arXiv subjects

Romain Teyssier

Publications and source records attributed to Romain Teyssier.

At least 199 records · Page 11Linked to original sources

arXiv2009

Morphological quenching of star formation: making early-type galaxies red

We point out a natural mechanism for quenching of star formation in early-type galaxies. It automatically links the color of a galaxy with its morphology and does not require gas consumption, removal or termination of gas supply. Given that star formation takes place in gravitationally unstable gas disks, it can be quenched when a disk becomes stable against fragmentation to bound clumps. This can result from the growth of a stellar spheroid, for instance by mergers. We present the concept of morphological quenching (MQ) using standard disk instability analysis, and demonstrate its natural occurrence in a cosmological simulation using an efficient zoom-in technique. We show that the transition from a stellar disk to a spheroid can be sufficient to stabilize the gas disk, quench star formation, and turn an early-type galaxy red and dead while gas accretion continues. The turbulence necessary for disk stability can be stirred up by sheared perturbations within the disk in the absence of bound star-forming clumps. While gas stripping processes are limited to dense groups and clusters, and other quenching mechanisms like AGN feedback, virial shock heating and gravitational heating, are limited to halos more massive than 10^12 Mo, the MQ can explain the appearance of red ellipticals even in less massive halos and in the field. The dense gas disks observed in some of today's red ellipticals may be the relics of this mechanism, whereas red galaxies with quenched gas disks are expected to be more frequent at high redshift.

astro-ph.CO↗

arXiv2009

Resolving Globular Cluster Formation within a Cosmological Context

We place constraints on the formation redshifts for blue globular clusters (BGCs), independent of the details of hydrodynamics and population III star formation. The observed radial distribution of BGCs in the Milky Way Galaxy suggests that they formed in biased dark matter halos at high redshift. As a result, simulations of a ~1 Mpc box up to z~10 must resolve BGC formation in LCDM. We find that most halo stars could be produced from destroyed BGCs and other low-mass clusters that formed at high redshift. We present a proof-of-concept simulation that captures the formation of globular-like star clusters.

astro-ph.CO↗

arXiv2009

Disk formation and the origin of clumpy galaxies at high redshift

Observations of high redshift galaxies have revealed a multitude of large clumpy rapidly star-forming galaxies. Their formation scenario and their link to present day spirals is still unknown. In this Letter we perform adaptive mesh refinement simulations of disk formation in a cosmological context that are unrivalled in terms of mass and spatial resolution. We find that the so called "chain-galaxies" and "clump-clusters" are a natural outcome of early epochs of enhanced gas accretion from cold dense streams as well as tidally and ram-pressured stripped material from minor mergers and satellites. Through interaction with the hot halo gas, this freshly accreted cold gas settles into a large disk-like system, not necessarily aligned to an older stellar component, that undergoes fragmentation and subsequent star formation, forming large clumps in the mass range 10^7-10^9 M_sun. Galaxy formation is a complex process at this important epoch when most of the central baryons are being acquired through a range of different mechanisms - we highlight that a rapid mass loading epoch is required to fuel the fragmentation taking place in the massive arms in the outskirts of extended disks, an accretion mode that occurs naturally in the hierarchical assembly process at early epochs.

astro-ph.GA↗

arXiv2009

Full-Sky Weak Lensing Simulation with 70 Billion Particles

We have performed a 70 billion dark-matter particles N-body simulation in a 2 $h^{-1}$ Gpc periodic box, using the concordance, cosmological model as favored by the latest WMAP3 results. We have computed a full-sky convergence map with a resolution of $Δθ\simeq 0.74$ arcmin$^{2}$, spanning 4 orders of magnitude in angular dynamical range. Using various high-order statistics on a realistic cut sky, we have characterized the transition from the linear to the nonlinear regime at $\ell \simeq 1000$ and shown that realistic galactic masking affects high-order moments only below $\ell < 200$. Each domain (Gaussian and non-Gaussian) spans 2 decades in angular scale. This map is therefore an ideal tool for testing map-making algorithms on the sphere. As a first step in addressing the full map reconstruction problem, we have benchmarked in this paper two denoising methods: 1) Wiener filtering applied to the Spherical Harmonics decomposition of the map and 2) a new method, called MRLens, based on the modification of the Maximum Entropy Method on a Wavelet decomposition. While the latter is optimal on large spatial scales, where the signal is Gaussian, MRLens outperforms the Wiener method on small spatial scales, where the signal is highly non-Gaussian. The simulated full-sky convergence map is freely available to the community to help the development of new map-making algorithms dedicated to the next generation of weak-lensing surveys.

astro-ph↗

arXiv2008

Large scale galactic turbulence: can self-gravity drive the observed HI velocity dispersions?

