10,449 research outputs found
Reconstruction of cosmological initial conditions from galaxy redshift catalogues
We present and test a new method for the reconstruction of cosmological
initial conditions from a full-sky galaxy catalogue. This method, called
ZTRACE, is based on a self-consistent solution of the growing mode of
gravitational instabilities according to the Zel'dovich approximation and
higher order in Lagrangian perturbation theory. Given the evolved
redshift-space density field, smoothed on some scale, ZTRACE finds via an
iterative procedure, an approximation to the initial density field for any
given set of cosmological parameters; real-space densities and peculiar
velocities are also reconstructed. The method is tested by applying it to
N-body simulations of an Einstein-de Sitter and an open cold dark matter
universe. It is shown that errors in the estimate of the density contrast
dominate the noise of the reconstruction. As a consequence, the reconstruction
of real space density and peculiar velocity fields using non-linear algorithms
is little improved over those based on linear theory. The use of a
mass-preserving adaptive smoothing, equivalent to a smoothing in Lagrangian
space, allows an unbiased (although noisy) reconstruction of initial
conditions, as long as the (linearly extrapolated) density contrast does not
exceed unity. The probability distribution function of the initial conditions
is recovered to high precision, even for Gaussian smoothing scales of ~ 5
Mpc/h, except for the tail at delta >~ 1. This result is insensitive to the
assumptions of the background cosmology.Comment: 19 pages, MN style, 12 figures included, revised version. MNRAS, in
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Joint formation of bright quasars and elliptical galaxies in the young Universe
We show that the mass function of black holes expected from the past quasar
activity (both visible and obscured) is consistent with the number of dormant
black holes found in the bulges of nearby galaxies. The joint formation of
quasars and bulges is addressed by means of an analytical model for galaxy
formation, based on the hierarchical clustering of cold dark matter halos. The
model is able to reproduce the main statistical properties of both populations
under the hypotheses that (i) star formation and quasar shining follow an
anti-hierarchical order, and (ii) galaxy morphology and final black hole mass
are determined by the same physical process.Comment: 5 pages, 3 postscript figures included, proceedings of the IGRAP
meeting "Clustering at high redshift", Marseille, June 199
Josephson Vortex Qubit based on a Confocal Annular Josephson Junction
We report theoretical and experimental work on the development of a Josephson
vortex qubit based on a confocal annular Josephson tunnel junction (CAJTJ). The
key ingredient of this geometrical configuration is a periodically variable
width that generates a spatial vortex potential with bistable states. This
intrinsic vortex potential can be tuned by an externally applied magnetic field
and tilted by a bias current. The two-state system is accurately modeled by a
one-dimensional sine-Gordon like equation by means of which one can numerically
calculate both the magnetic field needed to set the vortex in a given state as
well as the vortex depinning currents. Experimental data taken at 4.2K on
high-quality Nb/Al-AlOx/Nb CAJTJs with an individual trapped fluxon advocate
the presence of a robust and finely tunable double-well potential for which
reliable manipulation of the vortex state has been classically demonstrated.
The vortex is prepared in a given potential by means of an externally applied
magnetic field, while the state readout is accomplished by measuring the
vortex-depinning current in a small magnetic field. Our proof of principle
experiment convincingly demonstrates that the proposed vortex qubit based on
CAJTJs is robust and workable.Comment: 20 pages, 11 figure
Flux Flow Effects in Annular Josephson Tunnel Junctions
We investigate Josephson flux-flow in annular Josephson tunnel junctions
(AJTJs) under the application of magnetic fields generating finite-voltage
steps in their current-voltage characteristics. Experimental data are presented
for confocal AJTJs which are the natural generalization of the well studied
circular AJTJs for which flux flow effects have never been reported. Displaced
linear slopes, Fiske step staircases and Eck steps were sequentially recorded
at with high-quality Nb/Al-AlOx/Nb confocal AJTJs when increasing the
strength of a uniform magnetic field applied in the plane of the junction.
Their amplitude was found to strongly depend not only on the strength, but also
on the orientation, of the external field. Extensive numerical simulations
based on a phenomenological sine-Gordon model developed for confocal AJTJs were
carried out to disclose the basic flux-flow mechanism responsible for the
appearance of magnetically induced steps and to elucidate the role of several
critical parameters, namely, the field orientation, the system loss and the
annulus eccentricity. It was found that in a topologically closed system, such
as the AJTJ, where the number of trapped fluxons is conserved and new fluxons
can be created only in the form of fluxon-antifluxon pairs, the existence of a
steady viscous flow of Josephson vortices only relies on the capability of the
fluxons and antifluxons to be generated and to annihilate each other inside the
junction. This also implies that flux-flow effects are not observable in
circular AJTJs.Comment: 26 pages, 8 figure
Field Cooled Annular Josephson Tunnel Junctions
We investigate the physics of planar annular Josephson tunnel junctions
quenched through their transition temperature in the presence of an external
magnetic field. Experiments carried out with long Nb/Al-AlOx/Nb annular
junctions showed that the magnetic flux trapped in the high-quality
doubly-connected superconducting electrodes forming the junction generates a
persistent current whose associated magnetic field affects the both the static
and dynamics properties of the junctions. More specifically, the field trapped
in the hole of one electrode combined with a d.c. bias current induces a
viscous flow of dense trains of Josephson vortices which manifests itself
through the sequential appearance of displaced linear slopes, Fiske step
staircases and Eck steps in the junction's current-voltage characteristic.
