Oort constants

Abstract: We re-examine the stellar kinematics of the Solar neighbourhood in terms of the velocity of the Sun with respect to the local standard of rest. We show that the classical determination of its component V_sun in the direction of Galactic rotation via Stroemberg's relation is undermined by the metallicity gradient in the disc, which introduces a correlation between the colour of a group of stars and the radial gradients of its properties. Comparing the local stellar kinematics to a chemodynamical model which accounts for these effects, we obtain (U,V,W)_sun = (11.1 +/- 0.74, 12.24 +/- 0.47, 7.25 +/-0.37) km/s, with additional systematic uncertainties of ~ (1,2,0.5) km/s. In particular, V_sun is 7 km/s larger than previously estimated. The new values of solar motion are extremely insensitive to the metallicity gradient within the disc.

Abstract: From the parallaxes and proper motions of a kinematically unbiased subsample of the Hipparcos Catalogue, we have re-determined as a function of colour the kinematics of main-sequence stars. Whereas the radial and vertical components of the mean heliocentric velocity of stars show no trend with colour, the component in the direction of Galactic rotation nicely follows the asymmetric drift relation, except for stars bluer than B − V = 0.1 mag. Extrapolating to zero dispersion yields, for the velocity of the Sun with respect to the local standard of rest (LSR) in km s−1, U0 = 10.00 ± 0.36 (radially inwards), V0 = 5.25 ± 0.62 (in the direction of Galactic rotation) and W0 = 7.17 ± 0.38 (vertically upwards). Parenago's discontinuity is beautifully visible in a plot of velocity dispersion against colour: the dispersion, which is essentially constant for late spectral types, decreases towards early spectral types blueward of B − V ≈ 0.61 mag. We determine the velocity dispersion tensor σ2 as function of colour. The mixed moments involving vertical motion are zero within the errors, while σ2xy is non-zero at about (10 km s−1)2 independently of colour. The resulting vertex deviations are about 20° for early-type stars and 10° ± 4° for old-disc stars. The persistence of the vertex deviation to late-type stars implies that the Galactic potential is significantly non-axisymmetric at the solar radius. If spiral arms are responsible for the non-axisymmetry, they cannot be tightly wound. Except for stars bluer than B − V= 0.1 mag, the ratios of the principal velocity dispersions are given by σ1:σ2:σ3 ≈ 2.2 : 1.4 : 1, while the absolute values increase with colour from σ1 ≈ 20 km s−1 at B − V = 0.2 mag to σ1 ≈ 38 km s−1 at Parenago's discontinuity and beyond. These ratios imply significant heating of the disc by spiral structure and that R0/Rd ≃ 3 to 3.5, where Rd is the scalelength of the disc.

Abstract: We are using the VLBA and the Japanese VERA project to measure trigonometric parallaxes and proper motions of masers found in high-mass star-forming regions across the Milky Way. Early results from 18 sources locate several spiral arms. The Perseus spiral arm has a pitch angle of 16 +/- 3 degrees, which favors four rather than two spiral arms for the Galaxy. Combining positions, distances, proper motions, and radial velocities yields complete 3-dimensional kinematic information. We find that star forming regions on average are orbiting the Galaxy ~15 km/s slower than expected for circular orbits. By fitting the measurements to a model of the Galaxy, we estimate the distance to the Galactic center R_o = 8.4 +/- 0.6 kpc and a circular rotation speed Theta_o = 254 +/- 16 km/s. The ratio Theta_o/R_o can be determined to higher accuracy than either parameter individually, and we find it to be 30.3 +/- 0.9 km/s/kpc, in good agreement with the angular rotation rate determined from the proper motion of Sgr A*. The data favor a rotation curve for the Galaxy that is nearly flat or slightly rising with Galactocentric distance. Kinematic distances are generally too large, sometimes by factors greater than two; they can be brought into better agreement with the trigonometric parallaxes by increasing Theta_o/R_o from the IAU recommended value of 25.9 km/s/kpc to a value near 30 km/s/kpc. We offer a "revised" prescription for calculating kinematic distances and their uncertainties, as well as a new approach for defining Galactic coordinates. Finally, our estimates of Theta_o and To/R_o, when coupled with direct estimates of R_o, provide evidence that the rotation curve of the Milky Way is similar to that of the Andromeda galaxy, suggesting that the dark matter halos of these two dominant Local Group galaxy are comparably massive.

Abstract: We derive new constraints on the mass of the Milky Way's dark matter halo, based on 2401 rigorously selected blue horizontal-branch halo stars from SDSS DR6. This sample enables construction of the full line-of-sight velocity distribution at different galactocentric radii. To interpret these distributions, we compare them to matched mock observations drawn from two different cosmological galaxy formation simulations designed to resemble the Milky Way. This procedure results in an estimate of the Milky Way's circular velocity curve to ~60 kpc, which is found to be slightly falling from the adopted value of 220 km s?1 at the Sun's location, and implies -->M( Vcir(r) , derived in statistically independent bins, is found to be consistent with the expectations from an NFW dark matter halo with the established stellar mass components at its center. If we assume that an NFW halo profile of characteristic concentration holds, we can use the observations to estimate the virial mass of the Milky Way's dark matter halo, -->Mvir = 1.0+ 0.3?0.2 ? 1012 M?, which is lower than many previous estimates. We have checked that the particulars of the cosmological simulations are unlikely to introduce systematics larger than the statistical uncertainties. This estimate implies that nearly 40% of the baryons within the virial radius of the Milky Way's dark matter halo reside in the stellar components of our Galaxy. A value for -->Mvir of only ~ -->1 ? 1012 M? also (re)opens the question of whether all of the Milky Way's satellite galaxies are on bound orbits.