The ever increasing size and complexity of data coming from simulations of cosmic structure formation demand equally sophisticated tools for their analysis. During the past decade, the art of object ...finding in these simulations has hence developed into an important discipline itself. A multitude of codes based upon a huge variety of methods and techniques have been spawned yet the question remained as to whether or not they will provide the same (physical) information about the structures of interest. Here we summarize and extent previous work of the 'halo finder comparison project': we investigate in detail the (possible) origin of any deviations across finders. To this extent, we decipher and discuss differences in halo-finding methods, clearly separating them from the disparity in definitions of halo properties. We observe that different codes not only find different numbers of objects leading to a scatter of up to 20 per cent in the halo mass and V
max function, but also that the particulars of those objects that are identified by all finders differ. The strength of the variation, however, depends on the property studied, e.g. the scatter in position, bulk velocity, mass and the peak value of the rotation curve is practically below a few per cent, whereas derived quantities such as spin and shape show larger deviations. Our study indicates that the prime contribution to differences in halo properties across codes stems from the distinct particle collection methods and - to a minor extent - the particular aspects of how the procedure for removing unbound particles is implemented. We close with a discussion of the relevance and implications of the scatter across different codes for other fields such as semi-analytical galaxy formation models, gravitational lensing and observables in general.
We present a parallel implementation of the friends-of-friends algorithm and an innovative technique for reducing complex-shaped data to a user-friendly format. This code, named pFoF, contains an ...optimized post-processing workflow that reduces the input data coming from gravitational codes, arranges them in a user-friendly format and detects groups of particles using percolation and merging methods. The pFoF code also allows for detecting structures in sub- or non-cubic volumes of the comoving box. In addition, the code offers the possibility of performing new halo-findings with a lower percolation factor, useful for more complex analysis. In this paper, we give standard test results and show performance diagnostics to stress the robustness of pFoF. This code has been extensively tested up to 32768 MPI processes and has proved to be highly scalable with an efficiency of more than 75%. It has been used for analysing the Dark Energy Universe Simulation: Full Universe Runs (DEUS-FUR) project, the first cosmological simulations of the entire observable Universe, modelled with more than half a trillion dark matter particles.
Context. It has been found from ISO, Spitzer, and Herschel observations that molecular hydrogen, H2, can form on warm grains. Numerical models of interstellar chemistry have failed to reproduce the ...observed formation rates of H2, which remains a difficulty when interpreting observations of photon-dominated regions (PDRs). Aims: We attempt to include as much experimental and theoretical information as possible to describe H2 formation in astrophysical environments to solve this problem. Methods: We modified our "Meudon PDR code" to include a detailed treatment of H2 formation mechanisms including: i) the Langmuir-Hinshelwood mechanism taking into account the contribution of the different sizes of dust grains in the diffusion processes; and ii) the Eley-Rideal mechanism. Results: We are able to form H2 even in regions where the dust temperature is higher than 25 K. We also show that formation by the Eley-Rideal mechanism can be a significant source of gas heating. We derive line intensities for various astrophysical conditions. Conclusions: Our approach results in a higher H2 formation rate than for the "standard" 3 × 10-17 nH n(H) cm3 s-1 expression.
Haloes gone MAD: The Halo-Finder Comparison Project Knebe, Alexander; Knollmann, Steffen R.; Muldrew, Stuart I. ...
Monthly notices of the Royal Astronomical Society,
08/2011, Letnik:
415, Številka:
3
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
We present a detailed comparison of fundamental dark matter halo properties retrieved by a substantial number of different halo finders. These codes span a wide range of techniques including ...friends‐of‐friends, spherical‐overdensity and phase‐space‐based algorithms. We further introduce a robust (and publicly available) suite of test scenarios that allow halo finder developers to compare the performance of their codes against those presented here. This set includes mock haloes containing various levels and distributions of substructure at a range of resolutions as well as a cosmological simulation of the large‐scale structure of the universe.
All the halo‐finding codes tested could successfully recover the spatial location of our mock haloes. They further returned lists of particles (potentially) belonging to the object that led to coinciding values for the maximum of the circular velocity profile and the radius where it is reached. All the finders based in configuration space struggled to recover substructure that was located close to the centre of the host halo, and the radial dependence of the mass recovered varies from finder to finder. Those finders based in phase space could resolve central substructure although they found difficulties in accurately recovering its properties. Through a resolution study we found that most of the finders could not reliably recover substructure containing fewer than 30–40 particles. However, also here the phase‐space finders excelled by resolving substructure down to 10–20 particles. By comparing the halo finders using a high‐resolution cosmological volume, we found that they agree remarkably well on fundamental properties of astrophysical significance (e.g. mass, position, velocity and peak of the rotation curve).
