Intergalactic Lyman continuum photon budget in the past 5 billion years

Abstract

We constrain the H I photoionization rate $(\Gamma_{\rm HI})$ at $z \lesssim 0.45$ by comparing the flux probability distribution function and power spectrum of the Ly-$\alpha$ forest data along 82 QSO sightlines obtained using Cosmic Origins Spectrograph with models generated from smoothed particle hydrodynamic simulations. We have developed a module named "Code for Ionization and Temperature Evolution (CITE)" for calculating the intergalactic medium (IGM) temperature evolution from high to low redshifts by post-processing the GADGET-2 simulation outputs. Our method, that produces results consistent with other simulations, is computationally less expensive thus allowing us to explore a large parameter space. It also allows rigorous estimation of the error covariance matrix for various statistical quantities of interest. We find that the best-fit $\Gamma_{\rm HI}(z)$ increases with $z$ and follows $(4 \pm 0.1) \times 10^{-14}\:(1+z)^{4.99 \pm 0.12}$ s$^{-1}$. At any given $z$ the typical uncertainties $\Delta \Gamma_{\rm HI} / \Gamma_{\rm HI}$ are $\sim 25$ per cent which contains not only the statistical errors but also those arising from possible degeneracy with the thermal history of the IGM and cosmological parameters and uncertainties in fitting the QSO continuum. These values of $\Gamma_{\rm HI}$ favour the scenario where only QSOs contribute to the ionizing background at $z<2$. Our derived $3\sigma$ upper limit on average escape fraction is $0.008$, consistent with measurements of low-$z$ galaxies.