Professor James Binney FRS

Abstract:

(shortened) From a kinematically unbiased subsample of the Hipparcos catalogue we have redetermined as a function of colour the kinematics of main-sequence stars. The stars' mean heliocentric velocity nicely follows the asymmetric drift relation, except for stars blueward of B-V=0.1. Extrapolating to zero dispersion yields for the velocity of the Sun w.r.t. the LSR in km/s: U_0=10.00+/-0.36 (radially inwards), V_0=5.23+/-0.62 (in direction of galactic rotation), and W_0=7.17+/-0.38 (vertically upwards). A plot of velocity dispersion vs. colour beautifully shows Parenago's discontinuity: the dispersion is constant for B-V>0.62 and decreases towards bluer colour. We determine the velocity-dispersion tensor sigma^2_ij as function of B-V. The mixed moments involving vertical motion are zero within the errors, while sigma^2_xy is non-zero at about (10km/s)^2 independent of colour. The resulting vertex deviations are about 20 deg for early-type stars and 10+/-4 deg 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 this, they cannot be tightly wound. Except for stars bluer than B-V=0.1 the ratios of the principal velocity dispersions are 2.2 : 1.4 :1, while the absolute values increase with colour from sigma_1=20km/s at B-V=0.2 to sigma_1=38km/s at Parenago's discontinuity and beyond. These ratios imply significant heating of the disc by spiral structure and that R_0/R_d=3 to 3.5, where R_d is the scale length of the disc.

Abstract:

We show that the usual relation between redshift and angular-diameter distance can be obtained by considering light from a source to be gravitationally lensed by material that lies in the telescope beam as it passes from source to observer through an otherwise empty universe. This derivation yields an equation for the dependence of angular diameter on redshift in an inhomogeneous universe. We use this equation to model the distribution of angular-diameter distance for redshift z=3 in a realistically clustered cosmology. The distribution is such that attempts to determine q_0 from angular-diameter distances will systematically underestimate q_0 by ~0.15, and large samples would be required to beat down the intrinsic dispersion in measured values of q_0.

Abstract:

We study the persistence of warps in galactic discs in the presence of massive halos. A disc is approximated by a set of massive rings, while a halo is represented by a conventional n-body simulation. We confirm the conclusion of Nelson & Tremaine (1995) that a halo responds strongly to an embedded precessing disc. This response invalidates the approximations made by in the derivation of classical `modified tilt' modes. We show that the response of the halo causes the line of nodes of a disc that starts from a modified tilt mode to wind up within a few dynamical times. We explain this finding in terms of the probable spectrum of true normal modes of a combined disc-halo system.

Abstract:

We analyze a kinematically unbiased sample of 5610 stars around the south celestial pole that (i) have MK spectral types in the Michigan catalogues with luminosity class V and (ii) had photometric parallaxes that placed them within 80 pc of the Sun. We bin the stars by B - V and determine for each bin the solar motion from proper motions alone. As expected, the U and W components of the derived solar motions do not vary significantly from bin to bin, while the V component varies systematically. As the classic Stromberg relation predicts, V is a linear function of the variance S-2 within each bin around the solar motion. Extrapolating V (S-2) to S = 0 we determine the solar motion with respect to the LSR, obtaining a significantly smaller value of V than is usually employed. Parenago's discontinuity in the dependence of S-2 on spectral type emerges with exceptional clarity.