462 research outputs found
Cosmological Origin of the Stellar Velocity Dispersions in Massive Early-Type Galaxies
We show that the observed upper bound on the line-of-sight velocity
dispersion of the stars in an early-type galaxy, sigma<400km/s, may have a
simple dynamical origin within the LCDM cosmological model, under two main
hypotheses. The first is that most of the stars now in the luminous parts of a
giant elliptical formed at redshift z>6. Subsequently, the stars behaved
dynamically just as an additional component of the dark matter. The second
hypothesis is that the mass distribution characteristic of a newly formed dark
matter halo forgets such details of the initial conditions as the stellar
"collisionless matter" that was added to the dense parts of earlier generations
of halos. We also assume that the stellar velocity dispersion does not evolve
much at z<6, because a massive host halo grows mainly by the addition of
material at large radii well away from the stellar core of the galaxy. These
assumptions lead to a predicted number density of ellipticals as a function of
stellar velocity dispersion that is in promising agreement with the Sloan
Digital Sky Survey data.Comment: ApJ, in press (2003); matches published versio
Coherent Umklapp Scattering of Light from Disordered Photonic Crystals
A theoretical study of the coherent light scattering from disordered photonic
crystal is presented. In addition to the conventional enhancement of the
reflected light intensity into the backscattering direction, the so called
coherent backscattering (CBS), the periodic modulation of the dielectric
function in photonic crystals gives rise to a qualitatively new effect:
enhancement of the reflected light intensity in directions different from the
backscattering direction. These additional coherent scattering processes,
dubbed here {\em umklapp scattering} (CUS), result in peaks, which are most
pronounced when the incident light beam enters the sample at an angle close to
the the Bragg angle. Assuming that the dielectric function modulation is weak,
we study the shape of the CUS peaks for different relative lengths of the
modulation-induced Bragg attenuation compared to disorder-induced mean free
path. We show that when the Bragg length increases, then the CBS peak assumes
its conventional shape, whereas the CUS peak rapidly diminishes in amplitude.
We also study the suppression of the CUS peak upon the departure of the
incident beam from Bragg resonance: we found that the diminishing of the CUS
intensity is accompanied by substantial broadening. In addition, the peak
becomes asymmetric.Comment: LaTeX, 8 two-column pages, 6 figures include
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
Bar Diagnostics in Edge-On Spiral Galaxies. II. Hydrodynamical Simulations
We develop diagnostics based on gas kinematics to identify the presence of a
bar in an edge-on spiral galaxy and determine its orientation. We use
position-velocity diagrams (PVDs) obtained by projecting edge-on
two-dimensional hydrodynamical simulations of the gas flow in a barred galaxy
potential. We show that when a nuclear spiral is formed, the presence of a gap
in the PVDs, between the signature of the nuclear spiral and that of the outer
parts of the disk, reliably indicates the presence of a bar. This gap is due to
the presence of shocks and inflows in the simulations, leading to a depletion
of the gas in the outer bar region. If no nuclear spiral signature is present
in a PVD, only indirect arguments can be used to argue for the presence of a
bar. The shape of the signature of the nuclear spiral, and to a lesser extent
that of the outer bar region, allows to determine the orientation of the bar
with respect to the line-of-sight. The presence of dust can also help to
discriminate between viewing angles on either side of the bar. Simulations
covering a large fraction of parameter space constrain the bar properties and
mass distribution of observed galaxies. The strongest constraint comes from the
presence or absence of the signature of a nuclear spiral in the PVD.Comment: 25 pages (AASTeX, aaspp4.sty), 11 jpg figures. Accepted for
publication in The Astrophysical Journal. Online manuscript with PostScript
figures available at: http://www.strw.leidenuniv.nl/~bureau/pub_list.htm
Density Waves Inside Inner Lindblad Resonance: Nuclear Spirals in Disk Galaxies
We analyze formation of grand-design two-arm spiral structure in the nuclear
regions of disk galaxies. Such morphology has been recently detected in a
number of objects using high-resolution near-infrared observations. Motivated
by the observed (1) continuity between the nuclear and kpc-scale spiral
structures, and by (2) low arm-interarm contrast, we apply the density wave
theory to explain the basic properties of the spiral nuclear morphology. In
particular, we address the mechanism for the formation, maintenance and the
detailed shape of nuclear spirals. We find, that the latter depends mostly on
the shape of the underlying gravitational potential and the sound speed in the
gas. Detection of nuclear spiral arms provides diagnostics of mass distribution
within the central kpc of disk galaxies. Our results are supported by 2D
numerical simulations of gas response to the background gravitational potential
of a barred stellar disk. We investigate the parameter space allowed for the
formation of nuclear spirals using a new method for constructing a
gravitational potential in a barred galaxy, where positions of resonances are
prescribed.Comment: 18 pages, 9 figures, higher resolution available at
http://www.pa.uky.edu/~ppe/papers/nucsp.ps.g
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
Distortion Isomerism of Cu(II) Chloride Adducts with Bis(2-benzimidazolyl)ethane. Synthesis, Characterization, X-ray Structures and Spectroscopy of Four Different Isomers
Metals in Catalysis, Biomimetics & Inorganic Material
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
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
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