Publications by Julien Devriendt

A Galaxy in the Making


A Slyz, J Devriendt, L Powell

Modeling high-redshift galaxies: what can we learn from high and ultra-high resolution hydrodynamical simulations?


J Devriendt, A Slyz, L Powell, C Pichon, R Teyssier

The impact of thermally pulsing asymptotic giant branch stars on hierarchical galaxy formation models

Monthly Notices of the Royal Astronomical Society: Letters 396 (2009) L36-L40

C Tonini, C Maraston, J Devriendt, D Thomas, J Silk

The spectro-photometric properties of galaxies in galaxy formation models are obtained by combining the predicted history of star formation and mass accretion with the physics of stellar evolution through stellar population models. In the recent literature, significant differences have emerged regarding the implementation of the thermally pulsing asymptotic giant branch phase of stellar evolution. The emission in the TP-AGB phase dominates the bolometric and near-IR spectrum of intermediate-age (~1 Gyr) stellar populations, hence it is crucial for the correct modelling of the galaxy luminosities and colours. In this paper, for the first time, we incorporate a full prescription of the TP-AGB phase in a semi-analytic model of galaxy formation. We find that the inclusion of the TP-AGB in the model spectra dramatically alters the predicted colour-magnitude relation and its evolution with redshift. When the TP-AGB phase is active, the rest-frame V - K galaxy colours are redder by almost 2 mag in the redshift range z ~ 2-3 and by 1 mag at z ~ 1. Very red colours are produced in disc galaxies, so that the V - K colour distributions of disc and spheroids are virtually undistinguishable at low redshifts. We also find that the galaxy K-band emission is more than 1 mag higher in the range z ~ 1-3. This may alleviate the difficulties met by the hierarchical clustering scenario in predicting the red galaxy population at high redshifts. The comparison between simulations and observations has to be revisited in the light of our results. © 2009 The Authors. Journal compilation © 2009 RAS.

Building merger trees from cosmological N-body simulations

Astronomy and Astrophysics 506 (2009) 647-660

D Tweed, J Devriendt, J Blaizot, S Colombi, A Slyz

Context. In the past decade or so, using numerical N-body simulations to describe the gravitational clustering of dark matter (DM) in an expanding universe has become the tool of choice for tackling the issue of hierarchical galaxy formation. As mass resolution increases with the power of supercomputers, one is able to grasp finer and finer details of this process, resolving more and more of the inner structure of collapsed objects. This begs one to revisit time and again the post-processing tools with which one transforms particles into "invisible" dark matter haloes and from thereon into luminous galaxies.Aims. Although a fair amount of work has been devoted to growing Monte-Carlo merger trees that resemble those built from an N-body simulation, comparatively little effort has been invested in quantifying the caveats one necessarily encounters when one extracts trees directly from such a simulation. To somewhat revert the tide, this paper seeks to provide its reader with a comprehensive study of the problems one faces when following this route.Methods. The first step in building merger histories of dark matter haloes and their subhaloes is to identify these structures in each of the time outputs (snapshots) produced by the simulation. Even though we discuss a particular implementation of such an algorithm (called AdaptaHOP) in this paper, we believe that our results do not depend on the exact details of the implementation but instead extend to most if not all (sub)structure finders. To illustrate this point in the appendix we compare AdaptaHOP's results to the standard friend-of-friend (FOF) algorithm, widely utilised in the astrophysical community. We then highlight different ways of building merger histories from AdaptaHOP haloes and subhaloes, contrasting their various advantages and drawbacks.Results. We find that the best approach to (sub)halo merging histories is through an analysis that goes back and forth between identification and tree building rather than one that conducts a straightforward sequential treatment of these two steps. This is rooted in the complexity of the merging trees that have to depict an inherently dynamical process from the partial temporal information contained in the collection of instantaneous snapshots available from the N-body simulation. However, we also propose a simpler sequential "Most massive Substructure Method" (MSM) whose trees approximate those obtained via the more complicated non sequential method. © 2009 ESO.

