Epoch of reionisation

One of the most exciting applications of LOFAR will be the search for redshifted 21cm line emission from the Epoch of Reionisation (EoR). It is currently believed that the Dark Ages, the period after recombination when the Universe turned neutral, lasted until around z=20. WMAP polarization results appear to suggest that there may have been extended, or even multiple phases of Reionisation, the start possibly being around z~15-20 and ending at z~6. Using LOFAR the redshift range from z=11.4 (115 MHz) to z=6 (180 MHz) can be probed.

The cosmic web Matter in the local Universe (hot and warm baryons) is known to lies in a large-scale filamentary structuresand this cosmic web is believed to extend to the highest redshifts. This picture was taken from Cen & Ostriker 1999 and illustrates the results of numerical N-body hydrodynamical models. The seeds of the cosmic web were implanted during the dark ages, just prior to reionisation. This primeval era will be accessible to LOFAR.

LOFAR will address a number of key questions related to the EoR, including:
• What is the redshift range in which the bulk of the neutral hydrogen became ionized? Can a clear ‘Redshift of Reionisation’ be identified or defined or are there multiple phases of Reionisation?
• What are the characteristics of the spatial distribution of heated and still cold IGM and how do these evolve during the era of Reionisation?
• Which objects (Pop III stars, galaxies, quasars) or processes are responsible for re-ionizing the Universe? Ultimately, the final answer here will come from a close comparison of theoretical predictions with the most detailed measured characteristics.

Snapshot simulation of the HI universe evolving with time (from left to right). The dark regions correspond to highly ionized regions (such as those around protogalaxies) and the bright regions are dense, neutral pockets of gas. Credit: Steve Furlanetto et al 2004, Ap J

Three stages of increasing complexity, and phased in time accordingly, can be identified in the analysis of the spectral data cubes (with two spatial and one redshift = frequency dimension). These are:
• Global Signal The EoR 'transition' is expected to occur globally at approximately the same, but a-priori unknown, redshift. However, it could span an extended ‘era’.
• Statistical Analysis will be used in order to extract the principal structural measures of the Reionisation signal from the noisy and foreground contaminated data. Techniques that can be used include power spectrum and angular correlation estimation.
• EoR Map Reconstruction The ultimate goal is to reconstruct a large-sky map of the EoR signal.

More information: see NL science case for LOFAR (pdf)

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