6,591 research outputs found
Self-similar cosmologies in 5D: spatially flat anisotropic models
In the context of theories of Kaluza-Klein type, with a large extra
dimension, we study self-similar cosmological models in 5D that are
homogeneous, anisotropic and spatially flat. The "ladder" to go between the
physics in 5D and 4D is provided by Campbell-Maagard's embedding theorems. We
show that the 5-dimensional field equations determine the form of
the similarity variable. There are three different possibilities: homothetic,
conformal and "wave-like" solutions in 5D. We derive the most general
homothetic and conformal solutions to the 5D field equations. They require the
extra dimension to be spacelike, and are given in terms of one arbitrary
function of the similarity variable and three parameters. The Riemann tensor in
5D is not zero, except in the isotropic limit, which corresponds to the case
where the parameters are equal to each other. The solutions can be used as 5D
embeddings for a great variety of 4D homogeneous cosmological models, with and
without matter, including the Kasner universe. Since the extra dimension is
spacelike, the 5D solutions are invariant under the exchange of spatial
coordinates. Therefore they also embed a family of spatially {\it
inhomogeneous} models in 4D. We show that these models can be interpreted as
vacuum solutions in braneworld theory. Our work (I) generalizes the 5D
embeddings used for the FLRW models; (II) shows that anisotropic cosmologies
are, in general, curved in 5D, in contrast with FLRW models which can always be
embedded in a 5D Riemann-flat (Minkowski) manifold; (III) reveals that
anisotropic cosmologies can be curved and devoid of matter, both in 5D and 4D,
even when the metric in 5D explicitly depends on the extra coordinate, which is
quite different from the isotropic case.Comment: Typos corrected. Minor editorial changes and additions in the
Introduction and Summary section
The Photonic Lantern
Photonic lanterns are made by adiabatically merging several single-mode cores
into one multimode core. They provide low-loss interfaces between single-mode
and multimode systems where the precise optical mapping between cores and
individual modes is unimportant.Comment: 45 pages; article unchanged, accepted for publication in Advances in
Optics and Photonic
The Fermi Problem in Discrete Systems
The Fermi two-atom problem illustrates an apparent causality violation in
Quantum Field Theory which has to do with the nature of the built in
correlations in the vacuum. It has been a constant subject of theoretical
debate and discussions during the last few decades. Nevertheless, although the
issues at hand could in principle be tested experimentally, the smallness of
such apparent violations of causality in Quantum Electrodynamics prevented the
observation of the predicted effect. In the present paper we show that the
problem can be simulated within the framework of discrete systems that can be
manifested, for instance, by trapped atoms in optical lattices or trapped ions.
Unlike the original continuum case, the causal structure is no longer sharp.
Nevertheless, as we show, it is possible to distinguish between "trivial"
effects due to "direct" causality violations, and the effects associated with
Fermi's problem, even in such discrete settings. The ability to control
externally the strength of the atom-field interactions, enables us also to
study both the original Fermi problem with "bare atoms", as well as correction
in the scenario that involves "dressed" atoms. Finally, we show that in
principle, the Fermi effect can be detected using trapped ions.Comment: Second version - minor change
Aspects of Horava-Lifshitz cosmology
We review some general aspects of Horava-Lifshitz cosmology. Formulating it
in its basic version, we extract the cosmological equations and we use
observational data in order to constrain the parameters of the theory. Through
a phase-space analysis we extract the late-time stable solutions, and we show
that eternal expansion, and bouncing and cyclic behavior can arise naturally.
Concerning the effective dark energy sector we show that it can describe the
phantom phase without the use of a phantom field. However, performing a
detailed perturbation analysis, we see that Horava-Lifshitz gravity in its
basic version suffers from instabilities. Therefore, suitable generalizations
are required in order for this novel theory to be a candidate for the
description of nature.Comment: 10 pages, 4 figures, invited talk given at the 2nd International
Workshop on Dark Matter, Dark Energy and Matter-Antimatter Assymetry,
National Tsing Hua University, Hsinchu, Taiwan, November 5-6, 201
Long-term variability of the optical spectra of NGC 4151: II. Evolution of the broad Ha and Hb emission-line profiles
Results of the long-term (11 years, from 1996 to 2006) H and H
line variations of the active galactic nucleus of NGC 4151 are presented. High
quality spectra (S/N>50 and R~8A) of H and H were investigated.
We analyzed line profile variations during monitoring period. Comparing the
line profiles of H and H, we studied different details (bumps,
absorption features) in the line profiles. The variations of the different
H and H line profile segments have been investigated. Also, we
analyzed the Balmer decrement for whole line and for line segments. We found
that the line profiles were strongly changing during the monitoring period,
showing blue and red asymmetries. This indicates a complex BLR geometry of NGC
4151 with, at least, three kinematically distinct regions: one that contributes
to the blue line wing, one to the line core and one to the red line wing. Such
variation can be caused by an accelerating outflow starting very close to the
black hole, where the red part may come from the region {closer to the black
hole than the blue part, which is coming} from the region having the highest
outflow velocities. Taking into account the fact that the BLR of NGC 4151 has a
complex geometry (probably affected by an outflow) and that a portion of the
broad line emission seems to have not a pure photoionization origin, one can
ask the question whether the study of the BLR by reverberation mapping may be
valid in the case of this galaxy.Comment: 24 pages, 18 figures, accepted for publications in A&
Formation and evolution of clumpy tidal tails around globular clusters
We present some results of numerical simulations of a globular cluster
orbiting in the central region of a triaxial galaxy on a set of 'loop' orbits.
