Observational studies indicate that the intergalactic medium (IGM) is highly ionized up to redshifts just over 6. A number of models have been developed to describe the process of reionization and the e †ects of the ionizing photons from the Ðrst luminous objects. In this paper we study the impact of an X-ray background, such as high-energy photons from early quasars, on the temperature and ionization of the IGM prior to reionization, before the fully ionized bubbles associated with individual sources have overlapped. X-rays have large mean free paths relative to EUV photons, and their photoelectrons can have signiÐcant e †ects on the thermal and ionization balance. We Ðnd that hydrogen ionization is dominated by the X-ray photoionization of neutral helium and the resulting secondary electrons. Thus, the IGM may have been warm and weakly ionized prior to full reionization. We examine several related consequences, including the Ðltering of the baryonic Jeans mass scale, signatures in the cosmic microwave background, and the H~-catalyzed production of molecular hydrogen.

The escape of ionizing Lyman continuum (LyC) photons requires the existence of low-N H I sightlines, which also promote escape of Lyα. We use a suite of 2500 Lyα Monte-Carlo radiative transfer simulations through models of dusty, clumpy interstellar ("multiphase") media from Gronke & Dijkstra, and compare the escape fractions of Lyα ( a f esc Ly ) and LyC radiation ( f esc ion ). We find that f esc ion and a f esc Ly are correlated: galaxies with a low a f esc Ly consistently have a low f esc ion , while galaxies with a high a f esc Ly exhibit a large dispersion in f esc ion . We argue that there is increasing observational evidence that Lyα escapes more easily from UV-faint galaxies. The correlation between f esc ion and a f esc Ly then implies that UV-faint galaxies contribute more to the ionizing background than implied by the faint-end slope of the UV luminosity function. In multiphase gases, the ionizing escape fraction is most strongly affected by the cloud covering factor, f cl , which implies that f esc ion is closely connected to the observed Lyα spectral line shape. Specifically, LyC-emitting galaxies typically having narrower, more symmetric line profiles. This prediction is qualitatively similar to that for "shell models."

We critique the hypothesis that the first stars were very massive stars (VMS; M > 140 Msun). We review the two major lines of evidence for the existence of VMS: (1) that the relative metal abundances of extremely metal-poor Galactic halo stars show evidence of VMS enrichment, and (2) that the high electron-scattering optical depth (tau_e) to the CMB found by WMAP requires VMS for reionization in a concordance LambdaCDM cosmology. The supernova yield patterns of VMS are incompatible with the Fe-peak and r-process abundances in halo stars. Models including Type II supernovae and/or ``hypernovae'' from metal-free progenitors with 8 - 40 Msun can better explain the observed trends. With a simple metal-transport model, we estimate that halo enrichment curtails metal-free star formation after ~10^8 yr at z ~ 20. Because the ionizing photon efficiency of metal-free stars peaks at ~120 Msun and declines at higher mass, an IMF with a lower bound at M ~ 10 - 20 Msun and no VMS can maximize the ionizing photon budget and still be consistent with the nucleosynthetic evidence. An IMF devoid of low-mass stars is justified independently by models of the formation of primordial stars. Using a semi-analytic model for H I and He II reionization, we find that such an IMF can reproduce tau_e = 0.10 - 0.14, consistent with the range from WMAP. We conclude, on the basis of these results, that VMS are not necessary to meet the existing constraints commonly taken to motivate them.Comment: final version accepted for ApJ Volume 612, September 200

The first stars hold intrinsic interest for their uniqueness and for their potentially important contributions to galaxy formation, chemical enrichment, and feedback on the intergalactic medium (IGM). Although the sources of cosmological reionization are unknown at present, the declining population of large bright quasars at redshifts z > 3 implies that stars are the leading candidates for the sources that reionized the hydrogen in the IGM by z $ 6. The metal-free composition of the first stars restricts the stellar energy source to protonproton burning rather than the more efficient CNO cycle. Consequently, they are hotter, smaller, and have harder spectra than their present-day counterparts of finite metallicity. We present new results from a continuing study of metal-free stars from a cosmological point of view. We have calculated evolving spectra of Population III clusters, derived from a grid of zero-metallicity stellar evolutionary tracks. We find that H-ionizing photon production from metal-free stellar clusters takes twice as long as that of Population II to decline to 1/10 its peak value. In addition, metal-free stars produce substantially more photons than Population II in the He ii (E > 4 ryd) continuum. We suggest that large Ly equivalent widths (W Ly > 400 Å ) may provide a means of detecting metal-free stellar populations at high redshift and that He ii recombination lines (1640, 4686) may confirm identifications of Population III. While Population III clusters are intrinsically bluer than their Population II counterparts, nebular continuum emission makes up this difference and may confuse attempts to discern Population III stars with broadband colors. In a companion paper, we explore the consequences of evolving spectra of Population III for the reionization of the IGM in both H and He.

We examine the significance of the first metal-free stars (Population III) for the cosmological reionization of H i and He ii. These stars have unusually hard spectra, with the integrated ionizing photon rates from a Population III stellar cluster for H i and He ii being 1.6 and 10 5 times stronger, respectively, than those from a Population II cluster. For the currently favored cosmology, we find that Population III stars alone can reionize H i at redshifts of z ' 9 and 4.7 and He ii at z ' 5:1 and 0.7 for continuous and instantaneous modes of star formation, respectively. More realistic scenarios involving combinations of Population III and Population II stellar spectra yield similar results for hydrogen. Helium never reionizes completely in these cases; the ionization fraction of He iii reaches a maximum of about 60% at z $ 5:6 if Population III star formation lasts for 10 9 yr. Future data on H i reionization can test the amount of small-scale power available to the formation of the first objects and provide a constraint on values of 8 d0:7. Since current UV observations indicate an epoch of reionization for He ii at z $ 3, He ii may reionize more than once. Measurements of the He ii Gunn-Peterson effect in the intergalactic medium at redshifts ze3 may reveal the significance of Population III stars for He ii reionization, particularly in '' void '' regions that may contain '' relic '' ionization from early Population III stellar activity.