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Now showing items 1 - 16 of 61952

  • TREVR: A general N log(2) N radiative transfer algorithm

    Grond, J. J.   Woods, R. M.   Wadsley, J. W.   Couchman, H. M. P.  

    We present Tree-based REVerse Ray Tracing (TREVR), a general algorithm for computing the radiation field, including absorption, in astrophysical simulations. TREVR is designed to handle large numbers of sources and absorbers; it is based on a tree data structure and is thus suited to codes that use trees for their gravity or hydrodynamics solvers (e.g. adaptivemesh refinement). It achieves computational speed while maintaining a specified accuracy via controlled lowering of the resolution of both sources and rays from each source. TREVR computes the radiation field in order N log N-source time without absorption and order N log N(source)logN time with absorption. These scalings arise from merging sources of radiation according to an opening angle criterion and walking the tree structure to trace a ray to a depth that gives the chosen accuracy for absorption. The absorption-depth refinement criterion is unique to TREVR. We provide a suite of tests demonstrating the algorithm's ability to accurately compute fluxes, ionization fronts, and shadows.
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  • Chaos and variance in galaxy formation

    Keller, B. W.   Wadsley, J. W.   Wang, L.   Kruijssen, J. M. D.  

    The evolution of galaxies is governed by equations with chaotic solutions: gravity and compressible (magneto-)hydrodynamics. While this microscale chaos and stochasticity has been well studied, it is poorly understood how it couples to macroscale properties examined in simulations of galaxy formation, In this paper, we use tiny perturbations introduced by floating-point round-off, random number generators, and seemingly trivial differences in algorithmic behaviour as seeds for chaotic behaviour, These can ultimately grow to produce non-trivial differences in star formation histories, circumgalactic medium properties, and the distribution of stellar mass, We examine the importance of stochasticity due to discreteness noise, variations in merger timings, and how self-regulation moderates the effects of this stochasticity. We show that chaotic variations in stellar mass can be suppressed through gas exhaustion, or he maintained at a roughly constant variance through stellar feedback. We also find that galaxy mergers are critical points from which large (as much as a factor of 2) variations can grow in global quantities such as the total stellar mass. These variations can grow and persist for Gyr before regressing towards the mean. These results show that detailed comparisons of simulations require serious consideration of the magnitude of effects compared to run-to-run chaotic variation, and may significantly complicate interpreting the impact of different physical models. Understanding the results of simulations require us to understand that the process of simulation is not a mapping of an infinitesimal point in configuration space to another, final infinitesimal point. Instead, simulations map a point in a space of possible initial conditions points to a volume of possible final states,
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  • Lambda CDM is Consistent with SPARC Radial Acceleration Relation

    Keller, B. W.   Wadsley, J. W.  

    Recent analysis of the Spitzer Photometry and Accurate Rotation Curve (SPARC) galaxy sample found a surprisingly tight relation between the radial acceleration inferred from the rotation curves and the acceleration due to the baryonic components of the disk. It has been suggested that this relation may be evidence for new physics, beyond Lambda CDM. In this Letter, we show that 32 galaxies from the MUGS2 match the SPARC acceleration relation. These cosmological simulations of star-forming, rotationally supported disks were simulated with a WMAP3 Lambda CDM cosmology, and match the SPARC acceleration relation with less scatter than the observational data. These results show that this acceleration relation is a consequence of dissipative collapse of baryons, rather than being evidence for exotic dark-sector physics or new dynamical laws.
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  • Bars and spirals in tidal interactions with an ensemble of galaxy mass models

    Pettitt, Alex R.   Wadsley, J. W.  

    We present simulations of the gaseous and stellar material in several different galaxy mass models under the influence of different tidal fly-bys to assess the changes in their bar and spiral morphology. Five different mass models are chosen to represent the variety of rotation curves seen in nature. We find a multitude of different spiral and bar structures can be created, with their properties dependent on the strength of the interaction. We calculate pattern speeds, spiral wind-up rates, bar lengths, and angular momentum exchange to quantify the changes in disc morphology in each scenario. The wind-up rates of the tidal spirals follow the 2: 1 resonance very closely for the flat and dark matter-dominated rotation curves, whereas the more baryon-dominated curves tend to wind-up faster, influenced by their inner bars. Clear spurs are seen in most of the tidal spirals, most noticeable in the flat rotation curve models. Bars formed both in isolation and interactions agree well with those seen in real galaxies, with a mixture of 'fast' and 'slow' rotators. We find no strong correlation between bar length or pattern speed and the interaction strength. Bar formation is, however, accelerated/induced in four out of five of our models. We close by briefly comparing the morphology of our models to real galaxies, easily finding analogues for nearly all simulations presenter here, showing passages of small companions can easily reproduce an ensemble of observed morphologies.
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  • On the treatment of entropy mixing in numerical cosmology

    Wadsley, J. W.   Veeravalli, G.   Couchman, H. M. P.  

    For simulations of fluid dynamics in astrophysics, physical viscosity and diffusion are typically neglected. However, in this high Reynolds number regime, real fluids become highly turbulent and turbulent processes mediate substantial transport of momentum and heat that is diffusive in nature. In the absence of models for these processes, code-dependent numerical effects dominate how diffusion operates and may lead to physically incorrect simulation results. We highlight the qualitative difference in these numerical effects for smooth particle hydrodynamics (SPH) and grid-based Eulerian codes using two test problems: a buoyant gas bubble and gas in a galaxy cluster. Grid codes suffer from numerical diffusion in the absence of explicit terms, and small-scale diffusion of heat is completely absent in the Lagrangian SPH method. We find that SPH with heat diffusion added at a level similar to that expected from turbulence diffusion generates more physically appealing results. These results suggest, but do not confirm, that a flat entropy core is to be expected for gas in an idealized galaxy cluster (i.e. one without physics beyond that of a non-radiating gas). A goal of this work is thus to draw attention to the as yet unfulfilled need for models of turbulent diffusive processes in compressible gases in astrophysics.
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  • Professor J. W. Cuthbertson

    Gabe   D. R.  

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  • Professor J. W. Cuthbertson

    Gabe, D. R.  

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  • Quantum weirdness, by W. J. Mullin

    Zetie   K. P.  

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  • Quantum weirdness, by W. J. Mullin

    Zetie, K. P.  

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  • Bouman, W. P., & Arcelus, J. (Eds.). (2017).

    Dr. Gemma L. Witcomb  

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  • Bouman, W. P., & Arcelus, J. (Eds.). (2017).

    Witcomb   Gemma L.  

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  • Kleines Mittelhochdeutsches W?rterbuch () || J

    Hennig   Beate  

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  • Symbol and consciousness in phenomenology of J. W. Goethe

    Shadrina, Olga; Yazyev, B.  

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  • Late Roman Pottery. By J. W. Hayes

    D. P. S. Peacock;  

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  • Towards Modern Aesthetics: Charlotte Bronte and J. M. W. Turner

    Choe, Jian  

    This article aims to consider the intersections between Charlotte Bronte and William Turner, as shown in their text and image, centring upon the ways in which they worked towards the style that would be called impressionism. This particular aesthetic can clarify all the major dimensions of their mature art: epistemological, technical and even thematic. Their artistic careers coincided with a period of great social change and their innovative art was forged in response to the shifting conditions of their time. They witnessed the advent of the modern world, attempting to give form to the new mode of existence and consciousness. An inquiry into the parallels between the two eminent Victorians suggests that they carved out an aesthetic of modernity, which laid the foundation for the future course of art and literature.
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  • Towards Modern Aesthetics: Charlotte Bront? and J. M. W. Turner

    Choe   Jian  

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