Furthermore, a field shift is observed in the first lobe of the magnetic
diffraction pattern. The effects of the persistent current can be mitigated or
even canceled by an external magnetic field perpendicular to the junction
plane. The radial field associated with the persistent current can be
accurately modeled with the classical phenomenological sine-Gordon model for
extended one-dimensional Josephson junctions. Extensive numerical simulations
were carried out to disclose the basic flux-flow mechanism responsible for the
appearance of the magnetically induced steps and to elucidate the role of
geometrical parameters. It was found that the imprint of the field cooling is
enhanced in confocal annular junctions which are the natural generalization of
the well studied circular annular junctions.Comment: 26 pages, 10 figures. Supercond. Sci. Technol (2020
An Analytical Approach to Inhomogeneous Structure Formation
We develop an analytical formalism that is suitable for studying
inhomogeneous structure formation, by studying the joint statistics of dark
matter halos forming at two points. Extending the Bond et al. (1991) derivation
of the mass function of virialized halos, based on excursion sets, we derive an
approximate analytical expression for the ``bivariate'' mass function of halos
forming at two redshifts and separated by a fixed comoving Lagrangian distance.
Our approach also leads to a self-consistent expression for the nonlinear
biasing and correlation function of halos, generalizing a number of previous
results including those by Kaiser (1984) and Mo & White (1996). We compare our
approximate solutions to exact numerical results within the excursion-set
framework and find them to be consistent to within 2% over a wide range of
parameters. Our formalism can be used to study various feedback effects during
galaxy formation analytically, as well as to simply construct observable
quantities dependent on the spatial distribution of objects. A code that
implements our method is publicly available at
http://www.arcetri.astro.it/~evan/GeminiComment: 41 Pages, 11 figures, published in ApJ, 571, 585. Reference added,
Figure 2 axis relabele
Chemical composition of the stellar cluster Gaia1: No surprise behind Sirius
IndexaciĂłn: Web of Science; Scopus.We observed six He-clump stars of the intermediate-Age stellar cluster Gaia1 with the MIKE/Magellan spectrograph. A possible extra-galactic origin of this cluster, recently discovered thanks to the first data release of the ESA Gaia mission, has been suggested, based on its orbital parameters. Abundances for Fe, α, proton-And neutron-capture elements have been obtained. We find no evidence of intrinsic abundance spreads. The iron abundance is solar ([FeI/H] = + 0.00 ± 0.01; Ï = 0.03 dex). All the other abundance ratios are generally solar-scaled, similar to the Galactic thin disk and open cluster stars of similar metallicity. The chemical composition of Gaia1 does not support an extra-galactic origin for this stellar cluster, which can be considered as a standard Galactic open cluster.https://www.aanda.org/articles/aa/abs/2017/07/aa31009-17/aa31009-17.htm
The Local Galaxy Density and the Arm Class of Spiral Galaxies
We have examined the effect of the environmental density on the arm
classification of an extensive sample of spiral galaxies included in the Nearby
Galaxy Catalog (Tully, 1988a). We have also explored the dependence of the arm
class of a galaxy on other factors, such as its blue absolute magnitude and its
disk-to-total mass ratio, inferred in the literature either from the gradient
of a good galaxy rotation curve or from a photometric mass decomposition
method. We have found that the arm class is strongly related to the absolute
magnitude in the mid-type spirals (in the sense that grand design galaxies are,
on average, more luminous than flocculent objects), whilst this relation is
considerably weaker in the early and late types. In general the influence of
the local density on the arm structure appears to be much weaker than that of
the absolute magnitude. The local density acts essentially in strengthening the
arm class--absolute magnitude relation for the mid types, whereas no
environmental density effects are observed in the early and late types. Using
the most recent estimates of the disk-to-total mass ratio, we do not confirm
this ratio to be a significant factor which affects the arm class;
nevertheless, owing to poor statistics and large uncertanties, the issue
remains open. Neither a local density effect nor an unambiguous bar effect on
the disk-to-total mass ratio is detectable; the latter finding may challenge
some theoretical viewpoints on the formation of bar structures.Comment: 15 pages, Latex, SISSA 102/93/A openbib.sty and 4 POSTSCRIPT figures
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