Prospects of observing a quasar HII region during the Epoch of Reionization with redshifted 21cm

TL;DR

This study explores the potential to observe quasar-induced H II regions during the Epoch of Reionization using 21cm signals, assessing detectability with LOFAR and implications for understanding quasar properties.

Contribution

It demonstrates that large quasar H II regions are detectable with LOFAR and provides methods to constrain quasar lifetimes and ionizing photon rates from 21cm data.

Findings

Quasar H II regions are more spherical but not significantly larger than stellar H II regions.

Detection of ~25 Mpc radius H II regions is feasible with 1200 hours of LOFAR observation.

Realistic noise and foregrounds do not prevent detection or analysis of quasar properties.

Abstract

We present a study of the impact of a bright quasar on the redshifted 21cm signal during the Epoch of Reionization (EoR). Using three different cosmological radiative transfer simulations, we investigate if quasars are capable of substantially changing the size and morphology of the H II regions they are born in. We choose stellar and quasar luminosities in a way that is favourable to seeing such an effect. We find that even the most luminous of our quasar models is not able to increase the size of its native H II region substantially beyond those of large H II regions produced by clustered stellar sources alone. However, the quasar H II region is found to be more spherical. We next investigate the prospects of detecting such H II regions in the redshifted 21cm data from the Low Frequency Array (LOFAR) by means of a matched filter technique. We find that H II regions with radii ~ 25 comoving Mpc or larger should have a sufficiently high detection probability for 1200 hours of integration time. Although the matched filter can in principle distinguish between more and less spherical regions, we find that when including realistic system noise this distinction can no longer be made. The strong foregrounds are found not to pose a problem for the matched filter technique. We also demonstrate that when the quasar position is known, the redshifted 21cm data can still be used to set upper limits on the ionizing photon rate of the quasar. If both the quasar position and its luminosity are known, the redshifted 21 cm data can set new constrains on quasar lifetimes.