The Shape of Dark Matter Halos

Techniques for inferring the radial and geometric form of dark matter halos and the results they have produced to date are reviewed. Dark halos appear to extend to at least ~50 kpc with total enclosed masses that rise linearly with radius R. Whether this behavior can be extrapolated to distances as large as 200 kpc and beyond is controversial; results at this radius are model-dependent. Observationally, the geometrical form of the dark halo can be characterized by the equatorial axis ratio b/a (ovalness) and vertical-to-equatorial axis ratio c/a (flattening) of the total density. Different techniques consistently yield b/a > 0.7 (and thus b/a > 0.9 for the potential) at R~20 kpc, with more axisymmetric values, b/a >~ 0.8, being more likely. Results are less consistent for the vertical flattening, perhaps due to the difference in the spatial regions probed by different techniques or inappropriate assumptions. Techniques that probe furthest from the stellar plane z~15 kpc consistently implicate substantially flattened c/a = 0.5 +/- 0.2 dark halos. These axis ratios are in acceptable agreement with expectations from N-body simulations of cold dark matter mixed with ~10% dissipational gas.Comment: Invited Review to appear in Galaxy Dynamics, 1999, eds. D. Merritt, J.A. Sellwood and M. Valluri, ASP, LaTex using paspconf.sty, 3 figures in 5 postscript file

Planetary Microlensing: Present Status and Long-term Goals

Massive gravitational microlensing programs were begun about a decade ago as a means to search for compact baryonic dark matter in the Galaxy, but before the first events were detected the technique was also proposed as a means of detecting extra-solar planets in our Galaxy. Current microlensing planet searches, which have been underway for four years, are sensitive to jovian-mass planets orbiting a few to several AU from their parent Galactic stars. Within two years, sufficient data should be in hand to characterize or meaningfully constrain the frequency of massive planets in this range of parameter space, nicely complementing information about planets at smaller orbital radii now being provided by radial velocity searches. In principle, the technique could be pushed to smaller planetary masses, but only if a larger number of faint microlensed sources can be monitored with higher precision and temporal sampling. The VST on Paranal, with spectroscopic follow-up with the VLT, may be the ideal instrument for such an ambitious program.Comment: Invited Review at VLT Opening Symposium, Antofagasta, Chile, March 1999. To appear in the Springer-Verlag series ``ESO Astrophysics Symposia'

Another Flattened Dark Halo: Plar Ring Galaxy A0136-0801

Knowledge of the shape of dark matter halos is critical to our understanding of galaxy formation, dynamics, and of the nature of dark matter itself. Polar ring galaxies (PRGs) --- early-type galaxies defined by their outer rings of gas, dust and stars on orbits nearly perpendicular to those of the central host --- provide a rare probe of the vertical-to-radial axis ratio $(q_{\rho} = c/a)$ of dark halos. We present a Fabry-Perot velocity field for the H$\alpha$ gas in the kinematically-confirmed PRG \gal. By comparing ring orbits evolved in a generalized mass model to the observed ring velocity field and morphology of \gal, we conclude that $q_\rho \sim 0.5$ and rule out a spherical geometry.Comment: uuencoded gz-compressed file with figures include

Obtaining Atomic Matrix Elements from Vector Tune-Out Wavelengths using Atom Interferometry

Accurate values for atomic dipole matrix elements are useful in many areas of physics, and in particular for interpreting experiments such as atomic parity violation. Obtaining accurate matrix element values is a challenge for both experiment and theory. A new technique that can be applied to this problem is tune-out spectroscopy, which is the measurement of light wavelengths where the electric polarizability of an atom has a zero. Using atom interferometry methods, tune-out wavelengths can be measured very accurately. Their values depend on the ratios of various dipole matrix elements and are thus useful for constraining theory and broadening the application of experimental values. Tune-out wavelength measurements to date have focused on zeros of the scalar polarizability, but in general the vector polarizability also contributes. We show here that combined measurements of the vector and scalar polarizabilities can provide more detailed information about the matrix element ratios, and in particular can distinguish small contributions from the atomic core and the valence tail states. These small contributions are the leading error sources in current parity violation calculations for cesium.Comment: 11 pages, 3 figure