Collisionally Regenerated Dark Matter Structures in Galactic Nuclei

We show that the presence of a r−3/2 dark matter overdensity can be robustly predicted at the center of any galaxy old enough to have grown a power-law density cusp in the stars via the Bahcall-Wolf mechanism. Using both Fokker-Planck and direct N-body integrations, we demonstrate collisional generation of these dark matter “crests” (Collisionally REgenerated STtructures) even in the extreme case that the density of both stars and dark matter were previously lowered by slingshot ejection from a binary supermassive black hole. The time scale for collisional growth of the crest is approximately the two-body relaxation time as defined by the stars, which is . 10 Gyr at the centers of stellar spheroids with luminosities L . 109.5L⊙, including the bulge of the Milky Way. The presence of crests can robustly be predicted in such galaxies, unlike the steeper enhancements, called “spikes”, produced by the adiabatic growth of black holes. We discuss special cases where the prospects for detecting dark matter annihilations from the centers of galaxy haloes are significantly affected by the formation of crests

Die Entwicklung des Interstellaren Mediums in Galaxien

Im Rahmen dieser Arbeit wurde ein Programm zur Simulation der Entwicklung von Galaxien erstellt. Um eine Mehrphasen-Beschreibung des Interstellaren Mediums zu erreichen, wurde die TREESPH-Methode um ein sticky particle-Schema erweitert. Dabei wird mit SPH eine warme bis heiße diffuse Gasphase beschrieben, während die sticky particles eine kühle, in vielen Fragmenten vorliegende Komponente darstellen. Um einen Austausch von Materie und Impuls zwischen den beiden Gasphasen zu ermöglichen, wurden die Prozesse Kondensation, Evaporation und Reibung implementiert. Außerdem kann es durch Strahlungskühlung und inelastischen Stößen zwischen Wolken zu Energiedissipation kommen. Schließlich wird durch die Berücksichtigung der Sternentstehung und die damit verbundenen Rückkopplungsprozesse der Sterne auf das Interstellare Medium (ISM) der Materiekreislauf geschlossen. Dazu wurde ein neues Verfahren zur Sternentstehung entwickelt, bei dem der Prozess lokal für einzelne Wolken beschrieben wird. Durch eine von den örtlichen Bedingungen des ISMs abhängige Sternentstehungseffizienz wird außerdem die Möglichkeit zur Selbstregulierung gegeben. Das Programm wurde in einer Reihe von Simulationen auf die Entwicklung von isolierten Scheibengalaxien angewandt. Dabei wurde zunächst nur die Spätphase der Entwicklung - über einen Zeitraum von drei Milliarden Jahren - betrachtet. Die durchgeführten Modellrechnungen zeigen die erwartete stabile Entwicklung und ergeben weitgehende Übereinstimmung sowohl mit beobachteten Eigenschaften von Galaxien wie auch anderen numerischen oder theoretischen Modellen. So zeigen Beobachtungen z.B., dass die Sternentstehung in Galaxien durch ein Schmidt-Gesetz mit dem Index n zwischen 1.4 und 2 beschrieben werden kann; dies stimmt gut mit unseren Modelldaten überein. Ähnliches gilt für das Massenspektrum der Wolken, das in den Simulationen durch ein Potenzgesetz mit dem Index a~-2 beschrieben werden kann. Damit passt das Massenspektrum zu Beobachtungen und auch den Vorhersagen der entsprechenden theoretischen Modelle. Das diffuse ISM kann in eine warme und eine heiße Phase unterteilt werden. Erstere befindet sich in einer Scheibe während das heiße Gas überwiegend im Halo zu finden ist. In einem nächsten Schritt soll das Programm auf wechselwirkende Galaxien angewendet werden. Dabei soll u.a. untersucht werden unter welchen Voraussetzungen sogenannte star bursts ausgelöst werden können

SAPPORO: A way to turn your graphics cards into a GRAPE-6

We present Sapporo, a library for performing high-precision gravitational N-body simulations on NVIDIA Graphical Processing Units (GPUs). Our library mimics the GRAPE-6 library, and N-body codes currently running on GRAPE-6 can switch to Sapporo by a simple relinking of the library. The precision of our library is comparable to that of GRAPE-6, even though internally the GPU hardware is limited to single precision arithmetics. This limitation is effectively overcome by emulating double precision for calculating the distance between particles. The performance loss of this operation is small (< 20%) compared to the advantage of being able to run at high precision. We tested the library using several GRAPE-6-enabled N-body codes, in particular with Starlab and phiGRAPE. We measured peak performance of 800 Gflop/s for running with 10^6 particles on a PC with four commercial G92 architecture GPUs (two GeForce 9800GX2). As a production test, we simulated a 32k Plummer model with equal mass stars well beyond core collapse. The simulation took 41 days, during which the mean performance was 113 Gflop/s. The GPU did not show any problems from running in a production environment for such an extended period of time.Comment: 13 pages, 9 figures, accepted to New Astronom