Observations of turbulent velocity dispersions in the HI component of galactic disks show a characteristic floor in galaxies with low star formation rates and within individual galaxies the dispersion profiles decline with radius. We carry out several high resolution adaptive mesh simulations of gaseous disks embedded within dark matter haloes to explore the roles of cooling, star-formation, feedback, shearing motions and baryon fraction in driving turbulent motions. In all simulations the disk slowly cools until gravitational and thermal instabilities give rise to a multi-phase medium in which a large population of dense self-gravitating cold clouds are embedded within a warm gaseous phase that forms through shock heating. The diffuse gas is highly turbulent and is an outcome of large scale driving of global non-axisymmetric modes as well as cloud-cloud tidal interactions and merging. At low star-formation rates these processes alone can explain the observed HI velocity dispersion profiles and the characteristic value of ~10 km/s observed within a wide range of disk galaxies. Supernovae feedback creates a significant hot gaseous phase and is an important driver of turbulence in galaxies with a star-formation rate per unit area >10^-3 M_sun/yr/kpc^2.

astro-ph↗

arXiv2008

Hydrodynamical Adaptive Mesh Refinement Simulations of Disk Galaxies

To date, fully cosmological hydrodynamic disk simulations to redshift zero have only been undertaken with particle-based codes, such as GADGET, Gasoline, or GCD+. In light of the (supposed) limitations of traditional implementations of smoothed particle hydrodynamics (SPH), or at the very least, their respective idiosyncrasies, it is important to explore complementary approaches to the SPH paradigm to galaxy formation. We present the first high-resolution cosmological disk simulations to redshift zero using an adaptive mesh refinement (AMR)-based hydrodynamical code, in this case, RAMSES. We analyse the temporal and spatial evolution of the simulated stellar disks' vertical heating, velocity ellipsoids, stellar populations, vertical and radial abundance gradients (gas and stars), assembly/infall histories, warps/lopsideness, disk edges/truncations (gas and stars), ISM physics implementations, and compare and contrast these properties with our sample of cosmological SPH disks, generated with GCD+. These preliminary results are the first in our long-term Galactic Archaeology Simulation program.

astro-ph↗

arXiv2008

Numerical simulations of galaxy evolution in cosmological context

Large volume cosmological simulations succeed in reproducing the large-scale structure of the Universe. However, they lack resolution and may not take into account all relevant physical processes to test if the detail properties of galaxies can be explained by the CDM paradigm. On the other hand, galaxy-scale simulations could resolve this in a robust way but do not usually include a realistic cosmological context. To study galaxy evolution in cosmological context, we use a new method that consists in coupling cosmological simulations and galactic scale simulations. For this, we record merger and gas accretion histories from cosmological simulations and re-simulate at very high resolution the evolution of baryons and dark matter within the virial radius of a target galaxy. This allows us for example to better take into account gas evolution and associated star formation, to finely study the internal evolution of galaxies and their disks in a realistic cosmological context. We aim at obtaining a statistical view on galaxy evolution from z = 2 to 0, and we present here the first results of the study: we mainly stress the importance of taking into account gas accretion along filaments to understand galaxy evolution.

