Publications by Julien Devriendt


Towards simulating star formation in the interstellar medium

Monthly Notices of the Royal Astronomical Society 356 (2005) 737-752

AD Slyz, Julien Devriendt, Greg Bryan, Joseph Silk


Non-linear evolution of suppressed dark matter primordial power spectra

Monthly Notices of the Royal Astronomical Society 360 (2005) 282-287

C Boehm, H Mathis, J Devriendt, J Silk

We address the degree and rapidity of generation of small-scale power over the course of structure formation in cosmologies where the primordial power spectrum is strongly suppressed beyond a given wavenumber. We first summarize the situations where one expects such suppressed power spectra and point out their diversity. We then employ an exponential cut-off, which characterizes warm dark matter (WDM) models, as a template for the shape of the cut-off and focus on damping scales ranging from 10 6 to 10 9 h -1 M ⊙. Using high-resolution simulations, we show that the suppressed part of the power spectrum is quickly (re)generated and catches up with both the linear and the non-linear evolution of the unsuppressed power spectrum. From z = 2 onwards, a power spectrum with a primordial cut-off at 10 9 h -1 MŁódź, becomes virtually indistinguishable from an evolved cold dark matter (CDM) power spectrum. An attractor such as that described in Zaldarriaga, Scoccimarro & Hui for power spectra with different spectral indices also emerges in the case of truncated power spectra. Measurements of z ∼ 0 non-linear power spectra at ∼100 h -1 kpc cannot rule out the possibility of linear power spectra damped below ∼10 9 h -1 M ⊙. Therefore, WDM or scenarios with similar features should be difficult to exclude in this way. © 2005 RAS.


A simple model for the evolution of supermassive black holes and the quasar population

Monthly Notices of the Royal Astronomical Society 359 (2005) 1363-1378

JEG Devriendt, Mahmood, A., Silk, J.


The second generation VLT instrument MUSE: Science drivers and instrument design

P SOC PHOTO-OPT INS 5492 (2004) 1145-1149

R Bacon, S Bauer, R Bower, S Cabrit, M Cappellari, M Carollo, FO Combes, R Davies, B Delabre, H Dekker, J Devriendt, S Djidel, M Duchateau, JP Dubois, E Emsellem, P Ferruit, M Franx, G Gilmore, B Guiderdoni, F Henault, N Hubin, B Jungwiert, A Kelz, M Le Louarn, I Lewis, JL Lizon, R Mc Dermid, S Morris, U Laux, O Le Fevre, B Lantz, S Lilly, J Lynn, L Pasquin, A Pecontal, PPD Popovic, A Quirrenbach, R Reiss, M Roth, M Steinmetz, R Stuik, L Wisotzki, T de Zeeuw

The Multi Unit Spectroscopic Explorer (MUSE) is a second generation VLT panoramic integral-field spectrograph operating in the visible wavelength range. MUSE has a field of 1x1 arcmin(2) sampled at 0.20.2 arcsec(2) and is assisted by a ground layer adaptive optics system using four laser guide stars. The simultaneous spectral range is 0.465-0.93 mum, at a resolution of Rsimilar to3000. MUSE couples the discovery potential of a large imaging device to the measuring capabilities of a high-quality spectrograph, while taking advantage of the increased spatial resolution provided by adaptive optics. This makes MUSE a unique and tremendously powerful instrument for discovering and characterizing objects that lie beyond the reach of even the deepest imaging surveys. MUSE has also a high spatial resolution mode with 7.5x7.5 arcsec(2) field of view sampled at 25 milli-arcsec. In this mode MUSE should be able to get diffraction limited data-cube in the 0.6-1 mum wavelength range. Although MUSE design has been optimized for the study of galaxy formation and evolution, it has a wide range of possible applications; e.g. monitoring of outer planets atmosphere, young stellar objects environment, supermassive black holes and active nuclei in nearby galaxies or massive spectroscopic survey of stellar fields.


