Signatures of X-rays in the early Universe (1210.7319v3)

Abstract: [abridged] 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. We explore this in various signals: (i) Reionization history: including X-rays results in an earlier, more extended reionization. Efficient thermal feedback from X-ray heating could yield an extended, ~10% ionized epoch. (ii) Reionization morphology: a sizable (~10%) 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, xH. However, changes in morphology cannot be countered by increasing the bias of the ionizing sources, making them a robust signature. (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 microK^2. Our extreme model in which X-rays dominate reionization is the only one that is marginally consistent with upper limits from the South Pole Telescope, assuming no thermal Sunyaev-Zel'dovich (tSZ) - dusty galaxy correlation. Since this extreme model is unlikely, we conclude that there should be a sizable tSZ-dusty galaxy signal. (iv) The cosmic 21cm signal: the impact of X-rays on the 21cm power spectrum during the advanced stages of reionization (xH<0.7) is modest, except in extreme, X-ray dominated models. The largest impact of X-rays is to govern IGM heating. In fact, unless thermal feedback is efficient, the epoch of X-ray heating likely overlaps with the beginning of reionization (xH>0.9). This results in a 21cm power spectrum which is ~ 10-100 times higher than obtained from naive estimates ignoring this overlap. However, 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.

Signatures of X-rays in the Early Universe

The paper by Mesinger et al. investigates the influence of X-rays on the thermal and ionization history of the early Universe. By conducting semi-numeric simulations of the Dark Ages and the Epoch of Reionization (EoR), the authors aim to provide frameworks for interpreting upcoming observations related to cosmic X-ray contributions during reionization. The study explores various astrophysical scenarios, altering parameters such as X-ray efficiency, minimum halo virial temperature ($T_{\rm vir}$), and UV photon contribution, to elucidate the global and local implications of X-ray involvement in cosmic evolution.

Key Findings

• Reionization History: Inclusion of X-rays generates an earlier and more extended EoR. The simulations suggest increased thermal feedback from X-ray heating potentially lengthening the reionization period wherein the Universe remains partially ionized (approximately 10%). The timing of this epoch impacts observational constraints such as the integrated Thomson scattering optical depth ($\tau_e$).
• Reionization Morphology: A modest yet significant contribution of X-rays (around 10%) leads to more uniform ionization morphology. The X-rays contribute to larger mean free paths compared to UV photons, resulting in less small-scale ionization structure and a reduction of small-scale ionization power by a factor of two.
• Kinetic Sunyaev-Zel'dovich (kSZ) Effect: At a fixed reionization history, X-rays reduce the kSZ power at $l=3000$ by approximately 0.5 units. Models with higher X-ray dominance, particularly those driven entirely by X-rays, show compatibility with recent kSZ power spectrum constraints barring other correlations such as the thermal Sunyaev-Zel'dovich (tSZ) effect with dusty galaxies.
• Redshifted 21cm Signal: The influence of X-rays on the 21cm power spectrum during advanced reionization stages is modest. However, X-rays are crucial in dictating the timing and length of IGM heating. Efficient thermal feedback scenarios predict a potential overlap between X-ray heating and the onset of reionization, resulting in significantly heightened 21cm power at high neutral fractions.

Implications and Future Directions

Practically, the research highlights how X-ray driven reionization could alter the morphology and timeline of the EoR, challenging the assumption that stellar UV photons predominantly drive reionization processes. Theoretically, the study suggests that X-ray contributions provide distinctive signatures that could be observed through cosmological probes such as the kSZ effect and 21cm line signals. The presence of a partially-ionized cosmic 'haze' and the suppression of small-scale ionization morphology are indicative of substantial X-ray involvement.

Future developments in astronomical instrumentation, notably interferometers like LOFAR, MWA, and future prospects such as the SKA, will have the sensitivity to directly image these signatures at high resolution. These advancements will allow the validation of model predictions related to the timing and nature of reionization and the role of X-ray emissions. Additionally, next-generation all-sky experiments will offer insights into X-ray-driven IGM heating epochs by analyzing the global brightness temperature evolution—a domain where X-rays have the most substantial cosmological imprint.

In conclusion, Mesinger et al. advocate for a comprehensive approach to characterizing the EoR. Their study lends credence to the notion that X-rays, alongside UV emissions, could play a vital role in shaping the cosmos during its transitional phase from a neutral to ionized state, paving the way for refined observational strategies to uncover the complexities of the early Universe.