astro-ph↗

arXiv2008

Cosmic velocity--gravity relation in redshift space

We propose a simple way to estimate the parameter beta = Omega_m^(0.6)/b from three-dimensional galaxy surveys. Our method consists in measuring the relation between the cosmological velocity and gravity fields, and thus requires peculiar velocity measurements. The relation is measured *directly in redshift space*, so there is no need to reconstruct the density field in real space. In linear theory, the radial components of the gravity and velocity fields in redshift space are expected to be tightly correlated, with a slope given, in the distant observer approximation, by g / v = (1 + 6 beta / 5 + 3 beta^2 / 7)^(1/2) / beta. We test extensively this relation using controlled numerical experiments based on a cosmological N-body simulation. To perform the measurements, we propose a new and rather simple adaptive interpolation scheme to estimate the velocity and the gravity field on a grid. One of the most striking results is that nonlinear effects, including `fingers of God', affect mainly the tails of the joint probability distribution function (PDF) of the velocity and gravity field: the 1--1.5 sigma region around the maximum of the PDF is *dominated by the linear theory regime*, both in real and redshift space. This is understood explicitly by using the spherical collapse model as a proxy of nonlinear dynamics. Applications of the method to real galaxy catalogs are discussed, including a preliminary investigation on homogeneous (volume limited) `galaxy' samples extracted from the simulation with simple prescriptions based on halo and sub-structure identification, to quantify the effects of the bias between the galaxy and the total matter distibution, and of shot noise (ABRIDGED).

astro-ph↗

arXiv2008

Protostellar collapse: A comparison between SPH and AMR calculations

The development of parallel supercomputers allows today the detailed study of the collapse and the fragmentation of prestellar cores with increasingly accurate numerical simulations. Thanks to the advances in sub-millimeter observations, a wide range of observed initial conditions enable us to study the different modes of low-mass star formation. The challenge for the simulations is to reproduce the observational results. Two main numerical methods, namely AMR and SPH, are widely used to simulate the collapse and the fragmentation of prestellar cores. We compare thoroughly these two methods within their standard framework. We use the AMR code RAMSES and the SPH code DRAGON. Our physical model is as simple as possible and consists of an isothermal sphere rotating around the z-axis. We first study the conservation of angular momentum as a function of the resolution. Then, we explore a wide range of simulation parameters to study the fragmentation of prestellar cores. There seems to be a convergence between the two methods, provided resolution in each case is sufficient. Resolution criteria adapted to our physical cases, in terms of resolution per Jeans mass, for an accurate description of the formation of protostellar cores are deduced from the present study. This convergence is encouraging for future work in simulations of low-mass star formation, providing the aforementioned criteria are fulfilled. Higher resolution figures can be downloaded at http://www-dapnia.cea.fr/Projets/COAST/paper_amrvssph.pdf

astro-ph↗

arXiv2007

A radiative transfer scheme for cosmological reionization based on a local Eddington tensor

A radiative transfer scheme is presented, based on a moment description of the equation of radiative transfer and the so-called ``M1 closure model'' for the Eddington tensor. This model features a strictly hyperbolic transport step for radiation: it has been implemented using standard Godunov--like techniques in a new code called ATON. Coupled to simple models of ionization chemistry and photo-heating, ATON is able to reproduce the results of other schemes on a various set of standard tests such as the expansion of a HII region, the shielding of the radiation by dense clumps and cosmological ionization by multiple sources. Being simple yet robust, such a scheme is intended to be naturally and easily included in grid--based cosmological fluid solvers.

astro-ph↗

arXiv2007

On the onset of galactic winds in quiescent star forming galaxies

We studied the effect of supernovae feedback on a disk galaxy, taking into account the impact of infalling gas on both the star formation history and the corresponding outflow structure, the apparition of a supernovae-driven wind being highly sensitive to the halo mass, the galaxy spin and the star formation efficiency. We model our galaxies as cooling and collapsing NFW spheres. The dark matter component is modelled as a static external potential, while the baryon component is described by the Euler equations using the AMR code RAMSES. Metal-dependent cooling and supernovae-heating are also implemented using state-of-the-art recipes coming from cosmological simulations. We allow for 3 parameters to vary: the halo circular velocity, the spin parameter and the star formation efficiency. We found that the ram pressure of infalling material is the key factor limiting the apparition of galactic winds. We obtain a very low feedback efficiency, with supernovae to wind energy conversion factor around one percent, so that only low cicrular velocity galaxies give rise to strong winds. For massive galaxies, we obtain a galatic fountain, for which we discuss the observational properties. We conclude that for quiescent isolated galaxies, galactic winds appear only in very low mass systems. Although that can quite efficiently enrich the IGM with metals, they don't carry away enough cold material to solve the overcooling problem.

