Observed Properties of Dark Matter on Small Spatial Scales

The nature of dark matter is one of the outstanding questions of astrophysics. The internal motions of member stars reveal that the lowest luminosity galaxies in the Local Group are the most dark-matter dominated. New large datasets allow one to go further, and determine systematic properties of their dark matter haloes. We summarise recent results, emphasising the critical role of the dwarf spheroidal galaxies in understanding both dark matter and baryonic processes that shape galaxy evolution.Comment: To appear in proceedings of IAU Symposium 244, `Dark Galaxies & Lost Baryons', eds J. Davies & M. Disney. Eight page

How galaxies form: Mass assembly from chemical abundances in the era of large surveys

The chemical abundances in the atmosphere of a star provide unique information about the gas from which that star formed, and, modulo processes that are not important for the vast majority of stars, such as mass transfer in close binary systems, are conserved through a star's life. Correlations between chemistry and kinematics have been used for decades to trace dynamical evolution of the Milky Way Galaxy. I discuss how it should be possible to refine and extend such analyses, provided planned large-scale deep imaging surveys have matched spectroscopic surveys.Comment: 9 pages, 2 figures, invited review at `Chemical abundances in the Universe, connecting first stars to planets', Proceedings of IAU Symposium No. 265, K. Cunha, M. Spite and B. Barbuy, eds, Cambridge University Press, in pres

The R-Process Alliance: Discovery of a Low-α, r-process-enhanced Metal-poor Star in the Galactic Halo

A new moderately r-process-enhanced metal-poor star, RAVE J093730.5−062655, has been identified in the Milky Way halo as part of an ongoing survey by the R-Process Alliance. The temperature and surface gravity indicate that J0937−0626 is likely a horizontal branch star. At [Fe/H] = −1.86, J0937−0626 is found to have subsolar [X/Fe] ratios for nearly every light, α, and Fe-peak element. The low [α/Fe] ratios can be explained by an ~0.6 dex excess of Fe; J0937−0626 is therefore similar to the subclass of "iron-enhanced" metal-poor stars. A comparison with Milky Way field stars at [Fe/H] = −2.5 suggests that J0937−0626 was enriched in material from an event, possibly a Type Ia supernova, that created a significant amount of Cr, Mn, Fe, and Ni and smaller amounts of Ca, Sc, Ti, and Zn. The r-process enhancement of J0937−0626 is likely due to a separate event, which suggests that its birth environment was highly enriched in r-process elements. The kinematics of J0937−0626, based on Gaia DR2 data, indicate a retrograde orbit in the Milky Way halo; J0937−0626 was therefore likely accreted from a dwarf galaxy that had significant r-process enrichment

Correlations between age, kinematics, and chemistry as seen by the RAVE survey

We explore the connections between stellar age, chemistry, and kinematics across a Galactocentric distance of 7.5 < R(kpc) < 9.0, using a sample of ∼12 000 intermediate-mass (FGK) turn-off stars observed with the RAdial Velocity Experiment (RAVE) survey. The kinematics of this sample are determined using radial velocity measurements from RAVE, and parallax and proper motion measurements from the Tycho–Gaia Astrometric Solution (TGAS). In addition, ages for RAVE stars are determined using a Bayesian method, taking TGAS parallaxes as a prior. We divide our sample into young (0 < τ < 3 Gyr) and old (8 < τ < 13 Gyr) populations, and then consider different metallicity bins for each of these age groups. We find significant differences in kinematic trends of young and old, metal-poor and metal-rich, stellar populations. In particular, we find a strong metallicity dependence in the mean Galactocentric radial velocity as a function of radius (∂VR/∂R) for young stars, with metal-rich stars having a much steeper gradient than metal-poor stars. For ∂Vϕ/∂R, young, metal-rich stars significantly lag the LSR with a slightly positive gradient, while metal-poor stars show a negative gradient above the LSR. We interpret these findings as correlations between metallicity and the relative contributions of the non-axisymmetries in the Galactic gravitational potential (the spiral arms and the bar) to perturb stellar orbits.∂Vϕ/∂R, young, metal-rich stars significantly lag the LSR with a slightly positive gradient, while metal-poor stars show a negative gradient above the LSR. We interpret these findings as correlations between metallicity and the relative contributions of the non-axisymmetries in the Galactic gravitational potential (the spiral arms and the bar) to perturb stellar orbits

The r-process alliance: First release from the Northern search for r-process enhanced metal-poor stars in the Galactic Halo

This paper presents the detailed abundances and r-process classifications of 126 newly identified metal-poor stars as part of an ongoing collaboration, the R-Process Alliance

