Reionization by Hard Photons. I. X‐Rays from the First Star Clusters

Abstract

Observations of the Lya forest at z D 3 reveal an average metallicity Z D 10~2The high-redshift Z _ . supernovae that polluted the intergalactic medium also accelerated relativistic electrons. Since the energy density of the cosmic microwave background is proportional to (1 ] z)4, at high redshift these electrons cool via inverse Compton scattering. Thus, the Ðrst star clusters emit X-rays. Unlike stellar UV ionizing photons, these X-rays can escape easily from their host galaxies. This has a number of important physical consequences :1. Owing to their large mean free path, these X-rays can quickly establish a universal ionizing background and partially reionize the universe in a gradual, homogeneous fashion. If X-rays formed the dominant ionizing background, the universe would have more closely resembled a single-phase medium rather than a two-phase medium.2. X-rays can reheat the universe to higher temperatures than possible with UV radiation. 3. X-rays counter the tendency of UV radiation to photodissociate an important coolant in the H 2 , early universe, by promoting gas-phase formation. H 2The X-ray production efficiency is calibrated to local observations of starburst galaxies, which imply that D10% of the supernova energy is converted to X-rays. While direct detection of sources in X-ray emission is difficult, the presence of relativistic electrons at high redshift and thus a minimal level of X-ray emission may be inferred by synchrotron emission observations with the Square Kilometer Array. These sources may constitute a signiÐcant fraction of the unresolved hard X-ray background and can account for both the shape and amplitude of the gamma-ray background. This paper discusses the existence and observability of high-redshift X-ray sources, while a companion paper models the detailed reionization physics and chemistry.

Secondary anisotropies of the CMB

“…Ricotti & Ostriker 2004, Ricotti, Ostriker & Gnedin 2005, Madau et al 2004, Oh 2001, Dijkstra, Haiman & Loeb 2004. In view of the correlation between central black hole mass and spheroid velocity dispersion (Ferrarese & Merritt 2000, Gebhardt et al 2000, miniquasars are as plausible ionisation sources as are Population III stars, whose nucleosynthetic traces have not yet been seen even in the most metal-poor halo stars nor in the high z Lyman alpha forest.…”

The Era of Massive Population III Stars: Cosmological Implications and Self‐Termination

“…Furthermore, SN explosions cause nearly complete evacuation of the host halos (see Paper I). Therefore, regardless of the efficiency of the proposed positive feedback on molecular hydrogen formation (e.g., Ferrara 1998;Oh 2001), it should take some time for the heated and expelled gas to cool and condense. Subsequent star formation can take place only after the gas falls back into the halos again.…”

“…This would in turn promote the formation of H 2 , hence enabling enhanced cooling of primordial gas (e.g., Haiman, Rees, & Loeb 1996;Oh 2001;Glover & Brand 2003). Apart from early quasars (e.g., Eisenstein & Loeb 1995;Bromm & Loeb 2003;Madau et al 2004), SNRs are the only plausible X-ray sources at very high redshifts.…”

Leptonic dark matter annihilation in the evolving universe: constraints and implications

“…We assume that the X-ray emissivity is proportional to the star formation rate, which at high redshifts is proportional to the differential increase of baryon collapse fraction [62] and is given by [63](see also [64]),…”