We further suggest to utilize the peak of the rotation curve, vmax, as a proxy for mass, given the arbitrariness in defining a proper halo edge.
The Vishniac instability is thought to explain the complex structure of radiative supernova remnants in their Pressure-Driven Thin Shell (PDTS) phase after a blast wave (BW) has propagated from a ...central explosion. In this paper, the propagation of the BW and the evolution of the PDTS stage are studied numerically with the two-dimensional (2D) code HYDRO-MUSCL for a finite-thickness shell expanding in the interstellar medium (ISM). The overstable mechanism, previously demonstrated theoretically by E. T. Vishniac in 1983, is recovered numerically in the linear stage and is expected to produce and enhance anisotropies and clumps on the shock front, leading to the disruption of the shell in the nonlinear phase. As a result, our model shows that the supernova remnant returns to a stable evolution and the Vishniac instability does not lead to the fragmentation of the shock as predicted by the theory.
The formation of self-gravitating systems is studied by simulating the collapse of a set of N particles which are generated from several distribution functions. We first establish that the results of ...such simulations depend on N for small values of N. We complete a previous work by Aguilar & Merritt concerning the morphological segregation between spherical and elliptical equilibria. We find and interpret two new segregations: one concerns the equilibrium core size and the other the equilibrium temperature. All these features are used to explain some of the global properties of self-gravitating objects: origin of globular clusters and central black hole or shape of elliptical galaxies.
The Vishniac instability is supposed to explain the fragmentation of the thin shell of shocked matter in the radiative phase of supernova remnants. However its implication and its consequence on the ...morphological evolution of stellar systems is not fully demonstrated. The present paper tackles this subject by numerical simulations and focus on the role of the adiabatic index in the instability growth. The HYDRO-MUSCL 2D hydrodynamics code has been used to simulate the evolution of a supernova remnant thin shell and the triggering of the Vishniac instability in this thin shell. We have studied the temporal behavior of the perturbation. The first result of the numerical study is the existence of the Vishniac instability in the simulations. This result is proved by the overstability process observed in the simulations as predicted by the theoretical analysis. The second important result is the damping of the perturbation at late evolution and for all the set of parameters. Indeed the accretion of matter onto the shock damps the instability when theoretical analysis predicts its occurrence.
Haloes gone MAD14: The Halo-Finder Comparison Project Knebe, Alexander; Knollmann, Steffen R; Muldrew, Stuart I ...
Monthly notices of the Royal Astronomical Society,
06/2011, Letnik:
415, Številka:
3
Journal Article
Recenzirano
We present a detailed comparison of fundamental dark matter halo properties retrieved by a substantial number of different halo finders. These codes span a wide range of techniques including ...friends-of-friends, spherical-overdensity and phase-space-based algorithms. We further introduce a robust (and publicly available) suite of test scenarios that allow halo finder developers to compare the performance of their codes against those presented here. This set includes mock haloes containing various levels and distributions of substructure at a range of resolutions as well as a cosmological simulation of the large-scale structure of the universe.
All the halo-finding codes tested could successfully recover the spatial location of our mock haloes. They further returned lists of particles (potentially) belonging to the object that led to coinciding values for the maximum of the circular velocity profile and the radius where it is reached. All the finders based in configuration space struggled to recover substructure that was located close to the centre of the host halo, and the radial dependence of the mass recovered varies from finder to finder. Those finders based in phase space could resolve central substructure although they found difficulties in accurately recovering its properties. Through a resolution study we found that most of the finders could not reliably recover substructure containing fewer than 30-40 particles. However, also here the phase-space finders excelled by resolving substructure down to 10-20 particles. By comparing the halo finders using a high-resolution cosmological volume, we found that they agree remarkably well on fundamental properties of astrophysical significance (e.g. mass, position, velocity and peak of the rotation curve).
We further suggest to utilize the peak of the rotation curve, v
max, as a proxy for mass, given the arbitrariness in defining a proper halo edge.
Nous étudions la formation et les propriétés des systèmes auto-gravitants à l'aide de simulations numériques à N corps d'effondrements gravitationnels. Nous effectuons dans un premier temps une ...synthèse des principaux résultats analytiques concernant les équations de Boltzmann sans collisions et de Poisson, qui modélisent les systèmes gravitationnels non collisionnels ainsi que certaines solutions analytiques de ce système couplé d'équations. Nous présentons ensuite les codes de calcul utilisés pour les simulations. Nous avons parallélisé certains de ces codes, nous introduisons donc le calcul parallèle et la bibliothèque d'échange de message MPI. Nous exposons enfin les résultats de nos simulations, et leurs analyses. Nous déduisons de ces analyses divers résultats pouvant expliquer différentes caractéristiques des systèmes auto-gravitants ainsi que les conditions initiales nécessaires au déclenchement des instabilités d'Antonov et d'orbites radiales.
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