Misc,
Constraints on QSO emissivity using H I and He II Lyman alpha forest
(2017)cite arxiv:1702.03937Comment: 13 pages, 7 figures, submitted to MNRAS.
Abstract
The spectrum of cosmic ultraviolet background radiation at He II ionizing
energies (E > 4 Ryd) is important to study the He II reionization, thermal
history of the intergalactic medium (IGM) and metal lines observed in QSO
absorption spectra. It is determined by the emissivity of QSOs at E > 4 Ryd
obtained from their observed luminosity functions and the mean spectral energy
distribution (SED). The SED is approximated as a power-law at energies E > 1
Ryd, $f_E E^\alpha$, where the existing observations constrain the
power-law index $\alpha$ up to ~2.3 Ryd. Here, we constrain $\alpha$ for E > 4
Ryd using recently measured He II Lyman-alpha effective optical depths
($\tau_HeII$), H I photoionization rates and updated H I distribution in the
IGM. We find that -1.6 > $\alpha$ > -2 is required to reproduce the
$\tau_HeII$ measurements when we use QSO emissivity obtained from their
luminosity function using optical surveys. We also find that the models where
QSOs can alone reionize H I can not reproduce the $\tau_HeII$ measurements.
These models need modifications, such as a break in mean QSO SED at energies
greater than 4 Ryd. Even after such modifications the predicted He II
reionization history is significantly different than the standard models and
thermal history of the IGM will be crucial to distinguish them. We also provide
the photoionization rates of He II obtained from binned $\tau_HeII$
measurements.
