350 research outputs found
The Balance of Dark and Luminous Mass in Rotating Galaxies
A fine balance between dark and baryonic mass is observed in spiral galaxies.
As the contribution of the baryons to the total rotation velocity increases,
the contribution of the dark matter decreases by a compensating amount. This
poses a fine-tuning problem for \LCDM galaxy formation models, and may point to
new physics for dark matter particles or even a modification of gravity.Comment: 4 pages RevTeX. Phys. Rev. Letters, in pres
The scale-free character of the cluster mass function and the universality of the stellar IMF
Our recent determination of a Salpeter slope for the IMF in the field of 30
Doradus (Selman and Melnick 2005) appears to be in conflict with simple
probabilistic counting arguments advanced in the past to support observational
claims of a steeper IMF in the LMC field. In this paper we re-examine these
arguments and show by explicit construction that, contrary to these claims, the
field IMF is expected to be exactly the same as the stellar IMF of the clusters
out of which the field was presumably formed. We show that the current data on
the mass distribution of clusters themselves is in excellent agreement with our
model, and is consistent with a single spectrum {\it by number of stars} of the
type with beta between -1.8 and -2.2 down to the smallest clusters
without any preferred mass scale for cluster formation. We also use the random
sampling model to estimate the statistics of the maximal mass star in clusters,
and confirm the discrepancy with observations found by Weidner and Kroupa
(2006). We argue that rather than signaling the violation of the random
sampling model these observations reflect the gravitationally unstable nature
of systems with one very large mass star. We stress the importance of the
random sampling model as a \emph{null hypothesis} whose violation would signal
the presence of interesting physics.Comment: 9 pages emulateap
Self-consistent models of triaxial galaxies in MOND gravity
The Bekenstein-Milgrom gravity theory with a modified Poisson equation is
tested here for the existence of triaxial equilibrium solutions. Using the
non-negative least square method, we show that self-consistent triaxial
galaxies exist for baryonic models with a mild density cusp . Self-consistency is achieved for a wide range of central
concentrations, , representing
low-to-high surface brightness galaxies. Our results demonstrate for the first
time that the orbit superposition technique is fruitful for constructing galaxy
models beyond Newtonian gravity, and triaxial cuspy galaxies might exist
without the help of Cold dark Matter.Comment: 19 pages, 1 table, 7 figures, Accepted for publication in Ap
Comment on Viscous Stability of Relativistic Keplerian Accretion Disks
Recently Ghosh (1998) reported a new regime of instability in Keplerian
accretion disks which is caused by relativistic effects. This instability
appears in the gas pressure dominated region when all relativistic corrections
to the disk structure equations are taken into account. We show that he uses
the stability criterion in completely wrong way leading to inappropriate
conclusions. We perform a standard stability analysis to show that no unstable
region can be found when the relativistic disk is gas pressure dominated.Comment: 9 pages, 4 figures, uses aasms4.sty, submitted for ApJ Letter
The Effect of Resonances on Diffusive Scattering
The presence of resonances modifies the passage of light or of electrons
through a disordered medium. We generalize random matrix theory to account for
this effect. Using supersymmetry, we calculate analytically the mean density of
states, and the effective Lagrangean of the generating functional for the
two-point function. We show that the diffusion constant scales with the
effective mean level spacing. The latter exhibits a resonance dip. These facts
allow us to interpret experimental results on light scattering for different
concentrations of resonant scatterers.Comment: 12 pages, 1 Figure, to be published in Physical Review
Coherent Backscattering of Light by Cold Atoms
Light propagating in an optically thick sample experiences multiple
scattering. It is now known that interferences alter this propagation, leading
to an enhanced backscattering, a manifestation of weak localization of light in
such diffuse samples. This phenomenon has been extensively studied with
classical scatterers. In this letter we report the first experimental evidence
for coherent backscattering of light in a laser-cooled gas of Rubidium atoms.Comment: 4 pages REVTEX, 1 page color image GIF, accepted for publication in