MUSE: A second-generation integral-field spectrograph for the VLT

ESO Astrophysics Symposia 2008 (2008) 325-336

RM Mcdermid, R Bacon, S Bauer, P Boehm, D Boudon, S Brau-Nogué, P Caillier, L Capoani, CM Carollo, N Champavert, T Contini, E Daguisé, B Delabre, J Devriendt, S Dreizler, J Dubois, M Dupieux, JP Dupin, E Emsellem, P Ferruit, M Franx, G Gallou, J Gerssen, B Guiderdoni, T Hahn, D Hofmann, A Jarno, A Kelz, C Koehler, W Kollatschny, J Kosmalski, F Laurent, SJ Lilly, JL Lizon, M Loupias, A Manescau, C Monstein, H Nicklas, L Parès, L Pasquini, A Pécontal-rousset, E Pécontal, R Pello, C Petit, JP Picat, E Popow, A Quirrenbach, R Reiss, E Renault, M Roth, J Schaye, G Soucail, M Steinmetz, S Stroebele, R Stuik, P Weilbacher, L Wisotzki, H Wozniak, PT de Zeeuw

The Multi Unit Spectroscopic Explorer (MUSE) is a second-generation instrument in development for the Very Large Telescope (VLT) of the European Southern Observatory (ESO), due to begin operation in 2011/12. MUSE will be an extremely powerful integral-field spectrograph fed by a new multiple-laser adaptive optics system on the VLT. In its usual operating mode, MUSE will, in a single observation, produce a 3-dimensional data cube consisting of 90,000 R 3000 spectra, each covering a full spectral octave (480-930 nm), and fully sampling a contiguous 1×1 arcmin2 field with 0.2×0.2 arcsec2 apertures. A high-resolution mode will increase the spatial sampling to 0.025 arcsec per pixel. MUSE is built around a novel arrangement of 24 identical spectrographs (each comparable to a 1st generation VLT instrument), which are fed by a set of 24 precision image slicers. MUSE is designed for stability, with only 2 modes, and virtually no moving parts, allowing very long exposures to be accumulated. Together with high throughput, this ensures that MUSE will have extreme sensitivity for observing faint objects. We overview the technical and scientific aspects of MUSE, highlighting the key challenges for dealing with the unprecedented quantity and complexity of the data, and the integration with the VLT adaptive optics facility (AOF) - a key development on the path to extremely large telescopes (ELTs). © 2008 Springer-Verlag Berlin Heidelberg.

Integral field unit spectrograph for extremely large telescopes

Publications of the Astronomical Society of the Pacific 120 (2008) 634-643

I Montilla, E Pécontal, J Devriendt, R Bacon

We have carried out a concept study for a wide-field monolithic integral field unit (IFU) spectrograph for extremely large telescopes (ELTs). We target in this paper the technological challenges that have to be faced in order to build such an instrument, focusing on the adaptive optics (AO) requirements, the image slicer technology, and the detectors status. We also address the main science drivers, together with the concept design and the expected performance applied to the European-ELT (E-ELT) case. A monolithic wide-field spectrograph provides a continuous field of view (FOV) separated by a field splitter in several subfields, each of them feeding a module featuring an image slicer, a collimator and a spectrograph. The use of image slicers provides 3D spectrographic images of the complete FOV, allowing for detection and study of sources without need of targeting them, a very useful property especially for the deep observation of faint high-redshift objects, whose density on the sky is expected to be quite high. In light of this discussion, we suggest the advantages of using shorter wavelengths and its implication in both the scientific program and the budget. © 2008. The Astronomical Society of the Pacific. All rights reserved.

