ReHILAE: is the Re-ionisation of Hydrogen-I the sole consequence of Lyman-alpha Emitters?

Gold, James and Donovan, Connor and Bowden, Jack and Carr, James and Philips, Joe and Sobral, David (2020) ReHILAE: is the Re-ionisation of Hydrogen-I the sole consequence of Lyman-alpha Emitters? Notices of Lancaster Astrophysics (NLUAstro), 2. pp. 42-52.

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Abstract

The Epoch of Hydrogen Re-Ionisation (EoR) is an important stage in the evolution of the Universe, in which the neutral hydrogen in the Intergalactic Medium (IGM) becomes fully ionised. There are a number of ambiguities concerning the exact time period of the EoR, in addition to the exact nature of its causes. Previous methods describing this event use observations of Lyman Break Galaxies (LBGs), Lyman Alpha Emitters (LAEs - a subset of LGBs) or Active Galactic Nuclei (AGNs) as the predominant ionising sources of the EoR. With a few varying assumptions, galaxies appear to be the primary sources to consider. The UV-based framework currently used requires assumptions of the efficiency in converting between UV and Lyman-continuum (LyC) ionising photons (ξion), and the fraction of LyC photons that actually escape their sources (fesc). Direct measurements of these values using the UV-framework ap- pear to produce values far below what are require. Considering LAEs, which are a subset of the UV continuum-selected sources, we can use different observations to comfortably approximate them as the sources with the highest production of ionising photons per UV luminosity. Therefore, by only considering LAEs, we can eliminate the need for determining ξion entirely. Taking this approach, our own model for the fraction of ionised hydrogen in the Universe as a function of redshift (QHII ) is outlined. This model provided us with an approximate value of the LyC escape fraction as ~10%, which is a far more reasonable value than assumed in previous studies. Comparing final results for QHII directly to our own improved UV-framework model, we determined that the re-ionisation of hydrogen is very likely the sole consequence of LAEs.