1,003 research outputs found
The emergence of galaxies in the epoch of reionization
The field of high-redshift galaxy observation has seen incredible progress in the last decade and the recent launch of JWST is already providing us with data with unprecedented precision. In order to interpret this flood of data and understand the nature of the first galaxies in our Universe, we need to simulate a large number of galaxies to ensure statistically significant results with a mass resolution high enough to study the properties of low-mass sources and their assembly into massive systems. I use the ASTRAEUS framework, which couples a start-of-the-art N-body simulation with a galaxy formation semi-analytical model and a semi-numerical reionization scheme. ASTRAEUS includes all the key processes of galaxy evolution such as mergers, accretion, supernova, and reionization feedback. has three key strengths to tackle this challenge, (i) the simulation box is large enough to investigate the role of the environment; (ii) it tracks galaxies in a wide mass range; and (iii) it accounts for the radiative feedback from reionization and allows us to explore different scenario. Using this simulation, I address the following questions: What is the form of the star formation history of galaxies? What is the respective role of accretion and mergers, both minor and major, in the mass assembly? How is galaxy growth affected by their environment? I also outline a method to further improve the mass resolution of simulations at a low cost, by combining a machine learning algorithm with analytically generated merger trees
Diffractive orbits in the length spectrum of a 2D microwave cavity with a small scatterer
In a 2D rectangular microwave cavity dressed with one point-like scatterer, a
semiclassical approach is used to analyze the spectrum in terms of periodic
orbits and diffractive orbits. We show, both numerically and experimentally,
how the latter can be accounted for in the so-called length spectrum which is
retrieved from 2-point correlations of a finite range frequency spectrum.
Beyond its fundamental interest, this first experimental evidence of the role
played by diffractive orbits in the spectrum of an actual cavity, can be the
first step towards a novel technique to detect and track small defects in wave
cavities.Comment: 14 pages, format IO
Negative reflection of elastic guided waves in chaotic and random scattering media
The propagation of waves in complex media can be harnessed either by taming
the incident wave-field impinging on the medium or by forcing waves along
desired paths through its careful design. These two alternative strategies have
given rise to fascinating concepts such as time reversal or negative
refraction. Here, we show how these two processes are intimately linked through
the negative reflection phenomenon. A negative reflecting mirror converts a
wave of positive phase velocity into its negative counterpart and vice versa.
In this article, we experimentally demonstrate this phenomenon with elastic
waves in a 2D billiard and in a disordered plate by means of laser
interferometry. Despite the complexity of such configurations, the negatively
reflected wave field focuses back towards the initial source location, thereby
mimicking a phase conjugation operation while being a fully passive process.
The super-focusing capability of negative reflection is also highlighted in a
monochromatic regime. The negative reflection phenomenon is not restricted to
guided elastic waves since it can occur in zero-gap systems such as photonic
crystals, chiral metamaterials or graphene. Negative reflection can thus become
a tool of choice for the control of waves in all fields of wave physics.Comment: 9 pages, 6 figure
Localized Modes in a Finite-Size Open Disordered Microwave Cavity
We present measurements of the spatial intensity distribution of localized
modes in a two-dimensional open microwave cavity randomly filled with
cylindrical dielectric scatterers. We show that each of these modes displays a
range of localization lengths and successfully relate the largest value to the
measured leakage rate at the boundary. These results constitute unambiguous
signatures of the existence of strongly localized electromagnetic modes in
two-dimensionnal open random media
Approximate equivalence between guided modes in a low-contrast photonic bandgap fiber and Maxwell TM modes of a high-contrast two-dimensional photonic structure
We present a formal analogy between the eigenvalue problem for guided scalar
modes in a low-contrast photonic bandgap fiber and quasi-stationary TM modes of
a two-dimensional (2D) photonic structure. Using this analogy, we numerically
study the confinement losses of disordered microstructured fibers through the
leakage rate of an open 2D system with high refractive index inclusions. Our
results show that for large values of the disorder, the confinement losses
increase. However, they also suggest that losses might be improved in strongly
disordered fibers by exploring ranges of physical parameters where Anderson
localization sets in
Saint-Sauveur-le-Vicomte – Cours de la Douve
Date de l'opération : 2007 (PR) Inventeur(s) : Le Guyader Laurent (BEN) ; Legrand Sébastien ; Giacchetti Laurent Cette prospection subaquatique s’est déroulée sur la commune de Saint-Sauveur-le-Vicomte. Place stratégique au sein du « Clos du Cotentin », elle joua un rôle important dans la guerre de Cent ans. Malgré de nombreuses restaurations, elle reste marquée par les bombardements de la seconde guerre mondiale. La Douve traverse le site et relie sur 800 m l’abbaye au château médiéval (dont..
Astraeus - III. The environment and physical properties of reionization sources
In this work, we use the {\sc astraeus} (seminumerical rAdiative tranSfer
coupling of galaxy formaTion and Reionization in N-body dArk mattEr
simUlationS) framework which couples galaxy formation and reionization in the
first billion years. Exploring a number of models for reionization feedback and
the escape fraction of ionizing radiation from the galactic environment
(), we quantify how the contribution of star-forming galaxies
{(with halo masses M)} to reionization depends on the
radiative feedback model, , and the environmental over-density.
Our key findings are: (i) for constant models,
intermediate-mass galaxies (with halo masses of M
and absolute UV magnitudes of to ) in
intermediate-density regions drive reionization; (ii) scenarios where
increases with decreasing halo mass shift the galaxy
population driving reionization to lower-mass galaxies
(M) with lower luminosities ()
and over-densities; (iii) reionization imprints its topology on the ionizing
emissivity of low-mass galaxies (M) through
radiative feedback. Low-mass galaxies experience a stronger suppression of star
formation by radiative feedback and show lower ionizing emissivities in
over-dense regions; (iv) a change in with galaxy properties
has the largest impact on the sources of reionization and their detectability,
with the radiative feedback strength and environmental over-density playing a
sub-dominant role; (v) JWST-surveys (with a limiting magnitude of ) will be able to detect the galaxies providing () of reionization photons at for constant models
(scenarios where increases with decreasing halo mass).Comment: 14 pages, 13 figures, accepted for publication in MNRA
Negative refraction of Lamb modes: A theoretical study
This paper provides a theoretical investigation of negative refraction and
focusing of elastic guided waves in a free-standing plate with a step-like
thickness change. Under certain conditions, a positive phase velocity (forward)
Lamb mode can be converted into a negative phase velocity (backward) mode at
such interface, giving rise to negative refraction. A semi-analytical model is
developed in order to study the influence of various parameters such as the
material Poisson's coefficient, the step-like thickness, the frequency and the
incidence angle. To this end, all the Lamb and shear horizontal propagating
modes, but also a large number of their inhomogeneous and evanescent
counterpart,s are taken into account. The boundary conditions applied to the
stress-displacement fields at the thickness step yields an equation system. Its
inversion provides the transmission and reflection coefficients between each
mode at the interface. The step-like thickness and Poisson's ratio are shown to
be key parameters to optimize the negative refraction process. In terms of
material, Duralumin is found to be optimal as it leads to a nearly perfect
conversion between forward and backward modes over broad frequency and angular
ranges. An excellent focusing ability is thus predicted for a flat lens made of
two symmetric thickness steps. Theoretical results are confirmed by a numerical
FDTD simulation and experimental measurements made on an optimized Duralumin
flat lens by means of laser interferometry. This theoretical study paves the
way towards the optimization of elastic devices based on negative refraction,
in particular for cloaking or super-focusing purposes.Comment: 8 pages, 6 figure
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