Turbulent ambipolar diffusion: Numerical studies in two dimensions

ASTROPHYSICAL JOURNAL 603 (2004) 165-179

F Heitsch, EG Zweibel, AD Slyz, JEG Devriendt


Magnetic flux transport in the ISM through turbulent ambipolar diffusion

ASTROPHYS SPACE SCI 292 (2004) 45-51

F Heitsch, EG Zweibel, Adrianne, D Slyz, JEG Devriendt

Under ideal MHD conditions the magnetic field strength should be correlated with density in the interstellar medium ( ISM). However, observations indicate that this correlation is weaker than expected. Ambipolar diffusion can decrease the flux-to-mass ratio in weakly ionized media; however, it is generally thought to be too slow to play a significant role in the ISM except in the densest molecular clouds. Turbulence is often invoked in other astrophysical problems to increase transport rates above the ( very slow) diffusive values. Building on analytical studies, we test with numerical models whether turbulence can enhance the ambipolar diffusion rate sufficiently to explain the observed weak correlations. The numerical method is based on a gas-kinetic scheme with very low numerical diffusivity, thus allowing us to separate numerical and physical diffusion effects.


Turbulent ambipolar diffusion: Numerical studies in two dimensions

Astrophysical Journal 603 (2004) 165-179

F Heitsch, EG Zweibel, AD Slyz, JEG Devriendt

Under ideal MHD conditions the magnetic field strength should be correlated with density in the interstellar medium (ISM). However, observations indicate that this correlation is weak. Ambipolar diffusion can decrease the flux-to-mass ratio in weakly ionized media; however, it is generally thought to be too slow to play a significant role in the ISM except in the densest molecular clouds. Turbulence is often invoked in astrophysical problems to increase transport rates above the (very slow) laminar values predicted by kinetic theory. We describe a series of numerical experiments addressing the problem of turbulent transport of magnetic fields in weakly ionized gases. We show, subject to various geometrical and physical restrictions, that turbulence in a weakly ionized medium rapidly diffuses the magnetic flux-to-mass ratio B/ρ through the buildup of appreciable ion-neutral drifts on small scales. These results are applicable to the field strength-density correlation in the ISM, as well as the merging of flux systems such as protostar and accretion disk fields or protostellar jets with ambient matter, and the vertical transport of galactic magnetic fields.


Star formation in a multi-phase interstellar medium

Astrophysics and Space Science 284 (2003) 833-836

A Slyz, J Devriendt, G Bryan, J Silk

This contribution reports on our first efforts to simulate a multiphase interstellar medium on a kiloparsec scale in three dimensions with the stars and gas modeled self-consistently. Starting from inhomogenous initial conditions, our closed box simulations follow the gas as it cools and collapses under its own self-gravity to form stars which eventually return material and energy back through supernovae explosions and winds.


Non-standard structure formation scenarios

Astrophysics and Space Science 284 (2003) 335-340

A Knebe, B Little, R Islam, J Devriendt, A Mahmood, J Silk

Observations on galactic scales seem to be in contradiction with recent high resolution N-body simulations. This so-called cold dark matter (CDM) crisis has been addressed in several ways, ranging from a change in fundamental physics by introducing self-interacting cold dark matter particles to a tuning of complex astrophysical processes such as global and/or local feedback. All these efforts attempt to soften density profiles and reduce the abundance of satellites in simulated galaxy halos. In this contribution we are exploring the differences between a Warm Dark Matter model and a CDM model where the power on a certain scale is reduced by introducing a narrow negative feature ('dip'). This dip is placed in a way so as to mimic the loss of power in the WDM model: both models have the same integrated power out to the scale where the power of the Dip model rises to the level of the unperturbed CDM spectrum again. Using N-body simulations we show that that the new Dip model appears to be a viable alternative to WDM while being based on different physics: where WDM requires the introduction of a new particle species the Dip stems from a nonstandard inflationary period. If we are looking for an alternative to the currently challenged standard ΛCDM structure formation scenario, neither the ΛWDM nor the new Dip model can be ruled out with respect to the analysis presented in this contribution. They both make very similar predictions and the degeneracy between them can only be broken with observations yet to come.


