2,413 research outputs found
Clues to Quasar Broad Line Region Geometry and Kinematics
We present evidence that the high-velocity CIV lambda 1549 emission line gas
of radio-loud quasars may originate in a disk-like configuration, in close
proximity to the accretion disk often assumed to emit the low-ionization lines.
For a sample of 36 radio-loud z~2 quasars we find the 20--30% peak width to
show significant inverse correlations with the fractional radio core-flux
density, R, the radio axis inclination indicator. Highly inclined systems have
broader line wings, consistent with a high-velocity field perpendicular to the
radio axis. By contrast, the narrow line-core shows no such relation with R, so
the lowest velocity CIV-emitting gas has an inclination independent velocity
field. We propose that this low-velocity gas is located at higher
disk-altitudes than the high-velocity gas. A planar origin of the high-velocity
CIV-emission is consistent with the current results and with an accretion
disk-wind emitting the broad lines. A spherical distribution of randomly
orbiting broad-line clouds and a polar high-ionization outflow are ruled out.Comment: 5 Latex pages, 1 figure, accepted for publication in ApJ Letter
Multiband effective bond-orbital model for nitride semiconductors with wurtzite structure
A multiband empirical tight-binding model for group-III-nitride
semiconductors with a wurtzite structure has been developed and applied to both
bulk systems and embedded quantum dots. As a minimal basis set we assume one
s-orbital and three p-orbitals, localized in the unit cell of the hexagonal
Bravais lattice, from which one conduction band and three valence bands are
formed. Non-vanishing matrix elements up to second nearest neighbors are taken
into account. These matrix elements are determined so that the resulting
tight-binding band structure reproduces the known Gamma-point parameters, which
are also used in recent kp-treatments. Furthermore, the tight-binding band
structure can also be fitted to the band energies at other special symmetry
points of the Brillouin zone boundary, known from experiment or from
first-principle calculations. In this paper, we describe details of the
parametrization and present the resulting tight-binding band structures of bulk
GaN, AlN, and InN with a wurtzite structure. As a first application to
nanostructures, we present results for the single-particle electronic
properties of lens-shaped InN quantum dots embedded in a GaN matrix.Comment: 10 pages, 5 figures, two supplementary file
Environment of compact extragalactic radio sources
We have studied the interrelation of young AGN with their hosts. The objects
of study are the young and powerful GPS and CSS radio sources. Due to their
small size, GPS and CSS sources are excellent probes of this relation.
Furhthermore, their young age allows us to compare them to the larger, old
radio sources and establish a time-line evolution of this relation. Combining
imaging and spectroscopy at UV, optical and radio wavelengths we find evidence
of strong interaction between the host and the radio source. The presence and
expansion of the radio source clearly affects the properties and evolution of
the host. Furthermore, the radio source and host significantly affect each
other's evolution. We describe our results and how these interactions take
place.Comment: 6 pages. To appear in "Highlights of Spanisg astrophysics IV.
Proceedings of the VII scientific meeting of the Spanish Astronomical
Society". Editors: F. Figueras, J.M. Girart, M.Hernanz, C. Jordi. Springe
Star-forming QSO host galaxies
The recent finding of substantial masses of cold molecular gas as well as
young stellar populations in the host galaxies of quasars is at odds with
results of Hubble Space Telescope imaging studies, since the latter appear to
yield mature, quiescent early type hosts. It is demonstrated here that the
characterization as `quiescent' is incorrect. Radio and far-infrared properties
of both the HST sample and a larger comparison sample of uv-excess selected
radio-quiet QSOs are consistent with substantial recent star-formation
activity.Comment: Accepted for publication by Astronomy & Astrophysic
ALMA Resolves the Stellar Birth Explosions in Distant Radio-Loud Quasars
Far-infrared photometry with the Herschel Space Observatory has found many examples of ultra-luminous dust emission at around 40 K in the host galaxies of high-redshift, radio-loud 3C Active Galactic Nuclei (AGN). The dust heating could have its origin in the central black hole activity or extreme circumnuclear starbursts, or both. We have used the Atacama Large Millimeter/submillimeter Array (ALMA) in Cycle 3 to study the dust morphology on the kiloparsec scale in a sample of these AGN, and present the results for three well-known distant quasars: 3C298, 3C318, and 3C454. After correction for the non-thermal radiation at 1 mm, the observations imply a starburst origin for the cool thermal dust emission, and a symbiotic physical relationship with the AGN-driven radio source
Domain-wall melting in ultracold boson systems with holes and spin-flip defects
Quantum magnetism is a fundamental phenomenon of nature. As of late, it has
garnered a lot of interest because experiments with ultracold atomic gases in
optical lattices could be used as a simulator for phenomena of magnetic
systems. A paradigmatic example is the time evolution of a domain-wall state of
a spin-1/2 Heisenberg chain, the so-called domain-wall melting. The model can
be implemented by having two species of bosonic atoms with unity filling and
strong on-site repulsion U in an optical lattice. In this paper, we study the
domain-wall melting in such a setup on the basis of the time-dependent density
matrix renormalization group (tDMRG). We are particularly interested in the
effects of defects that originate from an imperfect preparation of the initial
state. Typical defects are holes (empty sites) and flipped spins. We show that
the dominating effects of holes on observables like the spatially resolved
magnetization can be taken account of by a linear combination of spatially
shifted observables from the clean case. For sufficiently large U, further
effects due to holes become negligible. In contrast, the effects of spin flips
are more severe as their dynamics occur on the same time scale as that of the
domain-wall melting itself. It is hence advisable to avoid preparation schemes
that are based on spin-flips.Comment: 15 pages, 12 figures. Supplemental Material: 2 animations (avi)
comparing the domain-wall melting with and without defects, corresponding to
figures 3, 4 and the discussion in section V.B; minor improvements; published
versio
Magnetism, coherent many-particle dynamics, and relaxation with ultracold bosons in optical superlattices
We study how well magnetic models can be implemented with ultracold bosonic
atoms of two different hyperfine states in an optical superlattice. The system
is captured by a two-species Bose-Hubbard model, but realizes in a certain
parameter regime actually the physics of a spin-1/2 Heisenberg magnet,
describing the second order hopping processes. Tuning of the superlattice
allows for controlling the effect of fast first order processes versus the
slower second order ones.
Using the density-matrix renormalization-group method, we provide the
evolution of typical experimentally available observables. The validity of the
description via the Heisenberg model, depending on the parameters of the
Hubbard model, is studied numerically and analytically. The analysis is also
motivated by recent experiments [S. Foelling et al., Nature 448, 1029 (2007);
S. Trotzky et al., Sience 319, 295 (2008)] where coherent two-particle dynamics
with ultracold bosonic atoms in isolated double wells were realized. We provide
theoretical background for the next step, the observation of coherent
many-particle dynamics after coupling the double wells. Contrary to the case of
isolated double wells, relaxation of local observables can be observed. The
tunability between the Bose-Hubbard model and the Heisenberg model in this
setup could be used to study experimentally the differences in equilibration
processes for nonintegrable and Bethe ansatz integrable models. We show that
the relaxation in the Heisenberg model is connected to a phase averaging
effect, which is in contrast to the typical scattering driven thermalization in
nonintegrable models. We discuss the preparation of magnetic groundstates by
adiabatic tuning of the superlattice parameters.Comment: 20 pages, 24 figures; minor changes, published versio
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