Signatures of X-rays in the early Universe

• Published in: Monthly Notices of the Royal Astronomical Society
• Main Authors: MESINGER, ANDREI ALBERT, FERRARA, ANDREA, Spiegel DS
• Other Authors: Mesinger, ANDREI ALBERT, Ferrara, Andrea, Spiegel, Ds
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2013
• Subjects: Cosmology: Theory; Dark ages; reionization; first star; Early Universe; Intergalactic medium; X-rays: Galaxie; South Pole; South pole
• Online Access: http://hdl.handle.net/11384/24628; 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, 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, ...