Tails start forming after about a quarter of the globular cluster orbital
period and develop, in most cases, along the cluster orbit, showing clumpy
substructures as observed, for example, in Palomar 5. If completely detectable,
clumps can contain about 7,000 solar masses each, i.e. about 10% of the cluster
mass at that epoch. The morphology of tails and clumps and the kinematical
properties of stars in the tails are studied and compared with available
observational data. Our finding is that the stellar velocity dispersion tends
to level off at large radii, in agreement to that found for M15 and Omega
Centauri.Comment: LaTeX 2e, uses AASTeX v5.x, 40 pages with 18 figures. Submitted to
The Astronomical Journa
The Sloan-Lens ACS Survey II: stellar populations and internal structure of early-type lens galaxies
We derive Fundamental Plane parameters of 15 early-type lens galaxies
identified by the Sloan Lens ACS (SLACS) Survey. The size of the sample allows
us to investigate for the first time the distribution of lens galaxies in the
FP space. After correcting for evolution, we find that lens galaxies occupy a
subset of the local FP. The edge-on projection (approximately M vs M/L) is
indistinguishable from that of normal early-type galaxies. However -- within
the fundamental plane -- the lens galaxies appear to concentrate at the edge of
the region populated by normal early-type galaxies. We show that this is a
result of our selection procedure (approximately velocity dispersion
sigma>240km/s). We conclude that SLACS lenses are a fair sample of high
velocity dispersion early-type galaxies. By comparing the central stellar
velocity dispersion that of the best fit lens model, we find
== =1.01+-0.02 with 0.065 rms scatter. We conclude that
within the Einstein radii the SLACS lenses are very well approximated by
isothermal ellipsoids, requiring a fine tuning of the stellar and dark matter
distribution (bulge-halo ``conspiracy''). Interpreting the offset from the
local FP in terms of evolution of the stellar mass-to-light ratio, we find for
the SLACS lenses d log M/L_B/dz=-0.69+-0.08 (rms 0.11) consistent with the rate
found for field early-type galaxies and with a scenario where most of the stars
were formed at high redshift (>2) with secondary episodes of star formation
providing less than ~10% of the stellar mass below z=1. We discuss star
formation history and structural homogeneity in the context of formation
mechanisms such as collisionless (``dry'') mergers. [Abridged]Comment: 2006, ApJ, 604, 622; 13 pages, 7 figures, 2 tables. Replaced Table 2,
since the previous version was incorrectly sorted. Updated references. No
changes in plots or content. More info available at SLACS website
www.slacs.or
Evidence for tidal interaction and merger as the origin of galaxy morphology evolution in compact groups
We present the results of a morphological study based on NIR images of 25
galaxies, with different levels of nuclear activity, in 8 Compact Groups of
Galaxies (CGs). We perform independently two different analysis: a isophotal
study and a study of morphological asymmetries. The results yielded by the two
analysis are highly consistent. For the first time, it is possible to show that
deviations from pure ellipses are produced by inhomogeneous stellar mass
distributions related to galaxy interactions and mergers. We find evidence of
mass asymmetries in 74% of the galaxies in our sample. In 59% of these cases,
the asymmetries come in pairs, and are consistent with tidal effects produced
by the proximity of companion galaxies. The symmetric galaxies are generally
small in size or mass, inactive, and have an early-type morphology. In 20% of
the galaxies we find evidence for cannibalism. In 36% of the early-type
galaxies the color gradient is positive (blue nucleus) or flat. Summing up
these results, as much as 52% of the galaxies in our sample could show evidence
of an on going or past mergers. Our observations suggest that galaxies in CGs
merge more frequently under ``dry'' conditions. The high frequency of
interacting and merging galaxies observed in our study is consistent with the
bias of our sample towards CGs of type B, which represents the most active
phase in the evolution of the groups. In these groups we also find a strong
correlation between asymmetries and nuclear activity in early-type galaxies.
This correlation allows us to identify tidal interactions and mergers as the
cause of galaxy morphology transformation in CGs.[abridge]Comment: 64 pages, 35 figures. Accepted for publication in Ap
Gradient expansion approach to multiple-band Fermi liquids
Promoted by the recent progress of Berry phase physics in spin
galvanomagnetic communities, we develop a systematic derivation of the reduced
Keldysh equation (RKE) which captures the low-energy dynamics of
quasi-particles constrained within doubly degenerate bands forming a single
Fermi surface. Specifically, we project out the fully occupied/empty band
degrees of freedom perturbatively in the gradient expansion, whose coupling
constant measures how a system is disequilibrated. As for the electron-electron
interactions, however, we only employ the so-called adiabatic assumption of the
Fermi liquid theory, so that the effect of electron correlations onto the
adiabatic transport of quasi-particles, i.e. the hermitian (real) part of the
self-energy, is taken into account in an unbiased manner.Comment: 29 pages, 7 figure
A Capture Scenario for Globular Cluster Omega Centauri
We explore an accretion origin for Omega Cen by N-body modeling of the
orbital decay and disruption of a Milky-Way dwarf satellite. This work is
focused on studying a particular satellite model that aims to reproduce the
present orbit of Omega Cen, as recently determined from absolute proper
motions. The model satellite is launched from 58 kpc from the Galactic Center,
on a radial, low-inclination orbit. We find that a capture scenario can produce
an Omega Cen-like object with the current low-energy orbit of the cluster. Our
best model is a nucleated galaxy with a Hernquist density profile that has a
mass of 8 10**8 Msun, and a half-mass radius of 1.4 kpc.Comment: 15 pages, 5 figures, accepted by ApJ
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