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Paper: Towards Understanding Simulated Feedback in AMR and SPH Codes and the Multi-Phase Nature of the ISM
Volume: 453, Advances in Computational Astrophysics: Methods, Tools, and Outcome
Page: 19
Authors: Mitchell, N. L.; Bower, R. G.; Theuns, T.; Vorobyov, E. I.
Abstract: Feedback from supernova is believed to be a key ingredient for regulating star formation within galaxies, however modelling it self-consistently is prohibitively expensive. Even superbubbles which are formed from multiple supernova occuring in close proximity, are only a few hundred parsecs across — tiny compared to the sizes of many galaxies. Thus any simulation which aims to study the large scale properties of galaxies, groups and clusters cannot currently resolve the ISM into its true multi-phase nature. In order to overcome this limitation, many cosmological simulations which are run in both AMR and SPH codes, adopt polytropic equations of state. These approximate the physics of the ISM below those scales which can be resolved where the ISM splits to become multi-phase. However we show that when identical sub-grid physical recipes for cooling, star formation and feedback are included into both SPH and AMR codes, they do not necessarily yield the same results. Instead, we find that energy is dissipated far more readily in an AMR code, allowing supernova driven winds to stall. This prevents supernova feedback in AMR simulations from removing sufficient gas to adaquately regulate the star formation rate. Whereas in SPH codes the winds can remove more gas, with wind particles able to stream more freely out of the galaxy. Determining which of these codes provides a more physically correct description is extremely difficult, however it clearly highlights the need for a more robust model for the ISM. For a better understanding of the means by which energy from feedback is redistributed within the ISM, we present our new multi-phase chemodynamic model in the FLASH AMR code. We seperate the ISM into a hot tenuous gas phase and an almost collisionless compact molecular cloud component. Both phases are modelled on the adaptive mesh, the hot gas being modelled by using the standard Euler equations for compressible fluid dynamics whilst the collisionless component is solved using a similar set of equations based upon the zeroth, first and second order moments of the collisionless Boltzmann equation (the stellar component is modelled in an identical fashion). Now cold molecular star forming material can continue to accrete towards the galactic centre whilst hot supernova winds can propagate outwards through the space in between them. Such a model allows us to investigate the exchange of mass and energy between the different phases of the ISM along with a multitude of physical processes including ram pressure, heat conduction and turbulence.
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