Signatures of X-rays in the early Universe

Abstract

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, xHI. Specifically, X-rays produce a dearth of fully neutral regions and a suppression of small-scale (k > 0.1Mpc-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 (xHI < 0.7) is modest, except in extreme, X-ray-dominated models. The largest impact of X-rays is to govern the timing and duration of IGM heating. In fact, unless thermal feedback is efficient, the epoch of X-ray heating likely overlaps with the beginning of reionization. This results in a 21 cm power spectrum which is ~10-100 times higher at xHI > 0.9 than obtained from naive estimates ignoring this overlap. On the other hand, if thermal feedback is efficient, the resulting extended epoch between X-ray heating and reionization could provide a clean probe of the matter power spectrum in emission, at redshifts more accessible than the dark ages. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.