Top-down fragmentation of a warm dark matter filament

Monthly Notices of the Royal Astronomical Society 345 (2003) 1285-1290

A Knebe, JEG Devriendt, BK Gibson, J Silk

We present the first high-resolution N-body simulations of the fragmentation of dark matter filaments. Such fragmentation occurs in top-down scenarios of structure formation, when the dark matter is warm instead of cold. In a previous paper, we showed that warm dark matter (WDM) differs from the standard cold dark matter (CDM) mainly in the formation history and large-scale distribution of low-mass haloes, which form later and tend to be more clustered in WDM than in CDM universes, tracing the filamentary structures of the cosmic web more closely. Therefore, we focus our computational effort in this paper on one particular filament extracted from a WDM cosmological simulation and compare in detail its evolution to that of the same CDM filament. We find that the mass distribution of the haloes forming via fragmentation within the filament is broadly peaked around a Jeans mass of a few 109 M ⊙, corresponding to a gravitational instability of smooth regions with an overdensity contrast around 10 at these redshifts. Our results confirm that WDM filaments fragment and form gravitationally bound haloes in a top-down fashion, whereas CDM filaments are built bottom-up, thus demonstrating the impact of the nature of the dark matter on dwarf galaxy properties.


Collision-induced galaxy formation: semi-analytic model and multiwavelength predictions

Monthly Notices of the Royal Astronomical Society 343 (2003) 107-115

JEG Devriendt, Balland, C., Silk, J.


Predicting multi-wavelength properties of Lyman break galaxies with GalICS

ASTROPHYS SPACE SCI 284 (2003) 373-376

J Blaizot, B Guiderdoni, JEG Devriendt, FR Bouchet, S Hatton

GalICS (for GALaxies In Cosmological Simulations) is a model of hierarchical galaxy formation which combines high resolution numerical simulations for the dark matter component with semi-analytic prescriptions for the baryonic matter. It provides us with an explicit cosmological framework to analyse observations of distant galaxies, and to understand how they evolve to become local galaxies. We use GalICS to build multi-wavelength mock galaxy catalogues which include clustering properties. We can compare them to the sample of Lyman Break Galaxies at z=3 ( Steidel et al., 1996), and to deep sub-mm surveys. The predictions of the model will be detailed, and show a good agreement with the available data.


GALICS: A direct link between theory and observations

Astrophysics and Space Science 284 (2003) 369-372

J Devriendt

This contribution advocates anew method for comparing theoretical predictions to observations. Properties of virtual galaxies are computed using the hybrid model for hierarchical galaxy formation GALICS (for Galaxies In Cosmological Simulations), which takes advantage of large cosmological N-body simulations to plug in simple semi-analytic recipes describing the fate of the baryons. From such a fake galaxy catalog, one can build light cones, and project them onto virtual CCD devices, taking into account the technical characteristics of the detector/telescope. As a result, realistic mock images can be produced, which can then be directly compared to real observations.


GALICS - I. A hybrid N-body/semi-analytic model of hierarchical galaxy formation

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 343 (2003) 75-106

S Hatton, JEG Devriendt, S Ninin, FR Bouchet, B Guiderdoni, D Vibert


GALLICS - I. A hybrid N-body/semi-analytic model of hierarchical galaxy formation

Monthly Notices of the Royal Astronomical Society 343 (2003) 75-106

S Hatton, JEG Devriendt, S Ninin, FR Bouchet, B Guiderdoni, D Vibert

This is the first paper of a series that describes the methods and basic results of the GALICS model (Galaxies In Cosmological Simulations). GALICS is a hybrid model for hierarchical galaxy formation studies, combining the outputs of large cosmological N-body simulations with simple, semi-analytic recipes to describe the fate of the baryons within dark matter haloes. The simulations produce a detailed merging tree for the dark matter haloes, including complete knowledge of the statistical properties arising from the gravitational forces. We intend to predict the overall statistical properties of galaxies, with special emphasis on the panchromatic spectral energy distribution emitted by galaxies in the ultraviolet/optical and infrared/submillimetre wavelength ranges. In this paper, we outline the physically motivated assumptions and key free parameters that go into the model, comparing and contrasting with other parallel efforts. We specifically illustrate the success of the model in comparison with several data sets, showing how it is able to predict the galaxy disc sizes, colours, luminosity functions from the ultraviolet to far infrared, the Tully-Fisher and Faber-Jackson relations, and the fundamental plane in the local Universe. We also identify certain areas where the model fails, or where the assumptions needed to succeed are at odds with observations, and pay special attention to understanding the effects of the finite resolution of the simulations on the predictions made. Other papers in this series will take advantage of different data sets available in the literature to extend the study of the limitations and predictive power of GALICS, with particular emphasis put on high-redshift galaxies.


