Search

Rotationally supported, cold, gaseous discs are ubiquitous in astrophysics and appear in a diverse set of systems, such as protoplanetary discs, accretion discs around black holes, or large spiral galaxies. Capturing the gas dynamics accurately in these systems is challenging in numerical simulations due to the low sound speed compared to the bulk velocity of the gas, the resolution limitations of full disc models, and the fact that numerical noise can easily source spurious growth of fluid instabilities if not suppressed sufficiently well, negatively interfering with real physical instabilities present in such discs (like the magnetorotational instability). Here, we implement the so-called shearing-box approximation in the moving-mesh code arepo in order to facilitate achieving high resolution in local regions of differentially rotating discs and to address these problems. While our new approach offers manifest translational invariance across the shearing-box boundaries and offers continuous local adaptivity, we demonstrate that the unstructured mesh of arepo introduces unwanted levels of 'grid-noise' in the default version of the code. We show that this can be rectified by high-order integrations of the flux over mesh boundaries. With our new techniques we obtain highly accurate results for shearing-box calculations of the magnetorotational instability that are superior to other Lagrangian techniques. These improvements are also of value for other applications of the code that feature strong shear flows. [ABSTRACT FROM AUTHOR]

Accurate numerical solutions of the equations of hydrodynamics play an ever more important role in many fields of astrophysics. In this work, we reinvestigate the accuracy of the movingmesh code AREPO and show how its convergence order can be improved for general problems. In particular, we clarify that for certain problems AREPO only reaches first-order convergence for its original formulation. This can be rectified by simple modifications we propose to the time integration scheme and the spatial gradient estimates of the code, both improving the accuracy of the code. We demonstrate that the new implementation is indeed second-order accurate under the L1 norm, and in particular substantially improves conservation of angular momentum. Interestingly, whereas these improvements can significantly change the results of smooth test problems, we also find that cosmological simulations of galaxy formation are unaffected, demonstrating that the numerical errors eliminated by the new formulation do not impact these simulations. In contrast, simulations of binary stars followed over a large number of orbital times are strongly affected, as here it is particularly crucial to avoid a long-term build up of errors in angular momentum conservation. [ABSTRACT FROM AUTHOR]

Lagrangian smoothed particle hydrodynamics (SPH) is a well-established approach to model fluids in astrophysical problems, thanks to its geometric flexibility and ability to automatically adjust the spatial resolution to the clumping of matter. However, a number of recent studies have emphasized inaccuracies of SPH in the treatment of fluid instabilities. The origin of these numerical problems can be traced back to spurious surface effects across contact discontinuities, and to SPH's inherent prevention of mixing at the particle level. We here investigate a new fluid particle model where the density estimate is carried out with the help of an auxiliary mesh constructed as the Voronoi tessellation of the simulation particles instead of an adaptive smoothing kernel. This Voronoi-based approach improves the ability of the scheme to represent sharp contact discontinuities. We show that this eliminates spurious surface tension effects present in SPH and that play a role in suppressing certain fluid instabilities. We find that the new ‘Voronoi Particle Hydrodynamics’ (VPH) described here produces comparable results to SPH in shocks, and better ones in turbulent regimes of pure hydrodynamical simulations. We also discuss formulations of the artificial viscosity needed in this scheme and how judiciously chosen correction forces can be derived in order to maintain a high degree of particle order and hence a regular Voronoi mesh. This is especially helpful in simulating self-gravitating fluids with existing gravity solvers used for N-body simulations. [ABSTRACT FROM AUTHOR]

