Evaluating the QSO contribution to the 21-cm signal from the Cosmic Dawn

Abstract

The upcoming radio interferometer Square Kilometre Array (SKA) is expected to directly detect the redshifted 21-cm signal from the neutral hydrogen present during the Cosmic Dawn. Temperature fluctuations from X-ray heating of the neutral intergalactic medium can dominate the fluctuations in the 21-cm signal from this time. This heating depends on the abundance, clustering, and properties of the X-ray sources present, which remain highly uncertain. We present a suite of three new large-volume, 349 Mpc a side, fully numerical radiative transfer simulations including QSO-like sources, extending the work previously presented in Ross et al. (2017). The results show that our QSOs have a modest contribution to the heating budget, yet significantly impact the 21-cm signal. Initially, the power spectrum is boosted on large scales by heating from the biased QSO-like sources, before decreasing on all scales. Fluctuations from images of the 21-cm signal with resolutions corresponding to SKA1-Low at the appropriate redshifts are well above the expected noise for deep integrations, indicating that imaging could be feasible for all the X-ray source models considered. The most notable contribution of the QSOs is a dramatic increase in non-Gaussianity of the signal, as measured by the skewness and kurtosis of the 21-cm probability distribution functions. However, in the case of late Lyman-α saturation, this non-Gaussianity could be dramatically decreased particularly when heating occurs earlier. We conclude that increased non-Gaussianity is a promising signature of rare X-ray sources at this time, provided that Lyman-α saturation occurs before heating dominates the 21-cm signal.

Constraining the intergalactic medium at z ≈ 9.1 using LOFAR Epoch of Reionization observations

“…The upper limits from Mertens et al (2020) at scales k = 0.075 and 0.1 h Mpc −1 are ∆ 2 = (58.97) 2 mK 2 and (95.21) 2 mK 2 respectively (see also Table 1). Before proceeding, it should be realized that these values are rather high compared to the power spectrum at redshift ≈ 9.1 predicted by various standard reionization scenarios, such as in Mellema et al (2006a), Iliev et al (2007), Greig & Mesinger (2015), Ghara et al (2015b), Hassan et al (2016), Bolgar et al (2018) and Ross et al (2019). For example, the predicted power spectra at z ≈ 9 at k = 0.1 h Mpc −1 are found to be 10 3 mK 2 .…”

Position-dependent power spectra of the 21-cm signal from the epoch of reionization

“…Refs. [50,51] showed that spin temperature fluctuations can have a strong impact on non-Gaussianity of the signal. Ref.…”

Impact of Ly α heating on the global 21-cm signal from the Cosmic Dawn

“…The strength of the redshifted 21-cm signal from the CD depends on the gas temperature and background Lyα flux which are determined by the radiation sources and the heating/cooling processes. Several heating processes such as X-ray heating (Pritchard & Furlanetto 2007;Mesinger et al 2013;Ghara et al 2015bGhara et al , 2016Ross et al 2019;Islam et al 2019), shock heating (Furlanetto & Loeb 2004) and heating due to resonance scattering of Lyα photons (hereafter 'Lyα heating') (Chen & Miralda-Escudé 2004;Chuzhoy & Shapiro 2007;Furlanetto & Pritchard 2006) can increase the kinetic temperature of the gas in the IGM during these epochs. However, the relative contribution of these mechanisms is uncertain.…”