Signatures of X-rays in the early Universe

• Published in: Monthly Notices of the Royal Astronomical Society
• Main Authors: Mesinger, Andrei, Ferrara, Andrea, Spiegel, David S.
• Format: Text
• Language: English
• Published: Oxford University Press 2013
• Subjects: Article; South Pole; South pole
• Online Access: http://mnras.oxfordjournals.org/cgi/content/short/stt198v1; https://doi.org/10.1093/mnras/stt198

With their long mean free paths and efficient heating of the intergalactic medium (IGM), X-rays could have a dramatic impact on the thermal and ionization history of the Universe. Here we run several semi-numerical simulations of the dark ages and the epoch of reionization (EoR), including both X-rays and ultraviolet radiation fields, attempting to provide an intuitive framework for interpreting upcoming observations. We explore the impact of X-rays on various signals. (i) Reionization history: including X-rays results in an earlier, slightly more extended EoR. However, efficient thermal feedback from X-ray heating could yield an extended epoch in which the Universe was ≈10 per cent ionized. (ii) Reionization morphology: a sizeable (∼10 per cent) contribution of X-rays to reionization results in a more uniform morphology, though the impact is modest when compared at the same global neutral fraction, <f>$\bar{x}_{\rm H\,{\small {i}}}$</f>. Specifically, X-rays produce a dearth of fully neutral regions and a suppression of small-scale ( k ≳ 0.1 Mpc−1) ionization power by a factor of ≲2. However, these changes in morphology cannot be countered by increasing the bias of the ionizing sources, making them a robust indicator of an X-ray contribution to the EoR. (iii) The kinetic Sunyaev–Zel’dovich (kSZ) effect: at a fixed reionization history, X-rays decrease the kSZ power at l = 3000 by ≈ 0.5 μK2. Our extreme model in which X-rays entirely drive reionization is the only one which is marginally consistent with the recent upper limits on this signal from the South Pole Telescope, assuming no thermal Sunyaev–Zel’dovich (tSZ)–dusty galaxy cross-correlation. Since this extreme model is unlikely, we conclude that there should be a sizeable tSZ–dusty galaxy cross-correlation. (iv) The redshifted 21 cm signal: the impact of X-rays on the 21 cm power spectrum during the advanced stages of reionization (<f>$\bar{x}_{\rm H\,{\small {i}}}\lesssim 0.7$</f>) is modest, except in extreme, X-ray-dominated ...