6,217 research outputs found
Direct Mapping of Massive Compact Objects in Extragalactic Dark Halos
A significant fraction of non-baryonic or baryonic dark matter in galactic
halos may consist of MASsive Compact Objects (MASCOs) with mass
M=10^{1-4}M_{sun}. Possible candidates for such compact objects include
primordial black holes or remnants of primordial (Population III) stars. We
propose a method for directly detecting MASCOs in extragalactic halos, using
the VLBI techniques with extremely high resolution. If a galactic halo
comprising a large number of MASCOs produces multiple images of a background
radio-loud QSO by gravitational lensing, then a high-resolution radio map of
each macro-lensed image should reveal microlensing effects by MASCOs. To assess
their observational feasibility, we simulate microlensing of the radio-loud,
four-image lensed QSO, B1422+231, assuming angular resolution of ~0.01 mas.
MASCOs are represented by point masses. For comparison, we also simulate
microlensing of B1422+231 by singular isothermal spheres. We find that the
surface brightness of the macro-lensed images shows distinct spatial patterns
on the scale of the Einstein radius of the perturbers. In the case of
point-mass perturbers, many tiny dark spots also appear in the macro-lensed
images associated with a decrease in the surface brightness toward the fringe
of the original QSO image, whereas no such spots are available in the SIS
models. Based on the size, position and magnified or demagnified patterns of
images, we shall be able to determine the mass and density profile of a MASCO
as well as its spatial distribution and abundance in a galactic halo.Comment: 4 pages, 3 figure
Tree-ring structure of Galactic bar resonance
Galaxy models have long predicted that galactic bars slow down by losing angular momentum to their postulated dark haloes. When the bar slows down, resonance sweeps radially outwards through the galactic disc while growing in volume, thereby sequentially capturing new stars at its surface/separatrix. Since trapped stars conserve their action of libration, which measures the relative distance to the resonance centre, the order of capturing is preserved: the surface of the resonance is dominated by stars captured recently at large radius, while the core of the resonance is occupied by stars trapped early at small radius. The slow down of the bar thus results in a rising mean metallicity of trapped stars from the surface towards the centre of the resonance as the Galaxyâs metallicity declines towards large radii. This argument, when applied to Solar neighbourhood stars, allows a novel precision measurement of the barâs current pattern speed Ωp=35.5±0.8kmsâ1kpcâ1â , placing the corotation radius at RCR=6.6±0.2kpcâ . With this pattern speed, the corotation resonance precisely fits the Hercules stream in agreement with kinematics. Beyond corroborating the slow bar theory, this measurement manifests the deceleration of the bar of more than 24 per cent since its formation and thus the angular momentum transfer to the dark halo by dynamical friction. The measurement therefore supports the existence of a standard dark-matter halo rather than alternative models of gravity
Three-dimensional Mapping of CDM Substructure at Submillimeter Wavelengths
The cold dark matter (CDM) structure formation model predicts that about 5-10
percent of a typical galactic halo of mass \sim 10^{12} \ms is in
substructures with masses \lesssim 10^8 \ms. To directly detect such
substructures, we propose to observe dust continuum emission from a strongly
lensed QSO-host galaxy using a large submillimeter interferometer array with a
high angular resolution of arcsec such as the planned Atacama Large
Submillimeter Array (ALMA). To assess their observational feasibility, we
numerically simulate millilensing of an extended circular source by a CDM
substructure modeled as a tidally truncated singular isothermal sphere (SIS)
embedded in a typical QSO-galaxy lens system, B1422+231, modeled as a singular
isothermal ellipsoid (SIE) with an external constant shear and a constant
convergence. Assuming an angular resolution of 0.01arcsec, we find that the
angular positions of \sim 10^8 \ms substructures at several kpc from the
center of the macrolens halo can be directly measured if the size of the dust
continuum emission region and the gradient of the surface brightness at the
position of the perturber are sufficiently large. From the astrometric shift on
a scale of a few times mas of an image perturbed by a subhalo with respect
to an unperturbed macrolensed image, we can break the degeneracy between
subhalo mass and distance provided that macrolensing parameters are determined
from positions and fluxes of multiple images.Comment: 7 pages, 7 EPS files. An assessment of our assumption of constancy in
shear and convergence has been included. Version accepted for publication in
Ap
Quintessence, the Gravitational Constant, and Gravity
Dynamical vacuum energy or quintessence, a slowly varying and spatially
inhomogeneous component of the energy density with negative pressure, is
currently consistent with the observational data. One potential difficulty with
the idea of quintessence is that couplings to ordinary matter should be
strongly suppressed so as not to lead to observable time variations of the
constants of nature. We further explore the possibility of an explicit coupling
between the quintessence field and the curvature. Since such a scalar field
gives rise to another gravity force of long range (\simg H^{-1}_0), the solar
system experiments put a constraint on the non-minimal coupling: |\xi| \siml
10^{-2}.Comment: 9 pages, a version to be published in Phys.Rev.
