46,980 research outputs found
Lensing Properties of Cored Galaxy Models
A method is developed to evaluate the magnifications of the images of
galaxies with lensing potentials stratified on similar concentric ellipses. A
simple contour integral is provided which enables the sums of the
magnifications of even parity or odd parity or the central image to be easily
calculated. The sums for pairs of images vary considerably with source
position, while the signed sums can be remarkably uniform inside the tangential
caustic in the absence of naked cusps. For a family of models in which the
potential is a power-law of the elliptic radius, the number of visible images
is found as a function of flattening, external shear and core radius. The
magnification of the central image depends on the core radius and the slope of
the potential. For typical source and lens redshifts, the missing central image
leads to strong constraints; the mass distribution in the lensing galaxy must
be nearly cusped, and the cusp must be isothermal or stronger. This is in
accord with the cuspy cores seen in high resolution photometry of nearby,
massive, early-type galaxies, which typically have the surface density falling
like distance^{-1.3} outside a break radius of a few hundred parsecs. Cuspy
cores by themselves can provide an explanation of the missing central images.
Dark matter at large radii may alter the slope of the projected density;
provided the slope remains isothermal or steeper and the break radius remains
small, then the central image remains unobservable. The sensitivity of the
radio maps must be increased fifty-fold to find the central images in
abundance.Comment: 42 pages, 11 figures, ApJ in pres
No Need for Dark Matter in Galaxies?
Unhappily, there has been a maelstrom of problems for dark matter theories
over the last few years and many serious difficulties still have no resolution
in sight. This article reviews the evidence for dark matter in galaxies. The
haloes built up by hierarchical merging in dark matter cosmogonies are cusped
and dominated by dark matter at the center. Evidence from the microlensing
optical depth towards Baade's Window and from dynamical modelling of the
Galactic bar already suggests that the Galactic halo is not cusped. Similarly,
evidence from the stability of unbarred disk galaxies, as well as the survival
of fast bars in barred galaxies, suggests that the this result holds good more
generally. Judged on the data from galactic scales alone, the case for dark
matter is weak and non-standard theories of gravity provide a better
description. Of course, non-standard theories of gravity have their own
problems, but not on galactic scales.Comment: 8 pages, invited review for "IDM 2000: Third International Workshop
on the Identification of Dark Matter", ed. N. Spooner (World Scientific
Microlensing Maps for the Galactic Bulge
Microlensing maps -- that is, contours of equal numbers of events per
source stars -- are provided for the inner Galaxy under two alternative
hypotheses : (1) the bulge is an oblate axisymmetric spheroid or (2) the bulge
is a prolate bar. Oblate spheroids yield a total of events per year
per stars at Baade's Window ( events if the disk is maximal).
The event rate is slightly lower for prolate bars viewed at and
the maps have a characteristic asymmetry between positive and negative
longitudes. Prolate bars can yield mild amplifications of the event rate if
viewed almost down the long axis. The disk provides the dominant lensing
population on the bulge major axis for |\ell | \gta 6^\circ. Measurements of
the rate at major axis windows can test for disk dark matter or maximal disk
models.Comment: 12 pages, Late
Axisymmetric Self-Similar Equilibria of Self-Gravitating Isothermal Systems
All axisymmetric self-similar equilibria of self-gravitating, rotating,
isothermal systems are identified by solving the nonlinear Poisson equation
analytically. There are two families of equilibria: (1) Cylindrically symmetric
solutions in which the density varies with cylindrical radius as R^(-alpha),
with 0 <= alpha <= 2. (2) Axially symmetric solutions in which the density
varies as f(theta)/r^2, where `r' is the spherical radius and `theta' is the
co-latitude. The singular isothermal sphere is a special case of the latter
class with f(theta)=constant. The axially symmetric equilibrium configurations
form a two-parameter family of solutions and include equilibria which are
surprisingly asymmetric with respect to the equatorial plane. The asymmetric
equilibria are, however, not force-free at the singular points r=0, infinity,
and their relevance to real systems is unclear. For each hydrodynamic
equilibrium, we determine the phase-space distribution of the collisionless
analog.Comment: 13 pages, 7 figures, uses emulateapj.sty. Submitted to Ap
RIP: The Macho Era (1974-2004)
This article reviews the life and death of a scientific theory.Comment: 10 pages, 3 figures, invited review for "The Fifth International
Workshop on the Identification of Dark Matter", eds N. Spooner, V.
Kudryavtsev (World Scientific, Singapore
A Dipole on the Sky: Predictions for Hypervelocity Stars from the Large Magellanic Cloud
We predict the distribution of hypervelocity stars (HVSs) ejected from the
Large Magellanic Cloud (LMC), under the assumption that the dwarf galaxy hosts
a central massive black hole (MBH). For the majority of stars ejected from the
LMC the orbital velocity of the LMC has contributed a significant fraction of
their galactic rest frame velocity, leading to a dipole density distribution on
the sky. We quantify the dipole using spherical harmonic analysis and contrast
with the monopole expected for HVSs ejected from the Galactic Center. There is
a tendril in the density distribution that leads the LMC which is coincident
with the well-known and unexplained clustering of HVSs in the constellations of
Leo and Sextans. Our model is falsifiable, since it predicts that Gaia will
reveal a large density of HVSs in the southern hemisphere.Comment: 6 pages, ApJ (Letters), in pres
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