This review discusses smoothed particle hydrodynamics (SPH) in the astrophysical context, with a focus on inviscid gas dynamics. The particle-based SPH technique allows an intuitive and simple formulation of hydrodynamics that has excellent conservation properties and can be coupled to self-gravity with high accuracy. The Lagrangian character of SPH allows it to automatically adjust its resolution to the clumping of matter, a property that makes the scheme ideal for many application areas in astrophysics, where often a large dynamic range in density is encountered. We discuss the derivation of the basic SPH equations in their modern formulation, and give an overview about extensions of SPH developed to treat physics such as radiative transfer, thermal conduction, relativistic dynamics, or magnetic fields. We also briefly describe some of the most important applications areas of SPH in astrophysical research. Finally, we provide a critical discussion of the accuracy of SPH for different hydrodynamical problems, including measurements of its convergence rate for important classes of problems. [ABSTRACT FROM AUTHOR]

Hydrodynamic cosmological simulations at present usually employ either the Lagrangian smoothed particle hydrodynamics (SPH) technique or Eulerian hydrodynamics on a Cartesian mesh with (optional) adaptive mesh refinement (AMR). Both of these methods have disadvantages that negatively impact their accuracy in certain situations, for example the suppression of fluid instabilities in the case of SPH, and the lack of Galilean invariance and the presence of overmixing in the case of AMR. We here propose a novel scheme which largely eliminates these weaknesses. It is based on a moving unstructured mesh defined by the Voronoi tessellation of a set of discrete points. The mesh is used to solve the hyperbolic conservation laws of ideal hydrodynamics with a finite-volume approach, based on a second-order unsplit Godunov scheme with an exact Riemann solver. The mesh-generating points can in principle be moved arbitrarily. If they are chosen to be stationary, the scheme is equivalent to an ordinary Eulerian method with second-order accuracy. If they instead move with the velocity of the local flow, one obtains a Lagrangian formulation of continuum hydrodynamics that does not suffer from the mesh distortion limitations inherent in other mesh-based Lagrangian schemes. In this mode, our new method is fully Galilean invariant, unlike ordinary Eulerian codes, a property that is of significant importance for cosmological simulations where highly supersonic bulk flows are common. In addition, the new scheme can adjust its spatial resolution automatically and continuously, and hence inherits the principal advantage of SPH for simulations of cosmological structure growth. The high accuracy of Eulerian methods in the treatment of shocks is also retained, while the treatment of contact discontinuities improves. We discuss how this approach is implemented in our new codearepo, both in 2D and in 3D, and is parallelized for distributed memory computers. We also discuss techniques for adaptive refinement or de-refinement of the unstructured mesh. We introduce an individual time-step approach for finite-volume hydrodynamics, and present a high-accuracy treatment of self-gravity for the gas that allows the new method to be seamlessly combined with a high-resolution treatment of collisionless dark matter. We use a suite of test problems to examine the performance of the new code and argue that the hydrodynamic moving-mesh scheme proposed here provides an attractive and competitive alternative to current SPH and Eulerian techniques. [ABSTRACT FROM AUTHOR]

Using a new numerical model for cosmic chemical evolution, we study the influence of hypernova feedback on the star formation and metal enrichment history of the universe. For assumptions which produce plausible results in idealized collapse models of individual galaxies, our cosmological simulations of the standard Λ cold dark matter (CDM) cosmology show a peak of the cosmic star formation rate at , with ∼10 per cent of the baryons turning into stars. We find that the majority of stars in present-day massive galaxies formed in much smaller galaxies at high redshifts, giving them a mean stellar age as old as 10 Gyr, despite their late assembly times. The hypernova feedback drives galactic outflows efficiently in low-mass galaxies, and these winds eject heavy elements into the intergalactic medium. The ejected baryon fraction is larger for less massive galaxies, correlates well with stellar metallicity and amounts to ∼20 per cent of all baryons in total. The resulting enrichment history is broadly consistent with the observed abundances of Lyman break galaxies, of damped Lyman α systems, and of the intergalactic medium. The metallicity of the cold gas in galaxies increases with galaxy mass, which is comparable to observations with a significant scatter. The stellar mass–metallicity relation of the observed galaxy population is well reproduced by the simulation model as a result of the mass-dependent galactic winds. However, star formation does not terminate in massive galaxies at late times in our model, and too few dwarf galaxies are still forming stars. These problems may be due to a lack of resolution, to inappropriate modelling of supernova feedback or to a neglect of other feedback processes such as active galactic nuclei. [ABSTRACT FROM AUTHOR]