Cooling, gravity, and geometry: Flow-driven massive core formation

Astrophysical Journal 674 (2008) 316-328

F Heitsch, LW Hartmann, AD Slyz, JEG Devriendt, A Burkert

We study numerically the formation of molecular clouds in large-scale colliding flows including self-gravity. The models emphasize the competition between the effects of gravity on global and local scales in an isolated cloud. Global gravity builds up large-scale filaments, while local gravity, triggered by a combination of strong thermal and dynamical instabilities, causes cores to form. The dynamical instabilities give rise to a local focusing of the colliding flows, facilitating the rapid formation of massive protostellar cores of a few hundred M⊙. The forming clouds do not reach an equilibrium state, although the motions within the clouds appear to be comparable to virial. The self-similar core mass distributions derived from models with and without self-gravity indicate that the core mass distribution is set very early on during the cloud formation process, predominantly by a combination of thermal and dynamical instabilities rather than by self-gravity. © 2008. The American Astronomical Society. All rights reserved.

WFSPEC - a multi-object AO instrument for the European extremely large telescope

Optics InfoBase Conference Papers (2007)

G Moretto, JG Cuby, E Prieto, F Hammer, P Jagourel, G Rousset, T Fusco, J Devriendt

WFSPEC is a multi-integral field spectrograph instrument concept combining local AO correction over a wide field of view. This local correction is achieved by a multi object adaptive optics system integrated into the instrument. Instrument concept, development and preliminary results on performance simulations are discussed. © 2007 Optical Society of America.

GaIICS: A hybrid approach to cosmological chemodynamics

EAS PUBLICATIONS 24 (2007) 215-220

JEG Devriendt

This contribution addresses the issue of metal enrichment and the distribution of metals in the lSN/IGM/ICM within the framework of a hybrid N-body plus semi-analytic method. It discusses its impact on galaxy bimodality and multi-wavelength galaxy counts.

UV-optical colors as probes of early-type galaxy evolution

Astrophysical Journal, Supplement Series 173 (2007) 619-642

S Kaviraj, K Schawinski, JEG Devriendt, I Ferreras, S Khochfar, SJ Yoon, SK Yi, JM Deharveng, A Boselli, T Barlow, T Conrow, K Forster, PG Friedman, DC Martin, P Morrissey, S Neff, D Schiminovich, M Seibert, T Small, T Wyder, L Bianchi, J Donas, T Heckman, YW Lee, B Madore, B Milliard, RM Rich, A Szalay

We have studied ∼2100 early-type galaxies in the SDSS DR3 which have been detected by the GALEX Medium Imaging Survey (MIS), in the redshift range O < z < 0.1.1. Combining GALEXUV photometry with corollary optical data from the SDSS, we find that, at a 95% confidence level, at least ∼30% of galaxies in this sample have UV to optical colors consistent with some recent star formation within the last Gyr. In particular, galaxies with an NUV - r color less than 5.5 are very likely to have experienced such recent star formation, taking into account the possibility of a contribution to NUV flux from the UV upturn phenomenon. We find quantitative agreement between the observations and the predictions of a semianalytical ACDM hierarchical merger model and deduce that early-type galaxies in the redshift range 0 < z < 0.11 have ∼ 1 % -3 % of their stellar mass in stars less than 1 Gyr old. The average age of this recently formed population is ∼300-500 Myr. We also find that "monolithically" evolving galaxies, where recent star formation can be driven solely by recycled gas from stellar mass loss, cannot exhibit the blue colors (NUV - r < 5.5) seen in a significant fraction (∼30%) of our observed sample. © 2007. The American Astronomical Society. All rights reserved.

Magnetized nonlinear thin-shell instability: Numerical studies in two dimensions

ASTROPHYSICAL JOURNAL 665 (2007) 445-456

F Heitsch, AD Slyz, JEG Devriendt, LW Hartmann, A Burkert

The horizon project database


J-P Le Fevre, H Wozniak, J Devriendt, R Teyssier

UV-optical colors as probes of early-type galaxy evolution


S Kaviraj, K Schawinski, JEG Devriendt, I Ferreras, S Khochfar, S-J Yoon, SK Yi, J-M Deharveng, A Boselli, T Barlow, T Conrow, K Forster, PG Friedman, DC Martin, P Morrissey, S Neff, D Schiminovich, M Seibert, T Small, T Wyder, L Bianchi, J Donas, T Heckman, Y-W Lee, B Madore, B Milliard, RM Rich, A Szalay

