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

Abstract

We assess the impact of Galactic synchrotron foreground removal on the observation of high-redshift quasar H ii regions in redshifted 21-cm emission. We consider the case where a quasar is observed in an intergalactic medium (IGM) whose ionization structure evolves slowly relative to the light crossing time of the H II region, as well as the case where the evolution is rapid. The latter case is expected towards the end of the reionization 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 H II region and surrounding IGM. However, we find that foreground removal lowers the observed contrast between the H II 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 H II 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.