We discuss the cosmological simulation code gadget-2, a new massively parallel TreeSPH code, capable of following a collisionless fluid with the N-body method, and an ideal gas by means of smoothed particle hydrodynamics (SPH). Our implementation of SPH manifestly conserves energy and entropy in regions free of dissipation, while allowing for fully adaptive smoothing lengths. Gravitational forces are computed with a hierarchical multipole expansion, which can optionally be applied in the form of a TreePM algorithm, where only short-range forces are computed with the ‘tree’ method while long-range forces are determined with Fourier techniques. Time integration is based on a quasi-symplectic scheme where long-range and short-range forces can be integrated with different time-steps. Individual and adaptive short-range time-steps may also be employed. The domain decomposition used in the parallelization algorithm is based on a space-filling curve, resulting in high flexibility and tree force errors that do not depend on the way the domains are cut. The code is efficient in terms of memory consumption and required communication bandwidth. It has been used to compute the first cosmological N-body simulation with more than 1010 dark matter particles, reaching a homogeneous spatial dynamic range of 105 per dimension in a three-dimensional box. It has also been used to carry out very large cosmological SPH simulations that account for radiative cooling and star formation, reaching total particle numbers of more than 250 million. We present the algorithms used by the code and discuss their accuracy and performance using a number of test problems. gadget-2 is publicly released to the research community. [ABSTRACT FROM AUTHOR]

The cold dark matter model has become the leading theoretical picture for the formation of structure in the Universe. This model, together with the theory of cosmic inflation, makes a clear prediction for the initial conditions for structure formation and predicts that structures grow hierarchically through gravitational instability. Testing this model requires that the precise measurements delivered by galaxy surveys can be compared to robust and equally precise theoretical calculations. Here we present a simulation of the growth of dark matter structure using 2,1603 particles, following them from redshift z= 127 to the present in a cube-shaped region 2.230 billion lightyears on a side. In postprocessing, we also follow the formation and evolution of the galaxies and quasars. We show that baryon-induced features in the initial conditions of the Universe are reflected in distorted form in the low-redshift galaxy distribution, an effect that can be used to constrain the nature of dark energy with future generations of observational surveys of galaxies. [ABSTRACT FROM AUTHOR]

Modern astrophysical simulations aim to accurately model an ever-growing array of physical processes, including the interaction of fluids with magnetic fields, under increasingly stringent performance and scalability requirements driven by present-day trends in computing architectures. Discontinuous Galerkin (DG) methods have recently gained some traction in astrophysics, because of their arbitrarily high order and controllable numerical diffusion, combined with attractive characteristics for high-performance computing. In this paper, we describe and test our implementation of a DG scheme for ideal magnetohydrodynamics (MHD) in the arepo-dg code. Our DG-MHD scheme relies on a modal expansion of the solution on Legendre polynomials inside the cells of an Eulerian octree-based adaptive mesh refinement grid. The divergence-free constraint of the magnetic field is enforced using one out of two distinct cell-centred schemes: either a Powell-type scheme based on non-conservative source terms, or a hyperbolic divergence cleaning method. The Powell scheme relies on a basis of locally divergence-free vector polynomials inside each cell to represent the magnetic field. Limiting prescriptions are implemented to ensure non-oscillatory and positive solutions. We show that the resulting scheme is accurate and robust: it can achieve high-order and low numerical diffusion, as well as accurately capture strong MHD shocks. In addition, we show that our scheme exhibits a number of attractive properties for astrophysical simulations, such as lower advection errors and better Galilean invariance at reduced resolution, together with more accurate capturing of barely resolved flow features. We discuss the prospects of our implementation, and DG methods in general, for scalable astrophysical simulations. [ABSTRACT FROM AUTHOR]

