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Reionisation simulations at ECCA

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ECCA Research

Reionisation of the Intergalactic Medium

A new component to studies of the IGM and of galaxy formation is the problem of radiative transfer. While long known to be important for understanding the evolution of the IGM, both its reionization after the Recombination Epoch and in suppressing early stages of galaxy formation, only recently has the increased power of parallel computers made it feasible to simulate these effects. Radiative transfer adds a new dimension in the structure formation problem, requiring energy-resolution of the evolving radiation field.

The transition from a predominantly neutral to a predominantly ionized state is a crucial event in the history of the Universe. Establishing the nature and origin of the reionisation of the Intergalactic Medium (IGM) are critical for interpreting a host of observed properties of the IGM: hydrogen and helium mean transmitted flux measurements, the characteristics of absorption lines in the Lyman-α forest (like their widths, which contain information concerning the temperature of the IGM), and metal absorption lines, the origin of which are still unknown (e.g. Songaila 2006). Reionisation will also have played a role in galaxy formation, at least at the low mass end, and places constraints on the sources of ionising photons, whether QSOs, starburst galaxies, Population III stars, or some as yet un-envisioned population.

Temperature of the Intergalactic Medium

We have coupled a radiative transfer (RT) code to a particle-mesh (PM) N-body code to compute the post-reionisation temperature of the IGM. We have performed a suite of simulations with different spectra of ionising radiation: a power law (fν~ ν-0.5), miniquasar, starburst, and a time-varying spectrum that evolves from a starburst spectrum to a power law. We find the post-ionisation temperature of the ionised IGM is sensitive to the spectrum of the source of ionising radiation, providing a strong constraint on the nature of the sources of reionisation.

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