2020ApJ...888..112M

We use hydrodynamics and radiative transfer simulations to study the 21 cm signal around a bright QSO at z ∼ 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 ionizing and heating the surrounding gas. As expected, a longer QSO lifetime, t_QSO, results in a 21 cm signal in emission located at increasingly larger angular radii, θ, and covering a wider range of θ. Similar features can be obtained with a higher galactic emissivity efficiency, f_UV, such that determining the origin of a large ionized bubble (i.e., QSO versus stars) is not straightforward. Such degeneracy could be reduced by taking advantage of the finite light travel time effect, which is expected to affect an H II 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_QSO could be detected by Square Kilometre Array1-low instrument with a high signal-to-noise ratio (S/N). As a reference, for t_QSO = 10 Myr, a S/N ∼ 8 is expected assuming that no pre-heating of the intergalactic medium has taken place due to high-z energetic sources, while it can reach values above 10 in cases 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_UV and t_QSO.