* Astronomy

Title: A new parametrisation of the reionisation history
Author: Marian Douspis, Nabila Aghanim, Stéphane Ili, Mathieu Langer

Motivated by the current constraints on the epoch of reionisation from recent cosmic microwave background observations, ionising background measurements of star-forming galaxies, and low redshifts line-of-sight probes, we propose a new data-motivated parameterisation of the history of the average ionisation fraction. This parameterisation describes a flexible redshift-asymmetric reionisation process in two regimes that is capable of fitting all the current constraints.

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Title: Exploring the Cosmic Reionisation Epoch in Frequency Space: An Improved Approach to Remove the Foreground in 21 cm Tomography
Authors: Jingying Wang, Haiguang Xu, Tao An, Junhua Gu, Xueying Guo, Weitian Li, Yu Wang, Chengze Liu, Olivier Martineau-Huynh, Xiang-Ping Wu

Aiming to correctly restore the redshifted 21 cm signals emitted by the neutral hydrogen during the cosmic reionisation processes, we re-examine the separation approaches based on the quadratic polynomial fitting technique in frequency space to investigate whether they works satisfactorily with complex foreground, by quantitatively evaluate the quality of restored 21 cm signals in terms of sample statistics. We construct the foreground model to characterise both spatial and spectral substructures of the real sky, and use it to simulate the observed radio spectra. By comparing between different separation approaches through statistical analysis of restored 21 cm spectra and corresponding power spectra, as well as their constraints on the mean halo bias b and average ionisation fraction x_e of the reionisation processes, at z=8 and the noise level of 60 mK we find that, although the complex foreground can be well approximated with quadratic polynomial expansion, a significant part of Mpc-scale components of the 21 cm signals (75% for \gtrsim 6h^{-1} Mpc scales and 34% for \gtrsim 1h^{-1} Mpc scales) is lost because it tends to be mis-identified as part of the foreground when single-narrow-segment separation approach is applied. The best restoration of the 21 cm signals and the tightest determination of b and x_e can be obtained with the three-narrow-segment fitting technique as proposed in this paper. Similar results can be obtained at other redshifts.

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Title: Detecting the Rise and Fall of the First Stars by Their Impact on Cosmic Reionisation
Authors: Kyungjin Ahn, Ilian T. Iliev, Paul R. Shapiro, Garrelt Mellema, Jun Koda, Yi Mao

The intergalactic medium was reionised before redshift z~6, most likely by starlight which escaped from early galaxies. The very first stars formed when hydrogen molecules (H2) cooled gas inside the smallest galaxies, minihalos of mass between 10^5 and 10^8 solar masses. Although the very first stars began forming inside these minihalos before redshift z~40, their contribution has, to date, been ignored in large-scale simulations of this cosmic reionisation. Here we report results from the first reionisation simulations to include these first stars and the radiative feedback that limited their formation, in a volume large enough to follow the crucial spatial variations that influenced the process and its observability. We show that reionisation began much earlier with minihalo sources than without, and was greatly extended, which boosts the intergalactic electron-scattering optical depth and the large-angle polarization fluctuations of the cosmic microwave background significantly. Although within current WMAP uncertainties, this boost should be readily detectable by Planck. If reionisation ended as late as z_ov<~7, as suggested by other observations, Planck will thereby see the signature of the first stars at high redshift, currently undetectable by any other probe.

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Distant Galaxies Reveal The Clearing of the Cosmic Fog

Scientists have used ESO's Very Large Telescope to probe the early Universe at several different times as it was becoming transparent to ultraviolet light. This brief but dramatic phase in cosmic history - known as reionisation - occurred around 13 billion years ago. By carefully studying some of the most distant galaxies ever detected, the team has been able to establish a timeline for reionisation for the first time. They have also demonstrated that this phase must have happened quicker than astronomers previously thought.
An international team of astronomers used the VLT as a time machine, to look back into the early Universe and observe several of the most distant galaxies ever detected. They have been able to measure their distances accurately and find that we are seeing them as they were between 780 million and a billion years after the Big Bang.
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