Implications of the z ~ 5 Lyman-α forest for oberserving the EoR 21-cm power spectrum

Wednesday 11 Aug 2021 @ 12:00 p.m., Zoom
Dr Janakee Raste, Tata Institue of Fundamental Research; Email: -%%janakee[at]


Most ongoing experiments targetting the fluctuating 21-cm cosmological signal aim to look for the signal at redshifts well above 6. This strategy is motivated by the traditional assumption that reionization ends at z >~ 6. However, recent constraints from Lyman-α and CMB data prefer a significantly delayed reionization scenario in which reionization is 50% complete at redshifts as low as z ~ 7. In these models, reionization ends at z ~ 5, with large 100 Mpc “islands” of cold, neutral hydrogen persisting in the IGM well below z = 6. We have studied the effect of these neutral hydrogen islands on the 21-cm power spectrum by analysing outputs of a state-of-the-art radiative transfer simulation of the IGM calibrated to the CMB and Lyman-α forest data. We calculate the power spectra of the 21-cm signal from this simulations and compare them with a more traditional reionization model in which reionization is completed by z = 6. Contrary to previous models, we find that thanks to the late
end of reionization the 21-cm power continues at be high (~ 1 mK^2) at k ~ 0.1 h/cMpc at z = 5-6. At z = 5.5, for example, the power spectrum can be two or more orders of magnitude higher than the traditional models. This enhanced 21-cm power spectrum signal should be easily detectable by HERA and SKA1-LOW for reasonable integration times, assuming optimistic foreground subtraction. We argue that the redshift range z = 5-6 is very attractive for 21-cm experiments due to easier thermal noise characteristics and synergies with abundant multi-wavelength observations.