Konrad Kuijken

Hubble Space Telescope WFPC2 Proper Motions in Two Bulge Fields: Kinematics and Stellar Population of the Galactic Bulge

We report proper-motion dispersions for stars in the direction of two fields of the Galactic bulge, using Hubble Space Telescope Wide Field Planetary Camera 2 images taken 6 years apart. Our two fields are Baades window [(l, b) = (113, -377)] and Sgr I [(l, b) = (125, -265)]. Our proper-motion dispersions are in good agreement with prior ground- and space-based proper-motion studies in bulge fields, but in contrast to some prior studies, we do not exclude any subset of stars from our sample. In Baades window, we find the l and b proper-motion dispersions are 2.9 and 2.5 mas yr-1, while in Sgr I, they are 3.3 and 2.7 mas yr-1, respectively. For the first time, we can clearly separate the foreground disk stars out from the bulge because of their large mean apparent proper motion. The population with nondisk kinematics (which we conclude to be the bulge) has an old main-sequence turnoff point, similar to those found in old, metal-rich bulge globular clusters, while those stars selected to have disk kinematics lie on a fully populated main sequence. Separating main-sequence stars by luminosity, we find strong evidence that the bulge population is rotating, largely explaining observations of proper-motion anisotropy in bulge fields. Because we have isolated such a pure sample of stars in the bulge, we have one of the clearest demonstrations that the old stellar population of the inner bulge/bar is in fact rotating.

Abelian Higgs hair for black holes

We find evidence for the existence of solutions of the Einstein and Abelian Higgs field equations describing a black hole pierced by a Nielsen-Olesen vortex. This situation falls outside the scope of the usual no-hair arguments due to the nontrivial topology of the vortex configuration and the special properties of its energy-momentum tenser. By a combination of numerical and perturbative techniques we conclude that the black hole horizon has no difficulty in supporting the long-range fields of the Nielsen-Olesen string. Moreover, the effect of the vortex can in principle be measured from infinity, thus justifying its characterization as black hole hair.

Lowered Evans models: analytic distribution functions of oblate halo potentials

We describe an analytic distribution function of a finite, oblate stellar system that is useful for the practical modelling of dark halos. The function is determined by lowering Evanss (1993) distribution function of a flattened, cored isothermal system in analogy to the lowering of the singular, isothermal sphere in the definition of the King (1966) models. We derive analytic expressions of the density, maximal streaming velocity and velocity dispersion profiles as a function of the potential. As for the King models, the potential must be calculated numerically. We also present a recipe for generating N-body realizations of this distribution function and examine the stability in three models with dimensionless spins

Dark Matter in the Milky Way

lambda=

Dynamical simulations of semilocal strings

0.0, 0.05 and 0.18 using N-body simulations with 50,000 particles. The

Role of W condensation in electroweak string stability.

lambda=0.18

The maximum optical depth toward bulge stars from axisymmetric models of the Milky Way

model is unstable to the formation of a triaxial bar within

Is There a Bulge Distinct from the Bar

sim 5

Counter-rotating populations in a disk galaxy

King radii while the other models appear stable. We conclude that the slowly rotating systems are useful for modelling flattened dark halos.

ON THE ELLIPTICITY OF THE GALACTIC DISK

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.