs and SRs) develops. Comparing to the ATLAS 3D survey, we show that Illustris reproduces similar elliptical galaxy rotation properties, quantified by the degree of ordered rotation, λ R . There is a clear segregation between low-mass (M * < 10 11 M ) ellipticals, which form a smooth distribution of FRs, and high-mass galaxies (M * > 10 11.5 M ), which are mostly SRs, in agreement with observations. We find that SRs are very gas poor, metal rich and red in colour, while FRs are generally more gas rich and still star forming. We suggest that ellipticals begin naturally as FRs and, as they grow in mass, lose their spin and become SRs. While at z = 1, the progenitors of SRs and FRs are nearly indistinguishable, their merger and star formation histories differ thereafter. We find that major mergers tend to disrupt galaxy spin, though in rare cases can lead to a spin-up. No major difference is found between the effects of gas-rich and gaspoor mergers and the amount of minor mergers seem to have little correlation with galaxy spin. In between major mergers, lower-mass ellipticals, which are mostly gasrich, tend to recover their spin by accreting gas and stars. For galaxies with M * above ∼ 10 11 M , this trend reverses; galaxies only retain or steadily lose their spin. More frequent mergers, accompanied by an inability to regain spin, lead massive ellipticals to lose most of ordered rotation and transition from FRs to SRs.

Most binaries are undetected. Astrometric reductions of a system using the assumption that the object moves like a single point mass can be biased by unresolved binary stars. The discrepancy between the centre of mass of the system (which moves like a point mass) and the centre of light (which is what we observe) introduces additional motion. We explore the extent to which binary systems affect single object models fit to astrometric data. This tells us how observations are diluted by binaries and which systems cause the largest discrepancies - but also allows us to make inferences about the binarity of populations based on observed astrometric error. By examining a sample of mock observations, we show that binaries with periods close to one year can mimic parallax and thus bias distance measurements, whilst long period binaries can introduce significant apparent proper motion. Whilst these changes can soak up some of the error introduced by the binary, the total deviation from the best fitting model can be translated into a lower limit on the on-sky separation of the pair. Throughout we link these predictions to data from the Gaia satellite, whilst leaving the conclusions generalizable to other surveys.

We present a new method to construct fully self-consistent equilibrium models of multicomponent disc galaxies similar to the Milky Way. We define distribution functions for the stellar disc and dark halo that depend on phase space position only through action coordinates. We then use an iterative approach to find the corresponding gravitational potential. We study the adiabatic response of the initially spherical dark halo to the introduction of the baryonic component and find that the halo flattens in its inner regions with final minor-major axis ratios q = 0.75 -0.95. The extent of the flattening depends on the velocity structure of the halo particles with radially biased models exhibiting a stronger response. In this latter case, which is according to cosmological simulations the most likely one, the new density structure resembles a "dark disc" superimposed on a spherical halo. We discuss the implications of these results for our recent estimate of the local dark matter density. The velocity distribution of the dark-matter particles near the Sun is very nonGaussian. All three principal velocity dispersions are boosted as the halo contracts, and at low velocities a plateau develops in the distribution of v z . For models similar to a state-of-the-art Galaxy model we find velocity dispersions around 180 km s −1 for v z and the tangential velocity, v ϕ , and 150 -205 km s −1 for the in-plane radial velocity, v R , depending on the anisotropy of the model.