We introduce a set of four very high resolution cosmological simulations exploring f (R) gravity, with 20483 particles in 768 and |$1536 \, h^{-1} \, {\rm Mpc}$| simulation boxes, for |$|\bar{f}_{R0}| = 10^{-5}$| and Λ cold dark matter (ΛCDM), making the set the largest simulations of f (R) gravity to date. To mimic real observations, the simulations include a continuous 2D- and 3D-light-cone output dedicated to study lensing and clustering statistics. We present a detailed analysis and resolution study for the matter power spectrum in f (R) gravity over a wide range of scales. We also analyse the angular matter power spectrum and lensing convergence on the light-cone. In addition, we investigate the impact of modified gravity on the halo mass function, matter, and halo autocorrelation functions, linear halo bias, and the concentration–mass relation. The impact of f (R) gravity is generally larger on smaller scales and smaller redshift. Comparing our simulations to state-of-the-art hydrodynamical simulations, we confirm a degeneracy between f (R) gravity and baryonic feedback in the matter power spectrum on small scales, but also find that scales around |$k = 1 \, h\, {\rm Mpc}^{-1}$| are promising to distinguish both effects. The lensing convergence power spectrum is increased in f (R) gravity. Numerical fits are in good agreement with our simulations for both standard and modified gravity, but tend to overestimate their relative difference on non-linear scales. The halo bias is lower in f (R) gravity, whereas halo concentrations are increased for unscreened haloes. [ABSTRACT FROM AUTHOR]

We use a very large simulation of the concordance Λ cold dark matter cosmogony to study the clustering of dark matter haloes. For haloes less massive than about the amplitude of the two-point correlation function on large scales depends strongly on halo formation time. Haloes that assembled at high redshift are substantially more clustered than those that assembled more recently. The effect is a smooth function of halo formation time and its amplitude increases with decreasing halo mass. At the ‘oldest’ 10 per cent of haloes are more than five times more strongly correlated than the ‘youngest’ 10 per cent. This unexpected result is incompatible with the standard excursion set theory for structure growth, and it contradicts a fundamental assumption of the halo occupation distribution models often used to study galaxy clustering, namely that the galaxy content of a halo of given mass is statistically independent of its larger scale environment. [ABSTRACT FROM AUTHOR]

We present an overview of galaxy evolution across cosmic time in the Illustris simulation. Illustris is an N-body/hydrodynamical simulation that evolves 2 × 18203 resolution elements in a (106.5 Mpc)3 box from cosmological initial conditions down to z = 0 using the arepo moving-mesh code. The simulation uses a state-of-the-art set of physical models for galaxy formation that was tuned to reproduce the z = 0 stellar mass function and the history of the cosmic star formation rate density. We find that Illustris successfully reproduces a plethora of observations of galaxy populations at various redshifts, for which no tuning was performed, and provide predictions for future observations. In particular, we discuss (a) the buildup of galactic mass, showing stellar mass functions and the relations between stellar mass and halo mass from z = 7 to 0, (b) galaxy number density profiles around massive central galaxies out to z = 4, (c) the gas and total baryon content of both galaxies and their haloes for different redshifts, and as a function of mass and radius, and (d) the evolution of galaxy specific star formation rates up to z = 8. In addition, we (i) present a qualitative analysis of galaxy morphologies from z = 5 to 0, for the stellar as well as the gaseous components, and their appearance in Hubble Space Telescope mock observations, (ii) follow galaxies selected at z = 2 to their z = 0 descendants, and quantify their growth and merger histories, and (iii) track massive z = 0 galaxies to high redshift and study their joint evolution in star formation activity and compactness. We conclude with a discussion of several disagreements with observations, and lay out possible directions for future research. [ABSTRACT FROM PUBLISHER]

