On the contribution of active galactic nuclei to reionization (Research Note)

Abstract

The electron scattering optical depth constraints on reionization suggest that there may be other sources that contribute to the ionization of hydrogen aside from observable star forming galaxies. Often the calculated value of the electron scattering optical depth, τ_{es}, falls below the measurements derived from observations of the CMB or an assumption about non-observable sources must be made in order to reach agreement. Here, we calculate the hydrogen ionization fraction as a function of redshift and the electron scattering optical depth from both galaxies and active galactic nuclei (AGN) factoring in the secondary collisional ionizations from the AGN X-ray emission. In this paper we use the most current determination of the evolving hard X-ray luminosity function and extrapolate its evolution beyond z = 6. The AGN spectral energy distributions (SEDs) include both UV and X-ray ionizing photons. To search for the largest possible effect, all AGN are assumed to have λ_{Edd} = 1.0 and be completely unobscured. The results show that AGNs produce a perturbative effect on the reionization of hydrogen and remains in agreement with current constraints. Our calculations find the epoch of reionization still ends at z \approx 6 and only increases the electron scattering optical depth by ~2.3% under the most optimal conditions. This can only be moderately increased by assuming a constant black hole mass of M_{BH} = 10^(5) solar masses. As a result, we conclude that there is a need for other sources beyond observable galaxies and AGNs that contribute to the reionization of hydrogen at z > 6.

6 pages, 4 figures, accepted by Astronomy & Astrophysics

References in corpus (3)

• Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Likelihoods and Parameters from the WMAP data
• Keeping the Universe ionised: Photo-heating and the clumping factor of the high-redshift intergalactic medium
• Relative Role of Stars and Quasars in Cosmic Reionization

Cited by in corpus (19)

• The Birth of a Galaxy - III. Propelling reionisation with the faintest galaxies
• Cosmic X-ray Surveys of Distant Active Galaxies: The Demographics, Physics, and Ecology of Growing Supermassive Black Holes
• Fluctuating feedback-regulated escape fraction of ionizing radiation in low-mass, high-redshift galaxies
• Subaru high-z exploration of low-luminosity quasars (SHELLQs). I. Discovery of 15 quasars and bright galaxies at 5.7 < z < 6.9
• The X-ray luminosity function of Active Galactic Nuclei in the redshift interval z=3-5
• Escape of ionizing radiation from high redshift dwarf galaxies: role of AGN feedback
• The deepest X-ray view of high-redshift galaxies: constraints on low-rate black-hole accretion
• Constraining the UV emissivity of AGN throughout cosmic time via X-ray surveys
• An Introductory Review on Cosmic Reionization
• UV absorption lines and their potential for tracing the Lyman continuum escape fraction
• The Chandra Deep Fields: Lifting the Veil on Distant Active Galactic Nuclei and X-Ray Emitting Galaxies
• Evaluating the QSO contribution to the 21-cm signal from the Cosmic Dawn
• Outside the Lyman-break box: detecting Lyman continuum emitters at $3.5<z<5.1$ with CLAUDS
• The Impact of Unresolved Turbulence on the Escape Fraction of Lyman Continuum Photons
• Effect of primordial non-Gaussianities on the far-UV luminosity function of high-redshift galaxies: implications for cosmic reionization
• Extending Semi-numeric Reionisation Models to the First Stars and Galaxies
• High-Redshift AGNs and the Next Decade of Chandra and XMM-Newton
• Predicting High-magnification Events in Microlensed Quasars in the Era of LSST using Recurrent Neural Networks
• The Contribution of AGN Accretion Disks to Hydrogen Reionization