In 2020, I have started a new Max Planck Society Research Group at MPIA on Reionization and the Intergalactic Medium (REIGM).
The goals of the group include, but are not limited to, three primary topics:
We have already begun to detect the presence of vast swathes of intergalactic neutral hydrogen from Lyman-α damping wings in spectroscopy of a handful of the most distant quasars, indicating that the epoch of reionization is far from over at redshifts 7–7.5.
However, this probe is far from being fully explored — dozens of quasars are known at slightly later times when reionization should still be incomplete, and the upcoming Euclid mission will discover many more at even earlier times. The group will further develop the theoretical modeling and statistical machinery involved in quasar damping wing analysis to not only constrain the reionization history but also distinguish between different models of the reionization process.
Relevant publications: Yang et al. (2020), Wang et al. (2020), Davies et al. (2018c)
The Lyman-α transparent proximity zones of the most distant quasars, i.e. the earliest known supermassive black hole, suggest that their optically-luminous phase is shorter than a few million years. However, they cannot grow via conventional Eddington-limited accretion in such a short time, implying they either grew through super-Eddington accretion or predominantly in a phase obscured by copious gas and dust. Additionally, a handful of quasars at slightly later times show evidence for even shorter activity timescales, on the order of only thousands of years.
Much of the history of the ionizing luminosity of quasars is encoded in the Lyman-α transmission features in their proximity zones, but only the tip of the iceberg has been unveiled by existing analyses. The group will develop theoretical models of quasar proximity zones and compare them to existing and upcoming spectroscopic datasets, both along the line of sight and in three dimensions, to measure the light curves of distant quasars during and after the reionization epoch.
Relevant publications: Davies et al. (2020), Davies et al. (2019), Eilers et al. (2020), Eilers et al. (2018)
The epochs of hydrogen and helium reionization alter the structure of intergalactic gas via dramatic injections of thermal energy as the ionization fronts sweep through the Universe, and this relic heat persists for hundreds of millions of years. Even after reionization is complete, the ionizing radiation fields remain inhomogeneous due to continued absorption by dense pockets of neutral gas and the highly clustered nature of early galaxies and quasars. These features are imprinted on cosmological scales, from 10s up to 100 megaparsecs, and may be responsible for the observed excess stochasticity of large-scale Lyman-α forest absorption at redshifts above 5.
Prior to any reionization event, during the cosmic Dark Ages, the cold baryons in the neutral Universe could collapse and form structure in lockstep with the dark matter. Afterwards, when the gas was heated to more than ten thousand Kelvin, it became more diffuse due to gas pressure acting against the influence of gravity. The pressure waves in the intergalactic medium responds from the heat injection by reionization are quite slow, suggesting that a remaining excess in small-scale structure may be visible at early times.
The combination of large-scale volumes and small-scale gas structure involved in characterizing the effects of reionization on the intergalactic medium require targeted simulation and analysis approaches focusing on specific questions. The group will embark on a multi-scale odyssey involving approximate semi-numerical methods, radiation hydrodynamics, and high-resolution simulations of small-scale structure to resolve the mystery of hydrogen and helium Lyman-α forest fluctuations and constrain the physical state of the Universe at very early times.
Relevant publications: Davies (2020), Oñorbe et al. (2019), Eilers et al. (2019), Becker et al. (2018), Davies et al. (2018a), Davies et al. (2018b)