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We infer the gravitational potential of the Galactic disk by analysing the phase-space densities of 120 stellar samples in 40 spatially separate sub-regions of the solar neighbourhood, using Gaia's second data release (DR2), in order to quantify spatially dependent systematic effects that bias this type of measurement. The gravitational potential was inferred under the assumption of a steady state in the framework of a Bayesian hierarchical model. We performed a joint fit of our stellar tracers' three-dimensional velocity distribution, while fully accounting for the astrometric uncertainties of all stars. The inferred gravitational potential is compared, post-inference, to a model for the baryonic matter and halo dark matter components. We see an unexpected but clear trend for all 40 spatially separate sub-regions: Compared to the potential derived from the baryonic model, the inferred gravitational potential is significantly steeper close to the Galactic mid-plane (, 19 pages, 17 figures, accepted for publication in A&A

The aim of this paper is to test the need for non-baryonic dark matter in the context of galactic rotation and the apparent difference between distributions of galactic mass and luminosity. We present a set of rotation curves and 3.6 &mu, m surface brightness profiles for a diverse sample of 214 galaxies. Using rotation curves as the sole input into our Newtonian disk model, we compute non-parametric radial profiles of surface mass density. All profiles exhibit lower density than parametric models with dark halos and provide a superior fit with observed rotation curves. Assuming all dynamical mass is in main-sequence stars, we estimate radial distributions of characteristic star mass implied by the corresponding pairs of density and brightness profiles. We find that for 132 galaxies or 62% of the sample, the relation between density and brightness can be fully explained by a radially declining stellar mass gradient. Such idealized stellar population fitting can also largely address density and brightness distributions of the remaining 82 galaxies, but their periphery shows, on average, 14 M⊙/pc2 difference between total density and light-constrained stellar density. We discuss how this density gap can be interpreted, by considering a low-luminosity baryonic matter, observational uncertainties, and visibility cutoffs for red dwarf populations. Lastly, we report tight correlation between radial density and brightness trends, and the discovered flattening of surface brightness profiles&mdash, both being evidence against dark matter. Our findings make non-baryonic dark matter unnecessary in the context of galactic rotation.

We analyze the distribution of G and K type stars towards the Galactic poles using RAVE and ELODIE radial velocities, 2MASS photometric star counts, and UCAC2 proper motions. The combination of photometric and 3D kinematic data allows us to disentangle and describe the vertical distribution of dwarfs, sub-giants and giants and their kinematics. We identify discontinuities within the kinematics and magnitude counts that separate the thin disk, thick disk and a hotter component. The respective scale heights of the thin disk and thick disk are 225$\pm$10 pc and 1048$\pm$36 pc. We also constrain the luminosity function and the kinematic distribution function. The existence of a kinematic gap between the thin and thick disks is incompatible with the thick disk having formed from the thin disk by a continuous process, such as scattering of stars by spiral arms or molecular clouds. Other mechanisms of formation of the thick disk such as `created on the spot' or smoothly `accreted' remain compatible with our findings., Comment: 15 pages, 13 EPS figures and 1 table

We present the first data release of the Radial Velocity Experiment (RAVE), an ambitious spectroscopic survey to measure radial velocities and stellar atmosphere parameters (temperature, metallicity, and surface gravity) of up to one million stars using the Six Degree Field multiobject spectrograph on the 1.2 m UK Schmidt Telescope of the Anglo-Australian Observatory. The RAVE program started in 2003, obtaining medium-resolution spectra (median R = 7500) in the Ca-triplet region (8410-8795 angstrom) for southern hemisphere stars drawn from the Tycho-2 and SuperCOSMOS catalogs, in the magnitude range 9

Reliable systemic radial velocities are almost impossible to secure for Wolf-Rayet stars, difficult for O stars. Therefore, to study the motions - both systematic in the Galaxy and peculiar - of these two related types of hot, luminous star, we have examined the Hipparcos proper motions of some 70 stars of each type. We find that (a) both groups follow Galactic rotation in the same way, (b) both have a similar fraction of "runaways", (c) mean kinetic ages based on displacement and motion away from the Galactic plane tend to slightly favour the cluster ejection over the the binary supernova hypothesis for their formation, and (d) those with significant peculiar supersonic motion relative to the ambient ISM, tend to form bow shocks in the direction of the motion.

