Title
Constraints on First-Light Ionizing Sources from Optical Depth of the Cosmic Microwave Background
Document Type
Article
Publication Date
2008
Abstract
We examine the constraints on high-redshift star formation, ultraviolet and X-ray preionization, and the epoch of reionization at redshift zr, inferred from the recent WMAP-5 measurement, τe = 0.084 ± 0.016, of the electron- scattering optical depth of the cosmic microwave background (CMB). Half of this scattering can be accounted for by the optical depth, τe = 0.04-0.05, of a fully ionized intergalactic medium (IGM) at z ≤ zGP ≈ 6-7, consistent with Gunn-Peterson absorption in neutral hydrogen. The required additional optical depth, Δτe = 0.03 ± 0.02 atz > zGP, constrains the ionizing contributions of 'first light' sources. WMAP-5 also measured a significant increase in small- scale power, which lowers the required efficiency of star formation and ionization from minihalos. Early massive stars (UV radiation) and black holes (X-rays) can produce a partially ionized IGM, adding to the residual electrons left from incomplete recombination. Inaccuracies in computing the ionization history, xe(z), and degeneracies in cosmo- logical parameters (Ωm, Ωb, σ8, ns) add systematic uncertainty to the measurement and modeling of τe. From the additional optical depth from sources at z > zGP, we limit the star formation efficiency, the rate of ionizing photon production for Population III and Population II stars, and the photon escape fraction, using standard histories of baryon collapse, minihalo star formation, and black hole X-ray preionization.
DOI
10.1086/590898
Recommended Citation
Shull, J., & Venkatesan, A. (2008). Constraints on first-light ionizing sources from optical depth of the cosmic microwave background. Astrophysical Journal, 685(1), 1-7. doi:10.1086/590898
Comments
Copyright 2008 American Astronomical Society.
Published article available at: http://dx.doi.org/10.1086/590898