Observing the redshifted 21 cm signal around a bright QSO at $z\sim 10$

TL;DR

This study uses simulations to analyze the 21 cm signal around a bright QSO at redshift ~10, showing how QSO lifetime and galactic emissivity influence the signal and how observations can constrain QSO properties.

Contribution

It demonstrates the impact of QSO lifetime and galactic emissivity on the 21 cm signal and proposes using light travel time effects to distinguish QSO from stellar sources.

Findings

Longer QSO lifetime increases the size of the 21 cm emission region.

Detection of the 21 cm signal by SKA1-low is feasible with high S/N.

Observations can constrain QSO lifetime and reduce source degeneracy.

Abstract

We use hydrodynamics and radiative transfer simulations to study the 21~cm signal around a bright QSO at $z \sim 10$. Due to its powerful UV and X-ray radiation, the QSO quickly increases the extent of the fully ionized bubble produced by the pre-existing stellar type sources, in addition to partially ionize and heat the surrounding gas. As expected, a longer QSO lifetime, $t_{\rm QSO}$, results in a 21~cm signal in emission located at increasingly larger angular radii, $\theta$, and covering a wider range of $\theta$. Similar features can be obtained with a higher galactic emissivity efficiency, $f_{\rm UV}$, so that determining the origin of a large ionized bubble (i.e. QSO vs stars) is not straightforward. Such degeneracy could be reduced by taking advantage of the finite light traveltime effect, which is expected to affect an HII region produced by a QSO differently from one created by stellar type sources. From an observational point of view, we find that the 21 cm signal around a QSO at various $t_{\rm QSO}$ could be detected by SKA1-low with a high signal-noise ratio (S/N). As a reference, for $t_{\rm QSO} = 10\,\rm Myr$, a S/N $\sim 8$ is expected assuming that no pre-heating of the IGM has taken place due to high-$z$ energetic sources, while it can reach value above 10 in case of pre-heating. Observations of the 21~cm signal from the environment of a high-$z$ bright QSO could then be used to set constraints on its lifetime, as well as to reduce the degeneracy between $f_{\rm UV}$ and $t_{\rm QSO}$.