The Impact of a Percolating IGM on Redshifted 21 cm Observations of Quasar HII Regions

TL;DR

This study models the impact of inhomogeneous reionization on detecting quasar HII regions via 21cm observations, showing that significant signals are detectable with planned radio telescopes, especially during certain reionization stages.

Contribution

It introduces a semi-numerical scheme to simulate 3D ionized structures around high-redshift quasars, assessing detection prospects with upcoming radio telescopes.

Findings

Detectable HII regions at 5 sigma significance within 100 hours for neutral fractions ≥30%.

Detection sensitivity improves with longer integrations and stacking multiple HII regions.

Measurement of the global neutral fraction is limited by line-of-sight fluctuations, with varying accuracy depending on the neutral fraction.

Abstract

We assess the impact of inhomogeneous reionization on detection of HII regions surrounding luminous high redshift quasars using planned low frequency radio telescopes. Our approach is to implement a semi-numerical scheme to calculate the 3-dimensional structure of ionized regions surrounding a massive halo at high redshift, including the ionizing influence of a luminous quasar. As part of our analysis we briefly contrast our scheme with published semi-numerical models. We calculate mock 21cm spectra along the line of sight towards high redshift quasars, and estimate the ability of the planned Murchison Widefield Array to detect the presence of HII regions. The signal-to-noise for detection will drop as the characteristic bubble size grows during reionization because the quasar's influence becomes less prominent. However, quasars will imprint a detectable signature on observed 21cm spectra that is distinct from a region of typical IGM. At epochs where the mean hydrogen neutral fraction is ~30% or greater we find that neutral gas in the IGM surrounding a single quasar will be detectable (at a significance of 5 sigma) within 100 hour integrations in more than 50% of cases. 1000 hour integrations will be required to detect a smaller neutral fraction of 15% in more than 50% of cases. A highly significant detection will be possible in only 100 hours for a stack of 10 smaller 3 proper Mpc HII regions. The accurate measurement of the global average neutral fraction (<x_HI>) will be limited by systematic fluctuations between lines of sight for single HII regions. We estimate the accuracy with which the global neutral fraction could be measured from a single HII region to be 50%, 30% and 20% for <x_HI> ~ 0.15, 0.3 and 0.5 respectively.