ABSTRACT Magnetic fields play an important role in astrophysics on a wide variety of scales, ranging from the Sun and compact objects to galaxies and galaxy clusters. Here we discuss a novel implementation of ideal magnetohydrodynamics (MHD) in the moving-mesh code arepo which combines many of the advantages of Eulerian and Lagrangian methods in a single computational technique. The employed grid is defined as the Voronoi tessellation of a set of mesh-generating points which can move along with the flow, yielding an automatic adaptivity of the mesh and a substantial reduction of advection errors. Our scheme solves the MHD Riemann problem in the rest frame of the Voronoi interfaces using the HLLD Riemann solver. To satisfy the divergence constraint of the magnetic field in multiple dimensions, the Dedner divergence cleaning method is applied. In a set of standard test problems, we show that the new code produces accurate results and that the divergence of the magnetic field is kept sufficiently small to closely preserve the correct physical solution. We also apply the code to two first application problems, namely supersonic MHD turbulence and the spherical collapse of a magnetized cloud. We verify that the code is able to handle both problems well, demonstrating the applicability of this MHD version of arepo to a wide range of problems in astrophysics. [ABSTRACT FROM AUTHOR]

ABSTRACT We examine predictions for the quasar luminosity functions (QLFs) and quasar clustering at high redshift ( z ≥ 4.75) using MassiveBlack, our new hydrodynamic cosmological simulation which includes a self-consistent model for black hole (BH) growth and feedback. We show that the model reproduces the Sloan QLF within observational constraints at z ≥ 5. We find that the high- z QLF is consistent with a redshift-independent occupation distribution of BHs among dark matter haloes (which we provide) such that the evolution of the QLF follows that of the halo mass function. The sole exception is the bright end at z = 6 and 7, where BHs in high-mass haloes tend to be unusually bright due to extended periods of Eddington growth caused by high-density cold flows into the halo centre. We further use these luminosity functions to make predictions for the number density of quasars in upcoming surveys, predicting that there should be ∼119 ± 28 (∼87 ± 28) quasars detectable in the F125 W band of the WIDE (DEEP) fields of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) from z = 5 to 6, ∼19 ± 7 (∼18 ± 9) from z = 6 to 7 and ∼1.7 ± 1.5 (∼1.5 ± 1.5) from z = 7 to 8. We also investigate quasar clustering, finding that the correlation length is fully consistent with current constraints for Sloan quasars ( r0 ∼ 17 h−1 Mpc at z = 4 for quasars above m i = 20.2) and grows slowly with redshift up to z = 6 ( r0 ∼ 22 h−1 Mpc). Finally, we note that the quasar clustering strength depends weakly on luminosity for low LBH, but gets stronger at higher LBH as the BHs are found in higher mass haloes. [ABSTRACT FROM AUTHOR]

ABSTRACT We analyse the coarse-grained phase-space structure of the six Galaxy-scale dark matter haloes of the Aquarius Project using a state-of-the-art 6D substructure finder. Within r50, we find that about 35 per cent of the mass is in identifiable substructures, predominantly tidal streams, but including about 14 per cent in self-bound subhaloes. The slope of the differential substructure mass function is close to −2, which should be compared to ∼−1.9 for the population of self-bound subhaloes. Near r50, about 60 per cent of the mass is in substructures, with about 30 per cent in self-bound subhaloes. The inner 35 kpc of the highest resolution simulation has only 0.5 per cent of its mass in self-bound subhaloes, but 3.3 per cent in detected substructure, again primarily tidal streams. The densest tidal streams near the solar position have a 3D mass density about 1 per cent of the local mean, and populate the high-velocity tail of the velocity distribution. [ABSTRACT FROM AUTHOR]

We have carried out a comparison study of hydrodynamical codes by investigating their performance in modelling 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 an SPH smoothing kernel radius over which interactions are severely damped. [ABSTRACT FROM AUTHOR]