Photon underproduction crisis: Are QSOs sufficient to resolve it?

Abstract

We investigate the recent claim of 'photon underproduction crisis' by Kollmeier et al. (2014) which suggests that the known sources of ultra-violet (UV) radiation may not be sufficient to generate the inferred hydrogen photoionization rate ($\Gamma_{\rm HI}$) in the low redshift inter-galactic medium. Using the updated QSO emissivities from the recent studies and our cosmological radiative transfer code developed to estimate the UV background, we show that the QSO contribution to $\Gamma_{\rm HI}$ is higher by a factor ~2 as compared to the previous estimates. Using self-consistently computed combinations of star formation rate density and dust attenuation, we show that a typical UV escape fraction of 4% from star forming galaxies should be sufficient to explain the inferred $\Gamma_{\rm HI}$ by Kollmeier et al. (2014). Interestingly, we find that the contribution from QSOs alone can explain the recently inferred $\Gamma_{\rm HI}$ by Shull et al. (2015) which used the same observational data but different simulation. Therefore, we conclude that the crisis is not as severe as it was perceived before and there seems no need to look for alternate explanations such as low luminosity hidden QSOs or decaying dark matter particles.