Cardinal kinematics – I. : rotation fields of the APOGEE survey

Correlations between stellar chemistry and kinematics have long been used to gain insight into the evolution of the Milky Way Galaxy. Orbital angular momentum is a key physical parameter and it is often estimated from three-dimensional space motions.We here demonstrate the lower uncertainties that can be achieved in the estimation of one component of velocity through selection of stars in key directions and use of line-of-sight velocity alone (i.e. without incorporation of proper motion data). In this first paper, we apply our technique to stars observed in the direction of Galactic rotation in the APOGEE (Apache Point Observatory Galactic Evolution Experiment) survey.We first derive the distribution of azimuthal velocities, vφ, then from these and observed radial coordinates, estimate the stellar guiding centre radii, Rg, within 6.9 ≤ R ≤ 10 kpc with uncertainties smaller than (or of the order of) 1 kpc. We show that there is no simple way to select a clean stellar sample based on low errors on proper motions and distances to obtain high-quality 3D velocities and hence one should pay particular attention when trying to identify kinematically peculiar stars based on velocities derived using the proper motions. Using our vφ estimations, we investigate the joint distribution of elemental abundances and rotational kinematics free from the blurring effects of epicyclic motions, and we derive the ∂vφ/∂[α/Fe] and ∂vφ/∂[Fe/H] trends for the thin and thick discs as a function of radius. Our analysis provides further evidence for radial migration within the thin disc and hints against radial migration playing a significant role in the evolution of the thick disc

Cardinal kinematics – I. : rotation fields of the APOGEE survey

Correlations between stellar chemistry and kinematics have long been used to gain insight into the evolution of the Milky Way Galaxy. Orbital angular momentum is a key physical parameter and it is often estimated from three-dimensional space motions.We here demonstrate the lower uncertainties that can be achieved in the estimation of one component of velocity through selection of stars in key directions and use of line-of-sight velocity alone (i.e. without incorporation of proper motion data). In this first paper, we apply our technique to stars observed in the direction of Galactic rotation in the APOGEE (Apache Point Observatory Galactic Evolution Experiment) survey.We first derive the distribution of azimuthal velocities, vφ, then from these and observed radial coordinates, estimate the stellar guiding centre radii, Rg, within 6.9 ≤ R ≤ 10 kpc with uncertainties smaller than (or of the order of) 1 kpc. We show that there is no simple way to select a clean stellar sample based on low errors on proper motions and distances to obtain high-quality 3D velocities and hence one should pay particular attention when trying to identify kinematically peculiar stars based on velocities derived using the proper motions. Using our vφ estimations, we investigate the joint distribution of elemental abundances and rotational kinematics free from the blurring effects of epicyclic motions, and we derive the ∂vφ/∂[α/Fe] and ∂vφ/∂[Fe/H] trends for the thin and thick discs as a function of radius. Our analysis provides further evidence for radial migration within the thin disc and hints against radial migration playing a significant role in the evolution of the thick disc

Cardinal kinematics – I. : rotation fields of the APOGEE survey

Correlations between stellar chemistry and kinematics have long been used to gain insight into the evolution of the Milky Way Galaxy. Orbital angular momentum is a key physical parameter and it is often estimated from three-dimensional space motions.We here demonstrate the lower uncertainties that can be achieved in the estimation of one component of velocity through selection of stars in key directions and use of line-of-sight velocity alone (i.e. without incorporation of proper motion data). In this first paper, we apply our technique to stars observed in the direction of Galactic rotation in the APOGEE (Apache Point Observatory Galactic Evolution Experiment) survey.We first derive the distribution of azimuthal velocities, vφ, then from these and observed radial coordinates, estimate the stellar guiding centre radii, Rg, within 6.9 ≤ R ≤ 10 kpc with uncertainties smaller than (or of the order of) 1 kpc. We show that there is no simple way to select a clean stellar sample based on low errors on proper motions and distances to obtain high-quality 3D velocities and hence one should pay particular attention when trying to identify kinematically peculiar stars based on velocities derived using the proper motions. Using our vφ estimations, we investigate the joint distribution of elemental abundances and rotational kinematics free from the blurring effects of epicyclic motions, and we derive the ∂vφ/∂[α/Fe] and ∂vφ/∂[Fe/H] trends for the thin and thick discs as a function of radius. Our analysis provides further evidence for radial migration within the thin disc and hints against radial migration playing a significant role in the evolution of the thick disc

Probing Galaxy Halos at Large Radii using Background QSOs

NRC publication: Ye