http://arxiv.org/abs/1602.04407
We constrain the contribution of high-$z$ galaxies and active galactic nuclei (AGNs) to reionization, by comparing numerically computed H/He reionization with the observed HI/HeII fractions at various redshifts and optical depth to Thomson scattering. In the model, the contribution of galaxies is controlled by a parameter $f_{\rm esc}$ which indicates the escape fraction of ionizing photons from the galaxies, adopting an observed cosmic star formation history. On the other hand, in order to take ionizing photons from ANGs into account, observed X-ray luminosity functions and a composite spectral energy density with the energies in the range of $13.6\rm eV$ to $100\rm keV$ are assumed at $z\leq3$, while the redshift evolution of AGN abundance at $z>3$ is assumed to be proportional to $(1+z)^\beta$, where $\beta$ is a parameter in the model. We find that there are observationally allowed sets of the parameters $f_{\rm esc}$ and $\beta$. According to the comparisons, $\beta$ should satisfy $-4.2<\beta<-1.5$ with $0.1< f_{\rm esc}<0.18$, otherwise the model fails to complete the He{II} reionization by $z\approx3$. High escape fractions of $f_{\rm esc}>0.18$ are also unfavorable, because employing such high values results in early H{I} reionization. Interestingly even if we adopt $f_{\rm esc}<0.01$, the observed H{I}/He{II} fractions can be reproduced by numerical results with $\beta\approx-1$ as similar to the “AGN-dominated scenario” reported by Madau \& Haardt (2015). Such a high abundance of AGNs could be achieved if there were numbers of undiscovered faint AGNs during the EoR. We also confirm whether or not measurements of X-ray Background (XRB) is available for a constraint on AGN abundance during the EoR, and conclude that XRB is possibly difficult to use for the constraint, because the contribution of AGNs at $z>3$ is too small to be distinguished.
S. Yoshiura, K. Hasegawa, K. Ichiki, et. al.
Tue, 16 Feb 16
44/71
Comments: 9 pages, 8 figures, submitted to MNRAS
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