Laboratoire Univers et Theories

Laboratoire Univers et Theories

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Devriendt J.,University of Oxford | Pichon C.,University of Oxford | Pichon C.,CNRS Paris Institute of Astrophysics | Teyssier R.,CEA Saclay Nuclear Research Center | And 11 more authors.
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2010

The largest galaxies acquire their mass early on, when the Universe is still youthful. Cold streams violently feed these young galaxies a vast amount of fresh gas, resulting in very efficient star formation. Using a well resolved hydrodynamical simulation of galaxy formation, we demonstrate that these mammoth galaxies are already in place a couple of billion years after the big bang. Contrary to local star-forming galaxies, where dust re-emits a large part of the stellar ultraviolet (UV) light at infrared and sub-millimetre wavelengths, our self-consistent modelling of dust extinction predicts that a substantial fraction of UV photons should escape from primordial galaxies. Such a model allows us to compute reliably the number of high-redshift objects as a function of luminosity, and yields galaxies whose UV luminosities closely match those measured in the deepest observational surveys available. This agreement is remarkably good considering our admittedly still simple modelling of the interstellar medium physics. The luminosity functions of virtual UV luminous galaxies coincide with the existing data over the whole redshift range from 4 to 7, provided cosmological parameters are set to their currently favoured values. Despite their considerable emission at short wavelengths, we anticipate that the counterparts of the brightest UV galaxies will be detected by future sub-millimetre facilities like ALMA. © 2010 The Authors. Journal compilation © 2010 RAS.

Roueff E.,Laboratoire Univers et Theories | Lique F.,CNRS Laboratory of Waves and Complex Media
Chemical Reviews | Year: 2013

Collisional excitation is the basic process, and its efficiency relies on the composition of the medium providing the density of the main perturbers and the temperature, which establishes the degree of excitation and, consequently, drives the intensities of the radiated emission. Nevertheless, other excitation mechanisms can be at work such as radiative or chemical pumping. The computation of collisional inelastic rate coefficients usually takes place within the Born-Oppenheimer approximation for the separation of electronic and nuclear motions. Scattering cross sections are thus obtained by solving the motion of the nuclei on an electronic potential energy surface (PES) that is independent of the masses and spins of the nuclei. Recent studies have demonstrated that computational techniques employing advanced treatments for both the electronic and nuclear motion problems can rival experimental measurements, in terms of the achieved accuracy.

Mottez F.,Laboratoire Univers et Theories
Annales Geophysicae | Year: 2012

In the auroral zone of the Earth, the electron acceleration by Alfvén waves is sometimes seen as a precursor of the non-propagating acceleration structures. In order to investigate how Alfvén waves could generate non-propagating electric fields, a series of simulations of counter-propagating waves in a homogeneous medium is presented. The waves propagate along the ambient magnetic field direction. It is shown that non-propagating electric fields are generated at the locus of the Alfvén waves crossing. These electric fields have a component orientated along the direction of the ambient magnetic field, and they generate a significant perturbation of the plasma density. The non-linear interaction of down and up-going Alfvén waves might be a cause of plasma density fluctuations (with gradients along the magnetic field) on a scale comparable to those of the Alfvén wavelengths. The present paper is mainly focused on the creation process of the non-propagating parallel electric field. © Author(s) 2012. CC Attribution 3.0 License.

Belmont G.,Ecole Polytechnique - Palaiseau | Chust T.,Ecole Polytechnique - Palaiseau | Mottez F.,Laboratoire Univers et Theories | Hess S.,University of Colorado at Boulder
Transport Theory and Statistical Physics | Year: 2011

The linear Landau effect is revisited by the means of numerical simulations and analytical calculations. The existence of non-Landau solutions to the Vlasov-Poisson system is emphasized and the consistency of these solutions with respect to the arguments based on energy is investigated. The present article briefly summarizes the content of two articles already published on the subject and introduces a discussion based on the exchanges that occurred at Marseille during the Vlasovia meeting. © 2011 Copyright Taylor and Francis Group, LLC.

Mottez F.,Laboratoire Univers et Theories
Journal of Plasma Physics | Year: 2015

It is shown that two circularly polarized Alfvén waves that propagate along the ambient magnetic field in an uniform plasma trigger non oscillating electromagnetic field components when they cross each other. The non-oscilliating field components can accelerate ions and electrons with great efficiency. This work is based on particle-in-cell (PIC) numerical simulations and on analytical non-linear computations. The analytical computations are done for two counter-propagating monochromatic waves. The simulations are done with monochromatic waves and with wave packets. The simulations show parallel electromagnetic fields consistent with the theory, and they show that the particle acceleration results in plasma cavities and, if the waves amplitudes are high enough, in ion beams. These acceleration processes could be relevant in space plasmas. For instance, they could be at work in the auroral zone and in the radiation belts of the Earth magnetosphere. In particular, they may explain the origin of the deep plasma cavities observed in the Earth auroral zone. © 2014 Cambridge University Press.

Docampo D.,University of Vigo | Egret D.,Laboratoire Univers et Theories | Cram L.,Australian National University
Scientometrics | Year: 2015

The growing influence of the idea of world-class universities and the associated phenomenon of international academic rankings are intriguing issues for contemporary comparative analyses of higher education. Although the Academic Ranking of World Universities (ARWU or the Shanghai ranking) was originally devised to assess the gap between Chinese universities and world-class universities, it has since been credited with roles in stimulating higher education change on many scales, from increasing the labor value of individual high-performing scholars to wholesale renovation of national university systems including mergers. This paper exhibits the response of the ARWU indicators and rankings to institutional mergers in general, and specifically analyses the universities of France that are engaged in a major amalgamation process motivated in part by a desire for higher international rankings. © 2015, Akadémiai Kiadó, Budapest, Hungary.

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