X-Ray Background at High Redshifts From Pop Iii Remnants: Results From Pop Iii Star Formation Rates in the Renaissance Simulations

Abstract

Due to their long mean free paths, X-rays are expected to have global impacts on the properties of the intergalactic medium (IGM) by their large scale heating and ionizing processes. At high redshifts, X-rays from Population (Pop) III binaries might have important effects on cosmic reionization and the Lyα forest. As a continuation of our previous work on Pop III binary X-rays , we use the Pop III distribution and evolution from the Renaissance Simulations, a suite of self-consistent cosmological radiation hydrodynamics simulations of the formation of the first galaxies, to calculate the X-ray luminosity density and background over the redshift range 20 ≥ z ≥ 7.6. As we find that Pop III star formation continues at a low, nearly constant rate to the end of reionization, X-rays are being continuously produced at significant rates compared to other possible X-ray sources, such as AGNs and normal X-ray binaries during the same period of time. We estimate that Pop III binaries produce approximately 6 eV of energy in the X-rays per hydrogen atom. We calculate the X-ray background for different monochromatic photon energies. KeV X-rays redshift and accumulate to produce a strong X-ray background spectrum extending to roughly 500 eV. The X-ray background is strong enough to heat the IGM to ∼ 1000 K and to ionize a few percent of the neutral hydrogen. These effects are important for an understanding of the neutral hydrogen hyperfine transition 21-cm line signatures, the Lyα forest, and the Thomson optical depth to the CMB.

Chasing the observational signatures of seed black holes at z > 7: candidate statistics

“…We predict a Pop III SFRD that is about 40− 100 times higher than that in the "rare peak" simulation by Xu et al (2013Xu et al ( , 2016b, and closer to the level obtained by Chon et al (2016) (assuming h = 0.73, see their figure 2). If compared with the "rare peak" simulation presented by Xu et al (2013Xu et al ( , 2016b, the higher SFRD that we find in our model reflects the conditions in the highly biased region that we are considering. If we estimate the over-density of our comoving vol-ume (50 Mpc 3 ) following the definition of Xu et al (2016a), < δ >=< ρ > /(Ω M ρ cr ) − 1, we find < δ >∼ 5, a factor of ∼ 7 higher than that of the "rare peak" simulation (< δ >∼ 0.68, see e.g.…”

“…With the above caveat in mind, we have compared our results with the recent enzo Renaissance simulation presented by Xu et al (2013Xu et al ( , 2016a ("rare peak" case) and with the Gadget-2 simulation by Chon et al (2016). We predict a Pop III SFRD that is about 40− 100 times higher than that in the "rare peak" simulation by Xu et al (2013Xu et al ( , 2016b, and closer to the level obtained by Chon et al (2016) (assuming h = 0.73, see their figure 2). If compared with the "rare peak" simulation presented by Xu et al (2013Xu et al ( , 2016b, the higher SFRD that we find in our model reflects the conditions in the highly biased region that we are considering.…”

Galactic wind X-ray heating of the intergalactic medium during the Epoch of Reionization

“…Possibilities include substantially more efficient HMXB formation than in the local Universe, a different wind environment or alternative wind production mechanisms, or additional sources of heating altogether. Other heating sources that have been considered include high mass X-ray binaries formed from Population III stars (Xu et al 2016) and an early population of mini-QSOs (Kuhlen et al 2006) or galaxies harbouring moderately massive black holes (Pacucci et al 2015). An initial mass function tilted towards massive Pop III stars could also lead to greater heating from pair-instability supernovae remnants.…”

X-ray background and its correlation with the 21 cm signal

“…It has been suggested that this high-z component could originate from accretion powered sources during the epoch of reionization (EoR), such as XRBs (Fragos et al 2013a,b;Xu et al 2016), and/or AGNs (Dijkstra et al 2004; Salvaterra et al 2005Salvaterra et al , 2007. Although they are also expected to contribute to the CXB, it is not possible to separate such components directly from the CXB measurements due to the lack of redshift information.…”