astro-ph↗

arXiv2006

Metal enrichment in galactic winds

Observations give evidences of the presence of metals in the intergalactic medium (IGM). The stars responsible for transforming hydrogen and helium into more complex atoms do not form outside the galaxies in the standard scenario of galaxy formation. Supernovae-driven winds and their associated feedback was proposed as a possible solution to explain such enrichment of the IGM. It turned out that a proper modelling of supernovae explosions within a turbulent interstellar medium (ISM) is a difficult task. Recent advances have been obtained using a multiphase approach to solve for the thermal state of the ISM, plus some additional recipes to account for the kinetic effect of supernovae on the galactic gas. We briefly describe here our implementation of supernovae feedback within the RAMSES code, and apply it to the formation and evolution of isolated galaxies of various masses and angular momenta. We have explored under what conditions a galactic wind can develop, if one considers only a quiescent mode of star formation. We have also characterized the distribution and evolution of metallicity in the gas outflow spreading in the IGM.

astro-ph↗

arXiv2006

Fundamental differences between SPH and grid methods

We have carried out a hydrodynamical code comparison study of interacting multiphase fluids. The two commonly used techniques of grid and smoothed particle hydrodynamics (SPH) show striking differences in their ability to model processes that are fundamentally important across many areas of astrophysics. Whilst Eulerian grid based methods are able to resolve and treat important dynamical instabilities, such as Kelvin-Helmholtz or Rayleigh-Taylor, these processes are poorly or not at all resolved by existing SPH techniques. We show that the reason for this is that SPH, at least in its standard implementation, introduces spurious pressure forces on particles in regions where there are steep density gradients. This results in a boundary gap of the size of the SPH smoothing kernel over which information is not transferred.

astro-ph↗

arXiv2006

Soft gamma-ray background and light Dark Matter annihilation

The bulk of the extragalactic background between 10 keV and 10 GeV is likely to be explained by the emission of Seyfert galaxies, type Ia supernovae, and blazars. However, as revealed by the INTEGRAL satellite, the bulge of our galaxy is an intense source of a 511 keV gamma-ray line, indicating the production of a large number of positrons that annihilate. The origin of the latter is debated, and they could be produced, in particular, by the (S- or P-wave) annihilation of light Dark Matter particles into e+e-. In any case, the cumulated effect of similar sources at all redshifts could lead to a new background of hard X-ray and soft gamma-ray photons. On the basis of the hierarchical model of galaxy formation, we compute analytically the SNIa contribution to the background, and add it to Seyfert and blazars emission models. We find that any extra contribution to this unresolved background at 511 keV should be lower than about 4 keV/cm^2/s/sr. We also estimate analytically the extragalactic background due to Dark Matter annihilation, increasing the accuracy of the earlier computations. Indeed, we take into account the large positron escape fraction from low mass dark matter halos, unable to confine a dense and magnetized interstellar medium. Our new background estimate turns out to be one order of magnitude lower, so that the hypothesis of a light Dark Matter candidate remains compatible with the observed extragalactic background for a wider range of particle masses and cross-sections.

astro-ph↗

arXiv2005

The History of the Baryon Budget: Cosmic Logistics in a Hierarchical Universe

Using a series of high-resolution N-body hydrodynamical numerical simulations, we investigate several scenarios for the evolution of the baryon budget in galactic halos. We derive individual halo star formation history (SFH), as well as the global star formation rate in the universe. We develop a simple analytical model that allows us to compute surprisingly accurate predictions, when compared to our simulations, but also to other simulations presented in Springel & Hernquist (2003). The model depends on two main parameters: the star formation time scale t* and the wind efficiency eta_w. We also compute, for halos of a given mass, the baryon fraction in each of the following phases: cold disc gas, hot halo gas and stars. Here again, our analytical model predictions are in good agreement with simulation results, if one correctly takes into account finite resolution effect. We compare predictions of our analytical model to several observational constraints, and conclude that a very narrow range of the model parameters is allowed. The important role played by galactic winds is outlined, as well as a possible `superwind' scenario in groups and clusters. The `anti-hierarchical' behavior of observed SFH is well reproduced by our best model with t*=3Gyr and eta_w=1.5. We obtain in this case a present-day cosmic baryon budget of Omega*= 0.004, Omega_cold=0.0004, Omega_hot=0.01 and Omega_back=0.02 (diffuse background).