Predicting the optical observables for nucleon scattering on even-even actinides
Previously derived Lane consistent dispersive coupled-channel optical model
for nucleon scattering on Th and U nuclei is extended to
describe scattering on even-even actinides with 90--98. A
soft-rotator-model (SRM) description of the low-lying nuclear structure is
used, where SRM Hamiltonian parameters are adjusted to the observed collective
levels of the target nucleus. SRM nuclear wave functions (mixed in quantum
number) have been used to calculate coupling matrix elements of the generalized
optical model. The "effective" deformations that define inter-band couplings
are derived from SRM Hamiltonian parameters. Conservation of nuclear volume is
enforced by introducing a dynamic monopolar term to the deformed potential
leading to additional couplings between rotational bands. Fitted static
deformation parameters are in very good agreement with those derived by Wang
and collaborators using the Weizs\"acker-Skyrme global mass model (WS4),
allowing to use the latter to predict cross section for nuclei without
experimental data. A good description of scarce "optical" experimental database
is achieved. SRM couplings and volume conservation allow a precise calculation
of the compound-nucleus formation cross sections, which is significantly
different from the one calculated with rigid-rotor potentials coupling the
ground-state rotational band. Derived parameters can be used to describe both
neutron and proton induced reactions.Comment: 6 pages, 4 figures, 5 table
Nucleon scattering on actinides using a dispersive optical model with extended couplings
Tamura coupling model has been extended to consider the coupling of
additional low-lying rotational bands to the ground state band. Rotational
bands are built on vibrational bandheads (even-even targets) or single particle
bandheads (odd- targets) including both axial and non-axial deformations.
These additional excitations are introduced as a perturbation to the underlying
axially-symmetric rigid rotor structure of the ground state rotational band.
Coupling matrix elements of the generalized optical model are derived for
extended multi-band transitions in even-even and odd- nuclei. Isospin
symmetric formulation of the optical model is employed.
A coupled-channels optical model potential (OMP) containing a dispersive
contribution is used to fit simultaneously all available optical experimental
databases including neutron strength functions for nucleon scattering on
Th, U and Pu nuclei and quasi-elastic (,)
scattering data on Th and U. Lane consistent OMP is derived for
all actinides if corresponding multi-band coupling schemes are defined.
Calculations using the derived OMP potential reproduce measured total
cross-section differences between several actinide pairs within experimental
uncertainty for incident neutron energies from 50 keV up to 150MeV. Multi-band
coupling is stronger in even-even targets due to the collective nature of the
coupling; the impact of extended coupling on predicted compound-nucleus
formation cross section reaches 5% below 3 MeV of incident neutron energy.
Coupling of ground-state rotational band levels in odd- nuclei is sufficient
for a good description of the compound-nucleus formation cross sections as long
as the coupling is saturated (a minimum of 7 coupled levels are typically
needed).Comment: 30 pages, 4 figures, 8 tables, 3 appendice
Brane-World Black Hole Solutions via a Confining Potential
Using a confining potential, we consider spherically symmetric vacuum (static
black hole) solutions in a brane-world scenario. Working with a constant
curvature bulk, two interesting cases/solutions are studied. A Schwarzschild-de
Sitter black hole solution similar to the standard solution in the presence of
a cosmological constant is obtained which confirms the idea that an extra term
in the field equations on the brane can play the role of a positive
cosmological constant and may be used to account for the accelerated expansion
of the universe. The other solution is one in which we can have a proper
potential to explain the galaxy rotation curves without assuming the existence
of dark matter and without working with new modified theories (modified
Newtonian dynamics).Comment: 12 pages, to appear in PR
Resonance sweeping by a decelerating Galactic bar
We provide the first quantitative evidence for the deceleration of the Galactic bar from local stellar kinematics in agreement with dynamical friction by a typical dark matter halo. The kinematic response of the stellar disc to a decelerating bar is studied using secular perturbation theory and test particle simulations. We show that the velocity distribution at any point in the disc affected by a naturally slowing bar is qualitatively different from that perturbed by a steadily rotating bar with the same current pattern speed Ωp and amplitude. When the bar slows down, its resonances sweep through phase space, trapping, and dragging along a portion of previously free orbits. This enhances occupation on resonances, but also changes the distribution of stars within the resonance. Due to the accumulation of orbits near the boundary of the resonance, the decelerating bar model reproduces with its corotation resonance the offset and strength of the Hercules stream in the local vR-vÏ plane and the double-peaked structure of mean vR in the LzâÏ plane. At resonances other than the corotation, resonant dragging by a slowing bar is associated with a continuing increase in radial action, leading to multiple resonance ridges in the action plane as identified in the Gaia data. This work shows models using a constant bar pattern speed likely lead to qualitatively wrong conclusions. Most importantly we provide a quantitative estimate of the current slowing rate of the bar ΩËp=(â4.5±1.4)kmsâ1kpcâ1Gyrâ1 with additional systematic uncertainty arising from unmodelled impacts of e.g. spiral arms
Massive stars and globular cluster formation
We first present chemodynamical simulations to investigate how stellar winds
of massive stars influence early dynamical and chemical evolution of forming
globular clusters (GCs). In our numerical models, GCs form in
turbulent,high-density giant molecular clouds (GMCs), which are embedded in a
massive dark matter halo at high redshifts. We show how high-density, compact
stellar systems are formed from GMCs influenced both by physical processes
associated with star formation and by tidal fields of their host halos. We also
show that chemical pollution of GC-forming GMCs by stellar winds from massive
stars can result in star-to-star abundance inhomogeneities among light elements
(e.g., C, N, and O) of stars in GCs. The present model with a canonical initial
mass function (IMF) also shows a C-N anticorrelation that stars with smaller
[C/Fe] have larger [N/Fe] in a GC. Although these results imply that
``self-pollution'' of GC-forming GMCs by stellar winds from massive stars can
cause abundance inhomogeneities of GCs, the present models with different
parameters and canonical IMFs can not show N-rich stars with [N/Fe] ~ 0.8
observed in some GCs (e.g., NGC 6752). We discuss this apparent failure in the
context of massive star formation preceding low-mass one within GC-forming GMCs
(``bimodal star formation scenario''). We also show that although almost all
stars (~97%) show normal He abundances (Y) of ~0.24 some stars later formed in
GMCs can have Y as high as ~0.3 in some models. The number fraction of He-rich
stars with Y >0.26 is however found to be small (~10^-3) for most models.Comment: 10 pages, 8 figures, accepted by Ap
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