Phys. Rev. Let
The properties of the Galactic bar implied by gas kinematics in the inner Milky Way
Longitude-velocity (l-V) diagrams of H I and CO gas in the inner Milky Way
have long been known to be inconsistent with circular motion in an axisymmetric
potential. Several lines of evidence suggest that the Galaxy is barred, and gas
flow in a barred potential could be consistent with the observed ``forbidden''
velocities and other features in the data. We compare the H I observations to
l-V diagrams synthesized from 2-D fluid dynamical simulations of gas flows in a
family of barred potentials. The gas flow pattern is very sensitive to the
parameters of the assumed potential, which allows us to discriminate among
models. We present a model that reproduces the outer contour of the H I l-V
diagram reasonably well; this model has a strong bar with a semimajor axis of
3.6 kpc, an axis ratio of approximately 3:1, an inner Lindblad resonance (ILR),
and a pattern speed of 42 km/s/kpc, and matches the data best when viewed from
34\deg to the bar major axis. The behavior of the models, combined with the
constraint that the shocks in the Milky Way bar should resemble those in
external barred galaxies, leads us to conclude that wide ranges of parameter
space are incompatible with the observations. In particular we suggest that the
bar must be fairly strong, must have an ILR, and cannot be too end-on, with the
bar major axis at 35\deg +/- 5\deg to the line of sight. The H I data exhibit
larger forbidden velocities over a wider longitude range than are seen in
molecular gas; this important difference is the reason our favored model
differs so significantly from other recently proposed models.Comment: 23 pages, 14 figures, 1 table, uses emulateapj and psfig, 640 kb.
Submitted to Ap
How universal are the density profiles of dark halos?
We investigate the formation of virialized halos due to the gravitational
collapse of collisionless matter using high-resolution N-body simulations. A
variety of formation scenarios are studied, ranging from hierarchical
clustering to monolithic radial collapse. The goal of these experiments was to
study departures from the universal density profiles recently found to arise in
cosmological settings. However, we found that even for models which exhibit
quite a different formation history, the density and velocity dispersion
profiles of the virialized halos are strikingly similar.
Power law density profiles do not result even in models with initial power
law profiles and no initial substructure or non-radial motions. Such initial
conditions give rise to a radial orbit instability which leads to curved
velocity dispersion and density profiles. The shapes of the density profiles in
all our models are well parameterized by the profiles of halos formed in a
generic cosmological setting. Our results show that the universality of dark
halo density profiles does not depend crucially on hierarchical merging as has
been suggested recently in the literature. Rather it arises because apparently
different collapse histories produces a near universal angular momentum
distribution among the halo particles. We conclude that the density and
velocity dispersion profiles of virialized halos in an expanding universe are
robust outcomes of gravitational collapse, nearly independent of the initial
conditions and the formation history.Comment: 10 pages, 4 figures included; Uses AASTeX macros v. 4.0; submitted to
ApJ; Also available at http://saguaro.as.arizona.edu/~matthia
Nothing moves a surface: vacancy mediated surface diffusion
We report scanning tunneling microscopy observations, which imply that all
atoms in a close-packed copper surface move frequently, even at room
temperature. Using a low density of embedded indium `tracer' atoms, we
visualize the diffusive motion of surface atoms. Surprisingly, the indium atoms
seem to make concerted, long jumps. Responsible for this motion is an ultra-low
density of surface vacancies, diffusing rapidly within the surface. This
interpretation is supported by a detailed analysis of the displacement
distribution of the indium atoms, which reveals a shape characteristic for the
vacancy mediated diffusion mechanism that we propose.Comment: 4 pages; for associated movie, see
http://www-lion.leidenuniv.nl/sections/cm/groups/interface/projects/therm
Cu(II) Compounds with Pyrimidine-Based Chelating Ligands, Bridged by a Flexible Alkyl Spacer: Synthesis, Characterisation and X-Ray Structures of Methoxide-Bridged Dinuclear-Based Species
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