Forming stars on an exponential timescale: the key to exponential stellar profiles in disc galaxies?

Monthly Notices of the Royal Astronomical Society 333 (2002) 894-910

AD Slyz, Julien Devriendt, Joseph Silk, Andreas Burkert


GALICS: Capturing the panchromaticity of galaxies

Astrophysics and Space Science 281 (2002) 505-508

J Devriendt

This contribution describes results obtained with the GALICS model (for Galaxies In Cosmological Simulations), which is a hybrid model for hierarchical galaxy formation studies, combining the outputs of large cosmological N-body simulations with simple, semi-analytic recipes to describe the fate of the baryons within dark matter halos. Designed to predict the overall statistical properties of galaxies, with special emphasis on the panchromatic spectral energy distribution emitted by galaxies in the UV/optical and IR/submm wavelength ranges, such an approach can be used to predict the galaxy luminosity function evolution from the ultraviolet to far infrared, along with individual galaxies star formation histories.


Source-lens clustering effects on the skewness of the lensing convergence

Monthly Notices of the Royal Astronomical Society 330 (2002) 365-377

T Hamana, ST Colombi, A Thion, JEGT Devriendt, Y Mellier, F Bernardeau

potentials causes a systematic effect on measurements of cosmic shear statistics, known as the source-lens clustering (SLC) effect. The SLC effect on the skewness of lensing convergence, S3, is examined using a non-linear semi-analytic approach and is checked against numerical simulations. The semi-analytic calculations have been performed in a wide variety of generic models for the redshift distribution of source galaxies and power-law models for the bias parameter between the galaxy and dark matter distributions. The semi-analytic predictions are tested successfully against numerical simulations. We find the relative amplitude of the SLC effect on S3 to be of the order of 5 -40 per cent. It depends significantly on the redshift distribution of sources and on the way in which the bias parameter evolves. We discuss possible measurement strategies to minimize the SLC effects.


Merger histories in warm dark matter structure formation scenarios

Monthly Notices of the Royal Astronomical Society 329 (2002) 813-828

JEG Devriendt, Knebe, A., Mahmood, A., Silk, J.


Probing galaxy formation with high energy gamma-rays

AIP CONF PROC 558 (2001) 463-478

JR Primack, RS Somerville, JS Bullock, JEG Devriendt

We discuss how measurements of the absorption of gamma -rays from GeV to TeV energies via pair production on the extragalactic background light (EBL) can probe important issues in galaxy formation. We use semi-analytic models (SAMs) of galaxy formation, set within the hierarchical structure formation scenario, to obtain predictions of the EEL from 0.1 to 1000 mum. SAMs incorporate simplified physical treatments of the key processes of galaxy formation - including gravitational collapse and merging of dark matter halos, gas cooling and dissipation, star formation, supernova feedback and metal production - and have been shown to reproduce key observations at low and high redshift. Here we also introduce improved modelling of the spectral energy distributions in the mid-to-far-IR arising from emission by dust grains. Assuming a flat ACDM cosmology with Omega (m) = 0.3 and Hubble parameter h = 0.65, we investigate the consequences of variations in input assumptions such as the stellar initial mass function (IMF) and the efficiency of converting cold gas into stars. We conclude that observational studies of the absorption of gamma -rays with energies from similar to 10 Gev to similar to 10 TeV will help to determine the EEL, and also help to explain its origin by constraining some of the most uncertain features of galaxy formation theory, including the IMF, the history of star formation, and the reprocessing of light by dust.

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