Measuring photo-ionization rate and mean free path of He ii ionizing photons at 2.5 ≤ z ≤ 3.6: evidence for late and rapid HeII reionization part II

We present measurements of the spatially averaged He ii photo-ionization rate (〈ΓHeII〉), mean free path of He ii ionizing photons (λmfp,HeII), and He ii fraction (f HeII) across seven redshift bins within the redshift range 2 < z < 4. The measurements are obtained by comparing the observed effective optical depth distribution of He ii (τ eff,HeII) with models generated by post-processing of the Sherwood simulation suite using our code ex-cite. With ex-cite, we efficiently explore a large parameter space (∼ 15000 models) by varying λmfp,HeII and 〈ΓHeII〉. We employ a non-parametric Anderson-Darling test for the cumulative distribution of τ eff,HeII to simultaneously measure λmfp, HeII and 〈ΓHeII〉. Our measurements account for possible observational and modeling uncertainties stemming mainly from the finite signal-to-noise ratio of the observed data and thermal parameter uncertainties. We find significant evolution, with the best-fit 〈ΓHeII〉 and λmfp,HeII decreasing by factors of ∼ 4.32 and ∼ 3.27, respectively, from z = 2.88 to z = 3.16. This decreasing trend in 〈ΓHeII〉 and λmfp,HeII suggests evolution of the size of ionized regions implying that HeII reionization is still ongoing at these redshifts. Based on these measurements, we constrain the emissivity at the He ii ionization frequency (ϵ 228) and He ii ionizing photon emission rate (ṅ), finding consistency with results from galaxy and QSO surveys. Comparison of our measured parameters with widely used uniform UVB models supports a scenario where He ii reionization is not completed before z ∼ 2.74. Our measured evolution is complementary and in good agreement with recent measurements of thermal parameters of the IGM, suggesting a coherent picture of rather late and rapid He ii reionization.