astro-ph↗

arXiv2001

Cosmological Hydrodynamics with Adaptive Mesh Refinement: a new high resolution code called RAMSES

A new N-body and hydrodynamical code, called RAMSES, is presented. It has been designed to study structure formation in the universe with high spatial resolution. The code is based on Adaptive Mesh Refinement (AMR) technique, with a tree based data structure allowing recursive grid refinements on a cell-by-cell basis. The N-body solver is very similar to the one developed for the ART code (Kravtsov et al. 97), with minor differences in the exact implementation. The hydrodynamical solver is based on a second-order Godunov method, a modern shock-capturing scheme known to compute accurately the thermal history of the fluid component. The accuracy of the code is carefully estimated using various test cases, from pure gas dynamical tests to cosmological ones. The specific refinement strategy used in cosmological simulations is described, and potential spurious effects associated to shock waves propagation in the resulting AMR grid are discussed and found to be negligible. Results obtained in a large N-body and hydrodynamical simulation of structure formation in a low density LCDM universe are finally reported, with 256^3 particles and 4.1 10^7 cells in the AMR grid, reaching a formal resolution of 8192^3. A convergence analysis of different quantities, such as dark matter density power spectrum, gas pressure power spectrum and individual haloes temperature profiles, shows that numerical results are converging down to the actual resolution limit of the code, and are well reproduced by recent analytical predictions in the framework of the halo model.

astro-ph↗

arXiv2000

Numerical and Analytical Predictions for the Large-Scale Sunyaev-Zel'dovich Effect

The hot gas embedded in the large-scale structures in the Universe produces secondary fluctuations in the Cosmic Microwave Background (CMB). Because it is proportional to the gas pressure integrated along the line of sight, this effect, the thermal Sunyaev-Zel'dovich (SZ) effect, provides a direct measure of large-scale structure and of cosmological parameters. We study the statistical properties of this effect using both hydrodynamical simulations and analytical predictions from an extended halo model. The Adaptive Mesh Refinement scheme, used in the newly developed code RAMSES, provides a dynamic range of 4 order of magnitudes, and thus allows us to significantly improve upon earlier calculations. After accounting for the finite mass resolution and box size of the simulation, we find that the halo model agrees well with the simulations. We discuss and quantify the uncertainty in both methods, and thus derive an accurate prediction for the SZ power spectrum in the 10^2<l<10^5 range of multipole. We show how this combined analytical and numerical approach is essential for accuracy, and useful for the understanding of the physical processes and scales which contribute to large-scale SZ anisotropies.

astro-ph↗

arXiv1998

Non equilibrium thermodynamics and cosmological pancakes formation

We investigate the influence of non equilibrium thermodynamics on cosmological structure formation. In this paper, we consider the collapse of planar perturbations usually called "Zel'dovich pancakes". We have developed for that purpose a new two fluids (gas and dark matter) hydrodynamical code, with three different thermodynamical species: electrons, ions and neutral particles (T_e\ne T_i \ne T_n). We describe in details the complex structure of accretion shock waves. We include several relevant processes for a low density, high temperature, collisional plasma such as non-equilibrium chemical reactions, cooling, shock heating, thermal energy equipartition between electrons, ions and neutral particles and electronic conduction. We find two different regions in the pancake structure: a thermal precursor ahead of the compression front and an equipartition wave after the compression front where electrons and ions temperatures differ significantly. This complex structure may have two interesting consequences: pre-heating of unshocked regions in the vicinity of massive X-ray clusters and ions and electrons temperatures differences in the outer regions of X-rays clusters.

astro-ph↗