Epoch of Reionisation 21cm Forecasting From MCMC-Constrained Semi-Numerical Models

TL;DR

This paper enhances semi-numerical models of cosmic reionization by including detailed ionization effects, constrains them with current observations, and forecasts how upcoming 21cm surveys can reveal reionization sources and topology.

Contribution

It introduces an improved semi-numerical model with time-integrated ionization and recombination effects, constrained by observations, to forecast 21cm signals and source properties.

Findings

Reionization occurred more suddenly with larger HII bubbles.

Local neutral hydrogen fluctuations dominate large-scale 21cm power.

Combining 21cm data with other observations constrains reionization parameters.

Abstract

The recent low value of Planck (2016) integrated optical depth to Thomson scattering suggests that the reionization occurred fairly suddenly, disfavoring extended reionization scenarios. This will have a significant impact on the 21cm power spectrum. Using a semi-numerical framework, we improve our model from Hassan et al. (2016) to include time-integrated ionisation and recombination effects, and find that this leads to more sudden reionisation. It also yields larger HII bubbles which leads to an order of magnitude more 21cm power on large scales, while suppressing the small scale ionization power. Local fluctuations in the neutral hydrogen density play the dominant role in boosting the 21cm power spectrum on large scales, while recombinations are subdominant. We use a Monte Carlo Markov Chain approach to constrain our model to observations of the star formation rate functions at z = 6,7,8 from Bouwens et al. (2015), the Planck (2016) optical depth measurements, and the Becker & Bolton (2013) ionising emissivity data at z~5. We then use this constrained model to perform 21cm forecasting for LOFAR, HERA, and SKA in order to determine how well such data can characterise the sources driving reionisation. We find that the 21cm power spectrum alone can somewhat constrain the halo mass dependence of ionising sources, the photon escape fraction and ionising amplitude, but combining the 21cm data with other current observations enables us to separately constrain all these parameters. Our framework illustrates how 21cm data can play a key role in understanding the sources and topology of reionisation as observations improve.