The Living Application: a Self-Organising System for Complex Grid Tasks

We present the living application, a method to autonomously manage applications on the grid. During its execution on the grid, the living application makes choices on the resources to use in order to complete its tasks. These choices can be based on the internal state, or on autonomously acquired knowledge from external sensors. By giving limited user capabilities to a living application, the living application is able to port itself from one resource topology to another. The application performs these actions at run-time without depending on users or external workflow tools. We demonstrate this new concept in a special case of a living application: the living simulation. Today, many simulations require a wide range of numerical solvers and run most efficiently if specialized nodes are matched to the solvers. The idea of the living simulation is that it decides itself which grid machines to use based on the numerical solver currently in use. In this paper we apply the living simulation to modelling the collision between two galaxies in a test setup with two specialized computers. This simulation switces at run-time between a GPU-enabled computer in the Netherlands and a GRAPE-enabled machine that resides in the United States, using an oct-tree N-body code whenever it runs in the Netherlands and a direct N-body solver in the United States.Comment: 26 pages, 3 figures, accepted by IJHPC

Simulating the universe on an intercontinental grid of supercomputers

Understanding the universe is hampered by the elusiveness of its most common constituent, cold dark matter. Almost impossible to observe, dark matter can be studied effectively by means of simulation and there is probably no other research field where simulation has led to so much progress in the last decade. Cosmological N-body simulations are an essential tool for evolving density perturbations in the nonlinear regime. Simulating the formation of large-scale structures in the universe, however, is still a challenge due to the enormous dynamic range in spatial and temporal coordinates, and due to the enormous computer resources required. The dynamic range is generally dealt with by the hybridization of numerical techniques. We deal with the computational requirements by connecting two supercomputers via an optical network and make them operate as a single machine. This is challenging, if only for the fact that the supercomputers of our choice are separated by half the planet, as one is located in Amsterdam and the other is in Tokyo. The co-scheduling of the two computers and the 'gridification' of the code enables us to achieve a 90% efficiency for this distributed intercontinental supercomputer.Comment: Accepted for publication in IEEE Compute

Performance Analysis of Direct N-Body Algorithms on Special-Purpose Supercomputers

Direct-summation N-body algorithms compute the gravitational interaction between stars in an exact way and have a computational complexity of O(N^2). Performance can be greatly enhanced via the use of special-purpose accelerator boards like the GRAPE-6A. However the memory of the GRAPE boards is limited. Here, we present a performance analysis of direct N-body codes on two parallel supercomputers that incorporate special-purpose boards, allowing as many as four million particles to be integrated. Both computers employ high-speed, Infiniband interconnects to minimize communication overhead, which can otherwise become significant due to the small number of "active" particles at each time step. We find that the computation time scales well with processor number; for 2*10^6 particles, efficiencies greater than 50% and speeds in excess of 2 TFlops are reached.Comment: 34 pages, 15 figures, submitted to New Astronom

Collisionally Regenerated Dark Matter Structures in Galactic Nuclei

We show that the presence of a rho~1/r^{3/2} dark matter overdensity can be robustly predicted at the center of any galaxy old enough to have grown a power-law density cusp in the stars via the Bahcall-Wolf mechanism. Using both Fokker-Planck and direct N-body integrations, we demonstrate collisional generation of these dark matter "crests" (Collisionally REgenerated STtructures) even in the extreme case that the density of both stars and dark matter were previously lowered by slingshot ejection from a binary supermassive black hole. The time scale for collisional growth of the crest is approximately the two-body relaxation time as defined by the stars, which is < 10 Gyr at the centers of stellar spheroids with luminosities comparable to that of the Milky Way bulge or less. The presence of crests can robustly be predicted in such galaxies, unlike the steeper enhancements, called "spikes," produced by the adiabatic growth of black holes. We discuss special cases where the prospects for detecting dark matter annihilations from the centers of galaxy haloes are significantly affected by the formation of crests.Comment: 11 pages, uses revtex

A multiphysics and multiscale software environment for modeling astrophysical systems

We present MUSE, a software framework for combining existing computational tools for different astrophysical domains into a single multiphysics, multiscale application. MUSE facilitates the coupling of existing codes written in different languages by providing inter-language tools and by specifying an interface between each module and the framework that represents a balance between generality and computational efficiency. This approach allows scientists to use combinations of codes to solve highly-coupled problems without the need to write new codes for other domains or significantly alter their existing codes. MUSE currently incorporates the domains of stellar dynamics, stellar evolution and stellar hydrodynamics for studying generalized stellar systems. We have now reached a "Noah's Ark" milestone, with (at least) two available numerical solvers for each domain. MUSE can treat multi-scale and multi-physics systems in which the time- and size-scales are well separated, like simulating the evolution of planetary systems, small stellar associations, dense stellar clusters, galaxies and galactic nuclei. In this paper we describe three examples calculated using MUSE: the merger of two galaxies, the merger of two evolving stars, and a hybrid N-body simulation. In addition, we demonstrate an implementation of MUSE on a distributed computer which may also include special-purpose hardware, such as GRAPEs or GPUs, to accelerate computations. The current MUSE code base is publicly available as open source at http://muse.liComment: 24 pages, To appear in New Astronomy Source code available at http://muse.l