11,932 research outputs found
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 of dark halos. We present a Fabry-Perot velocity field for the H
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 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
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