[Paper Review] Imaging HII Regions from Galaxies and Quasars During Reionisation with SKA

J. Stuart B. Wyithe, Paul M. Geil+1 more|arXiv (Cornell University)|Jan 17, 2015

Radio Astronomy Observations and Technology3 citations

TL;DR

This paper investigates the feasibility of imaging HII regions from galaxies and quasars during cosmic reionisation using the SKA1-LOW telescope. Using semi-numerical simulations, it demonstrates that SKA1-LOW’s resolution and sensitivity can resolve HII regions from feedback-affected galaxies and detect smaller regions in early reionisation, provided the field of view covers several degrees to image large-scale ionised structures.

ABSTRACT

The ionisation structure of the Intergalactic Medium (IGM) during reionisation is sensitive to the unknown galaxy formation physics that prevailed at that time. This structure introduces non-Gaussian statistics into the redshifted 21 cm fluctuation amplitudes that can only be studied through tomographic imaging, which will clearly discriminate between different galaxy formation scenarios. Imaging the ionisation structure and cosmological HII regions during reionisation is therefore a key goal for the SKA. For example, the SKA1-LOW baseline design with a 1 km diameter core will resolve HII regions expected from galaxy formation models which include strong feedback on low-mass galaxy formation. Imaging the smaller HII regions that result from galaxy formation in the absence of SNe feedback will also be possible for SKA1-LOW in the later stages of reionisation, but may require the greater sensitivity of SKA early in the reionisation era. In addition to having baselines long enough to resolve the HII regions, the field of view for SKA1-LOW reionisation experiments should be at least several degrees in order to image the largest HI structures towards the end of reionisation. The baseline design with 35 meter diameter stations has a field of view within a single primary pointing which is sufficient for this purpose.

Motivation & Objective

• To assess whether SKA1-LOW can image HII regions formed during cosmic reionisation, particularly those driven by galaxies and quasars.
• To determine the observational requirements—resolution, sensitivity, and field of view—needed to distinguish between different galaxy formation models during reionisation.
• To evaluate the role of tomographic imaging in detecting non-Gaussian statistics in 21 cm fluctuations, which are sensitive to ionisation structure and feedback physics.
• To investigate the detectability of HII regions under varying feedback scenarios, including strong supernova feedback and its absence.
• To establish the minimum field of view required for SKA1-LOW to image large-scale HI structures near the end of reionisation.

Proposed method

• Utilizes semi-numerical simulations to model the ionisation structure of the intergalactic medium (IGM) during reionisation, based on source catalogues and analytic HII region models.
• Applies a filtering technique based on the spherically averaged radial density profile of HII regions to estimate ionisation fields, ensuring photon conservation and self-consistent overlap.
• Evaluates angular resolution and sensitivity requirements for detecting HII regions of varying sizes, depending on feedback strength in galaxy formation models.
• Assesses the field of view needed to image large-scale HI structures (up to ~100 Mpc) at the end of reionisation, accounting for light-cone effects and cosmic variance.
• Compares the detectability of HII regions in the presence and absence of supernova feedback, linking resolution and sensitivity to observable features.
• Uses simulated 21 cm brightness temperature fluctuations to model observable signatures, focusing on tomographic imaging rather than power-spectrum statistics.

Experimental results

Research questions

• RQ1 Can SKA1-LOW resolve HII regions expected from galaxy formation models with strong feedback on low-mass galaxies?
• RQ2 What sensitivity and resolution are required to image smaller HII regions formed in the absence of supernova feedback during early reionisation?
• RQ3 What is the minimum field of view required for SKA1-LOW to image large-scale HI structures (up to ~100 Mpc) at the end of reionisation?
• RQ4 How do light-cone effects and cosmic variance constrain the observed size of HII regions during the overlap phase of reionisation?
• RQ5 To what extent can imaging of HII region morphology distinguish between 'inside-out' and 'outside-in' reionisation scenarios?

Key findings

• SKA1-LOW with a 1 km diameter core can resolve HII regions expected from galaxy formation models that include strong feedback on low-mass galaxies.
• Imaging of smaller HII regions from galaxies without supernova feedback may require the greater sensitivity of SKA, especially in the early-to-mid phases of reionisation.
• The field of view for SKA1-LOW reionisation experiments should be at least several degrees to image the largest HI structures, which can extend up to ~100 Mpc in size.
• The baseline SKA1-LOW design with 35 m diameter stations provides a field of view within a single pointing that is marginally sufficient for imaging large-scale structures.
• Non-Gaussian statistics in 21 cm fluctuation amplitudes—driven by the ionisation structure of the IGM—can only be studied through tomographic imaging, which discriminates between galaxy formation scenarios.
• The smooth edges of HII regions around massive galaxies, caused by source clustering, require high angular resolution to resolve, implying that detailed imaging of ionisation fronts will depend on SKA’s sensitivity and resolution.