Magnetized nonlinear thin-shell instability: Numerical studies in two dimensions

Astrophysical Journal 665 (2007) 445-456

F Heitsch, AD Slyz, JEG Devriendt, LW Hartmann, A Burkert

We revisit the analysis of the nonlinear thin shell instability (NTSI) numerically, including magnetic fields. The magnetic tension force is expected to work against the main driver of the NTSI - namely, transverse momentum transport. However, depending on the field strength and orientation, the instability may grow. For fields aligned with the inflow, we find that the NTSI is suppressed only when the Alfvén speed surpasses the (supersonic) velocities generated along the collision interface. Even for fields perpendicular to the inflow, which are the most effective at preventing the NTSI from developing, internal structures form within the expanding slab interface, probably leading to fragmentation in the presence of self-gravity or thermal instabilities. High Reynolds numbers result in local turbulence within the perturbed slab, which in turn triggers reconnection and dissipation of the excess magnetic flux. We find that when the magnetic field is initially aligned with the flow, there exists a (weak) correlation between field strength and gas density. However, for transverse fields, this correlation essentially vanishes. In light of these results, our general conclusion is that instabilities are unlikely to be erased unless the magnetic energy in clouds is much larger than the turbulent energy. Finally, while our study is motivated by the scenario of molecular cloud formation in colliding flows, our results span a larger range of applicability, from supernova shells to colliding stellar winds. © 2007. The American Astronomical Society. All rights reserved.

The birth of molecular clouds:formation of atomic precursors in colliding flows

Astrophysical Journal 648 (2006) 1052-1065

AD Slyz, Fabian Heitsch, Julien Devriendt, Lee Hartmann

Wide field spectrograph concepts for the European Extremely Large Telescope

Proceedings of SPIE - The International Society for Optical Engineering 6269 II (2006)

G Moretto, R Bacon, JG Cuby, F Hammer, P Amram, S Blais-Ouellette, PE Blanc, J Devriendt, B Epinat, T Fusco, P Jagourel, O Hernandez, JP Kneib, I Montilla, B Neichel, E Pécontal, E Prieto, M Puech

We report on the science case high level specifications for a wide field spectrograph instrument for an Extremely Large Telescope (ELT) and present possible concepts. Preliminary designs are presented which resort to different instrument concepts: monolithic integral field (IFU), multi-IFU, and a smart tunable filter. This work is part of the activities performed in the work package 'Instrumentation' of the 'ELT Design Study', a programme supported by the European Community, Framework Programme 6.

Modelling the galaxy bimodality: shutdown above a critical halo mass

Monthly Notices of the Royal Astronomical Society 370 (2006) 1651-1665

JEG Devriendt, Cattaneo, A., Dekel A., Guiderdoni B.

Non-Standard Structure Formation Scenarios

Astrophysics and Space Science Kluwer Academic Publishers 284 (2006) 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 non-standard inflationary period. If we are looking for an alternative to the currently challenged standard LCDM structure formation scenario, neither the LWDM 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.

Formation of structure in molecular clouds: A case study


F Heitsch, A Burkert, LW Hartmann, AD Slyz, JEG Devriendt

Formation of structure in molecular clouds: A case study

Astrophysical Journal 633 (2005)

F Heitsch, A Burkert, LW Hartmann, AD Slyz, JEG Devriendt

Molecular clouds (MCs) are highly structured and turbulent. Colliding gas streams of atomic hydrogen have been suggested as a possible source of MCs, imprinting the filamentary structure as a consequence of dynamical and thermal instabilities. We present a two-dimensional numerical analysis of MC formation via converging H I flows. Even with modest flow speeds and completely uniform inflows, nonlinear density perturbations arise as possible precursors of MCs. Thus, we suggest that MCs are inevitably formed with substantial structure, e.g., strong density and velocity fluctuations, which provide the initial conditions for subsequent gravitational collapse and star formation in a variety of Galactic and extragalactic environments. © 2005. The American Astronomical Society. All rights reserved.