Simulations Workshop Abstracts

While LambdaCDM cosmology has long predicted that the halos of spiral galaxies like our own should be strewn with the debris of minor mergers. It has only been recently that evidence of this has started to emerge. I will discuss how Wide Field Cameras have played a role in revealing these previously hidden structures in our Halo and highlight where the traps for young players lie.

Cosmological N-body simulations make very specific predictions about the central dark matter distribution in galaxies. Observations seem to be in stark conflict with these predictions. I review some of the difficulties that observers and modellers face in measuring the central dark matter distribution. The inclusion of baryon physics in both observational as well as computational analyses may hold the key to settling the cusp-core debate.

I will provide a layperson's description of the primary tools employed to simulate the non-linear regime of (collisionless) structure formation throughout the Universe. This will include the main features of cosmological N-body simulation codes: equations, numerical techniques, strengths, and limitations. The emphasis will be on conceptual understanding, with the ultimate goal being the breaking down of barriers between computational and observational astrophysics.

I will start by giving an overview of the N-body method for simulating the evolution of star clusters with a focus on some of the software and hardware issues involved. Then I will demonstrate how efforts to make these simulations as realistic as possible are paying off as we match model clusters to observed clusters to learn much about the initial conditions, evolution history and stellar populations of these objects.

I will show numerous examples of interacting galaxies, their gas distributions, stellar content and existing numerical simulations. The 21-cm line of atomic neutral hydrogen (HI) is an excellent tracer for galaxy interactions. HI gas is often found outside the stellar galaxy envelope and is easily disrupted. As a consequence we see peculiar features like tidal tails and gas bridges. Some HI clouds/islands can be separated from the host galaxy by several hundred kpc.

With new HI surveys like the Southern Galactic Plane Survey we are now able to image parsec scale features on kiloparsec scale HI shells. These new images reveal an amazing amount of detail, including a plethora of cold, apparently instability formed structures. I will present an observational view of small-scale structure in HI shells with an emphasis on what shell modelling can tell us about the physics of these remarkable objects.

Magellanic Bridge has long been considered to be a single, spatially coherent HI filament, connecting the Small and Large Magellanic Clouds. However, an analysis of the spatial power spectrum of the HI in the Bridge has shown that it consists of a identifyable northern and southern filament, which are very distinct in both morphological and velocity structure. Numerical simulations of the entire Magellanic system suggest that the SMC is in fact a two-armed galaxy, where one arms forms the actual SMC-LMC 'Bridge' link and the other extends almost radially away. Although the Bridge appears as a single feature on the sky, these two arms may be separated by as much as 20-25 kpc. A second statistical technique, the Spectral Correlation Function, appears to confirm the tidal mechanisms operating on the Bridge by showing that the structure of the HI in the Bridge tends to vary more slowly in the direction of the tidal perturbation than in the orthogonal direction. Due to the small dynamic range of HI brightness in the Bridge, this is a result which is invisible to standard analyses which use Fourier inversion of the dataset.

So you want to be a computational astrophysicist? What is one? How do you go about becoming one? What are the pitfalls? Why are people avoiding me at parties? In this talk I will combine a general introduction to computational fluid dynamics with some remarks about particle methods, and with some advice for new PhD students.

The N-body simulation is an established tool in numerical cosmology and consequently there is a rich and varied literature describing methods for integrating the equations of motion of large numbers of particles in an expanding Universe. In comparison, relatively little has been written on methods for generating initial conditions (ICs) required by these simulations or methods for identifying structure (dark matter haloes or galaxies) within the simulation outputs, despite their fundamental importance. During this lecture, I will tackle both problems, describing the basic concepts and the most popular and widely used methods, and I will highlight some useful starting points in the literature for those wishing to investigate further.

I shall outline the theory of interstellar shocks and give an overview of how steady, one-dimensional models are constructed. I will then present examples of multidimensional shock simulations and discuss the implications for fluid simulations in which shocks appear as part of a larger flow.

The coagulation theory of massive star formation (Bonnell, Bate, and Zinnecker 1998) predicts that most of the massive stars form in the central densest part of a protocluster by stellar collisions and tidal mergers. A further prediction is a high frequency of tight binary systems among massive stars due to tidal capture and the combination of tidal disruption followed by a star-disc encounter (failed stellar mergers). Recent observations have indeed confirmed this prediction (many young short-period SB2 systems among the O-stars in young clusters), but there is also observational evidence that seems to contradict the prediction (frequency of tight binaries higher in less dense young clusters). The jury is still out, whether stellar collisions are a viable theory for massive star formation. We will also briefly touch upon the implication of the observed very low binary frequency of O-type runaway stars and the high multiplicity of the four well-known Orion Trapezium high-mass OB stars.

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