The subtlety of Ly-a photons: changing the expected range of the 21-cm signal

Abstract

We present the evolution of the 21-cm signal from cosmic dawn and the epoch of reionization (EoR) in an upgraded model including three subtle effects of Ly-α radiation: Ly-α heating, CMB heating (mediated by Ly-α photons), and multiple scattering of Ly-α photons. Taking these effects into account we explore a wide range of astrophysical models and quantify the impact of these processes on the global 21-cm signal and its power spectrum at observable scales and redshifts. We find that, in agreement with the literature, Ly-α and CMB heating raise the gas temperature by up to O(100) degrees in models with weak X-ray heating and, thus, suppress the predicted 21-cm signals. Varying the astrophysical parameters over broad ranges, we find that in the upgraded model the absorption trough of the global signal reaches a lowest floor of −165 mK at redshifts z ≈ 15 − 19. This is in contrast with the predictions for a pure adiabatically cooling Universe, for which the deepest possible absorption is a monotonically decreasing function of cosmic time and is −178 mK at z = 19 and −216 mK at z = 15, dropping to even lower values at lower redshifts (e.g. −264 mK at z = 10). With the Ly-α and CMB heating included we also observe a strong suppression in the low-redshift power spectra, with the maximum possible power (evaluated over the ensemble of models) attenuated by a factor of 6.6 at z = 9 and k = 0.1 Mpc −1 . Finally, we find that at high redshifts corresponding to cosmic dawn, the heating terms have a subdominant effect while multiple scattering of Ly-α photons is important, leading to an amplification of the power spectrum by a factor of ∼ 2 − 5.

A Comprehensive Bayesian re-analysis of the SARAS2 data from the Epoch of Reionization

“…maximum predicted depth, ≈ 250 mK of the global signal from simulations like those in Cohen et al (2017). However, more recent simulations, including Lyman-𝛼 and CMB heating, by Reis et al (2021) report a maximum predicted depth of ≈ 165 mK approximately 3 times shallower than the reported ≈ 500 mK signal from EDGES (Bowman et al 2018).…”

“…Visbal et al the Global Epoch of Reionization Signature, Bowman et al 2018) In 2018 the EDGES collaboration reported the detection of an absorption trough at 78 MHz (Bowman et al 2018). However, the reported feature is approximately 3 times deeper 2 than the current standard theoretical predictions (Reis et al 2021). While the excess depth can be explained theoretically with an excess radio background above the CMB (Bowman et al 2018;Feng & Holder 2018;Ewall-Wice et al 2018;Jana et al 2019;Mirocha & Furlanetto 2019;Reis et al 2020b) or interactions between dark matter and baryons (Barkana 2018;Fialkov et al 2018;Barkana et al 2018;Berlin et al 2018;Kovetz et al 2018;Muñoz & Loeb 2018;Slatyer & Wu 2018;Liu et al 2019) there are concerns about the data analysis and potential presence of systematics in the publicly available EDGES data (Hills et al 2018;Bradley et al 2019;Singh & Subrahmanyan 2019;Sims & Pober 2020;Bevins et al 2021a).…”

“…We use models that include Lyman-𝛼 heating (Madau et al 1997;Chen & Miralda-Escudé 2004;Furlanetto & Pritchard 2006;Chuzhoy & Shapiro 2007), CMB heating (Venumadhav et al 2018) and multiple scattering of Lyman-𝛼 photons (Semelin, B. et al 2007;Naoz & Barkana 2008;Baek, S. et al 2009;Visbal & McQuinn 2018;Molaro et al 2019;Reis et al 2020a). The effects on the global signal of these physical processes have been understood for some time but the magnitude of those effects over a larger parameter space were not understood until recently (Villanueva-Domingo et al 2020;Mittal & Kulkarni 2020;Reis et al 2021). Additionally, we study astrophysical scenarios with a wide range of radio production efficiencies, 𝑓 radio , for early galaxies.…”

“…Here, we determine parameter constraints over broad prior ranges using the latest astrophysical models of the global 21-cm signal (Reis et al 2020b(Reis et al , 2021, representing an improved understanding of the standard astrophysical picture, and the nested sampling algorithm (Skilling 2004) (Handley et al 2015a,b). We use models that include Lyman-𝛼 heating (Madau et al 1997;Chen & Miralda-Escudé 2004;Furlanetto & Pritchard 2006;Chuzhoy & Shapiro 2007), CMB heating (Venumadhav et al 2018) and multiple scattering of Lyman-𝛼 photons (Semelin, B. et al 2007;Naoz & Barkana 2008;Baek, S. et al 2009;Visbal & McQuinn 2018;Molaro et al 2019;Reis et al 2020a).…”

“…In this work we use the recently developed emulator (Bevins et al 2021b) which we train on sets of signal models from Reis et al (2020b) and Reis et al (2021). It has been shown that global signal emulators such as 21 GEM (Cohen et al 2020) can be used for quick interpolation of the signal across the astrophysical parameter space (Monsalve et al 2019).…”