Polarised foreground removal at low radio frequencies using rotation measure synthesis: uncovering the signature of hydrogen reionisation

Measurement of redshifted 21-cm emission from neutral hydrogen promises to be the most effective method for studying the reionisation history of hydrogen and, indirectly, the first galaxies. These studies will be limited not by raw sensitivity to the signal, but rather, by bright foreground radiation from Galactic and extragalactic radio sources and the Galactic continuum. In addition, leakage due to gain errors and non-ideal feeds conspire to further contaminate low-frequency radio obsevations. This leakage leads to a portion of the complex linear polarisation signal finding its way into Stokes I, and inhibits the detection of the non-polarised cosmological signal from the epoch of reionisation. In this work, we show that rotation measure synthesis can be used to recover the signature of cosmic hydrogen reionisation in the presence of contamination by polarised foregrounds. To achieve this, we apply the rotation measure synthesis technique to the Stokes I component of a synthetic data cube containing Galactic foreground emission, the effect of instrumental polarisation leakage, and redshifted 21-cm emission by neutral hydrogen from the epoch of reionisation. This produces an effective Stokes I Faraday dispersion function for each line of sight, from which instrumental polarisation leakage can be fitted and subtracted. Our results show that it is possible to recover the signature of reionisation in its late stages (z ~ 7) by way of the 21-cm power spectrum, as well as through tomographic imaging of ionised cavities in the intergalactic medium.

Modification of the 21-cm power spectrum by quasars during the epoch of reionisation

We assess the effect of a population of high-redshift quasars on the 21-cm power spectrum during the epoch of reionisation. Our approach is to implement a semi-numerical scheme to calculate the three-dimensional structure of ionised regions surrounding massive halos at high redshift. We include the ionising influence of luminous quasars by populating a simulated overdensity field with quasars using a Monte Carlo Markov Chain algorithm. We find that quasars modify both the amplitude and shape of the power spectrum at a level which is of the same order as the fractional contribution to reionisation. The modification is found both at constant redshift and at constant global neutral fraction, and arises because ionising photons produced by quasars are biased relative to the density field at a level that is higher than steller ionising photons. Our results imply that quasar ionisation will need to be included in detailed modelling of observed 21-cm power spectra.

The effect of Galactic foreground subtraction on redshifted 21-cm observations of quasar HII regions

We assess the impact of Galactic synchrotron foreground removal on the observation of high-redshift quasar HII regions in redshifted 21-cm emission. We consider the case where a quasar is observed in an intergalactic medium (IGM) whose ionisation structure evolves slowly relative to the light crossing time of the HII region, as well as the case where the evolution is rapid. The latter case is expected towards the end of the reionisation era where the highest redshift luminous quasars will be observed. In the absence of foregrounds the fraction of neutral hydrogen in the IGM could be measured directly from the contrast between the HII region and surrounding IGM. However, we find that foreground removal lowers the observed contrast between the HII region and the IGM. This indicates that measurement of the neutral fraction would require modelling to correct for this systematic effect. On the other hand, foreground removal does not modify the most prominent features of the 21-cm maps. Using a simple algorithm we demonstrate that measurements of the size and shape of observed HII regions will not be affected by continuum foreground removal. Moreover, measurements of these quantities will not be adversely affected by the presence of a rapidly evolving IGM.

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

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 quasars 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.