It has been known that recent microlensing results toward the bulge imply mass densities that are surprisingly high, given dynamical constraints on the Milky Way mass distribution. We derive the maximum optical depth toward the bulge that may be generated by axisymmetric structures in the Milky Way, and we show that observations are close to surpassing these limits. This result argues in favor of a bar as a source of significantly enhanced microlensing. Several of the bar models in the literature are discussed.

It has been known that recent microlensing results towards the bulge imply mass densities that are surprisingly high given dynamical constraints on the Milky Way mass distribution. We derive the maximum optical depth towards the bulge that may be generated by axisymmetric structures in the Milky Way, and show that observations are close to surpassing these limits. This result argues in favor of a bar as a source of significantly enhanced microlensing. Several of the bar models in the literature are discussed., Comment: Latex, 6 pages, 4 figures, uses aas2pp4 and epsf style files. Accepted for publication in ApJ Letters

It has been known that recent microlensing results toward the bulge imply mass densities that are surprisingly high, given dynamical constraints on the Milky Way mass distribution. We derive the maximum optical depth toward the bulge that may be generated by axisymmetric structures in the Milky Way, and we show that observations are close to surpassing these limits. This result argues in favor of a bar as a source of significantly enhanced microlensing. Several of the bar models in the literature are discussed.

The Milky Way is often considered to be the best example of a spiral for which the dark matter not only dominates the outer kinematics but also plays a major dynamical role in the inner galaxy: the Galactic disk is therefore said to be ''submaximal.'' This conclusion is important to the understanding of the evolution of galaxies and the viability of particular dark matter models. The Galactic evidence rests on a number of structural and kinematic measurements, many of which have recently been revised. The new constraints not only indicate that the Galaxy is a more typical member of its class (Sb to Sc spirals) than previously thought but also require a reexamination of the question of whether the Milky Way disk is maximal. By applying to the Milky Way the same definition of ''maximal disk'' that is applied to external galaxies, it is shown that the new observational constraints are consistent with a Galactic maximal disk of reasonable M/L. In particular, the local disk column can be substantially less than the oft-quoted required Sigma(.) approximate to 100 M-. pc(-2)-as low as 40 M-. pc(-2) in the extreme case-and still be maximal, in the sense that the dark halo provides negligible rotation support in the inner Galaxy. This result has possible implications for any conclusion that rests on assumptions about the potential of the Galactic disk or dark halo and, in particular, for the interpretation of microlensing results along both LMC and bulge lines of sight.

Reliable systemic radial velocities are almost impossible to secure for Wolf-Rayet stars, difficult for O stars. Therefore, to study the motions - both systematic in the Galaxy and peculiar - of these two related types of hot, luminous star, we have examined the Hipparcos proper motions of some 70 stars of each type. We find that (a) both groups follow Galactic rotation in the same way, (b) both have a similar fraction of "runaways", (c) mean kinetic ages based on displacement and motion away from the Galactic plane tend to slightly favour the cluster ejection over the the binary supernova hypothesis for their formation, and (d) those with significant peculiar supersonic motion relative to the ambient ISM, tend to form bow shocks in the direction of the motion.

If the Newtonian inverse-square law is an appropriate description of gravity, then disk galaxies, and the Milky Way in particular, contain large amounts of dark matter: in general, the observed shapes of rotation curves do not correspond to the gravitational potential of the observed stars and gas. The nature of this dark matter is still a topic of debate, the outcome of which will have important consequences for our ideas of how galaxies and larger structures in the universe formed.