Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Observations of star formation rates (SFRs) in galaxies provide vital clues to the physical nature of the Hubble sequence and are key probes of the evolutionary histories of galaxies. The focus of this review is on the broad patterns in the star formation properties of galaxies along the Hubble sequence and their implications for understanding galaxy evolution and the physical processes that drive the evolution. Star formation in the disks and nuclear regions of galaxies are reviewed separately, then discussed within a common interpretive framework. The diagnostic methods used to measure SFRs are also reviewed, and a self-consistent set of SFR calibrations is presented as an aid to workers in the field. One of the most recognizable features of galaxies along the Hubble sequence is the wide range in young stellar content and star formation activity. This variation in stellar content is part of the basis of the Hubble classification itself (Hubble 1926), and understanding its physical nature and origins is fundamental to understanding galaxy evolution in its broader context. This review deals with the global star formation properties of galaxies, the systematics of those properties along the Hubble sequence, and their implications for galactic evolution. I interpret “Hubble sequence” in this context very loosely, to encompass not only morphological type but other properties such as gas content, mass, bar structure, and dynamical environment, which can strongly influence the largescale star formation rate (SFR).

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Star formation in galaxies along the hubble sequence

We describe the GALFORM semi-analytic model for calculating the formation and evolution of galaxies in hierarchical clustering cosmologies. It improves upon, and extends, the earlier scheme developed by Cole et al. (1994). The model employs a new Monte-Carlo algorithm to follow the merging evolution of dark matter halos with arbitrary mass resolution. It incorporates realistic descriptions of the density profiles of dark matter halos and the gas they contain; it follows the chemical evolution of gas and stars, and the associated production of dust; and it includes a detailed calculation of the sizes of disks and spheroids. Wherever possible, our prescriptions for modelling individual physical processes are based on results of numerical simulations. They require a number of adjustable parameters which we fix by reference to a small subset of local galaxy data. This results in a fully specified model of galaxy formation which can be �

Hierarchical galaxy formation

▪ Abstract The Universe is in transition. At early times, galactic evolution was dominated by hierarchical clustering and merging, processes that are violent and rapid. In the far future, evolution will mostly be secular—the slow rearrangement of energy and mass that results from interactions involving collective phenomena such as bars, oval disks, spiral structure, and triaxial dark halos. Both processes are important now. This review discusses internal secular evolution, concentrating on one important consequence, the buildup of dense central components in disk galaxies that look like classical, merger-built bulges but that were made slowly out of disk gas. We call these pseudobulges. We begin with an “existence proof”—a review of how bars rearrange disk gas into outer rings, inner rings, and stuff dumped onto the center. The results of numerical simulations correspond closely to the morphology of barred galaxies. In the simulations, gas is transported to small radii, where it reaches high densities and...

Secular Evolution and the Formation of Pseudobulges in Disk Galaxies

Supersymmetric Dark Matter

Basic dynamical mechanisms that produce an amplification of the accretion rate of gas clouds into the central regions of barred galaxies, and their subsequent effects on the evolution of barred galaxies, are discussed. Weakly dissipative orbits, representing gas clouds, are computed in a barred galaxy model with a central mass concentration such as a black hole, or a secondary small inner bar. Amplified accretion across resonances that is especially rapid inside the inner Lindblad resonance, large excursions outside the galactic plane, and the existence of nontrivial attractors like strange (chaotic) attractors or limit cycles are found. The underlying physical mechanisms are, in general, due to the presence of broad horizontal and vertical resonances through which weakly dissipative particles can rapidly traverse. The principal physical implications of these findings are discussed. 65 refs.

Dissipation in barred galaxies - The growth of bulges and central mass concentrations

The stellar dynamics at the transitions between a strongly barred and rotating potential and a spherical and dense central mass concentration is investigated in a model potential. Three different degrees of concentration of the central mass are studied. The main families of three-dimensonal periodic orbit are found by numerical means. The structure of phase space is presented by bifurcation diagrams and surfaces of section. A large region of instability develops near the inner Lindblad resonance (ILR) due to the strong bar potential. In such a mass configuration the radial ILR is typically near the corresponding vertical ILR, consequently the instability of orbits is radial as well as vertical, helping the heating of disk stars into a bulge

Galactic bars with central mass concentrations : three-dimensional dynamics

Induced Star Formation.- Observational Evidence for Interactions and Mergers.- Dynamics of Galaxy Interactions.

Galaxies: Interactions and Induced Star Formation

The Galactic Interstellar Medium

We characterize empirically the radial diffusion of stars in the plane of a typical barred disk galaxy by calculating the local spatial diffusion coefficient and diffusion time-scale for bulge-disk-halo N-body self-consistent systems which initially differ in the SafronovToomre-QT parameter. We find different diffusion scenarios that depend on the bar strength and on the degree of instability of the disk. Marginally stable disks, with QT ∼ 1, have two families of bar orbits with different values of angular momentum and energy, which determine a large diffusion in the corotation region. In hot disks, QT > 1, stellar diffusion is reduced with respect to the case of marginally stable disks. In cold models, we find that spatial diffusion is not constant in time and strongly depends on the activity of the bar, which can move stars all over the disk recurrently. We conclude that to realistically study the impact of radial migration on the chemical evolution modeling of the Milky Way the role of the bar has to be taken into account.

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Daniel Pfenniger

Papers

Dissipation in barred galaxies - The growth of bulges and central mass concentrations

Galactic bars with central mass concentrations : three-dimensional dynamics

Galaxies: Interactions and Induced Star Formation

The Galactic Interstellar Medium

Stellar diffusion in barred spiral galaxies