Kavli Affiliate: Martin Haehnelt
| Summary:
We present measurements of the spatially averaged HeII photo-ionization rate ($langle Γ_rm HeII rangle$), mean free path of HeII ionizing photons ($λ_rm mfp, HeII$), and HeII fraction ($f_rm HeII$) across seven redshift bins within the redshift range $2<z<4$. The measurements are obtained by comparing the observed effective optical depth distribution of HeII ($τ_rm eff, HeII$) with models generated by post-processing of the Sherwood simulation suite using our code EXCITE. With EXCITE, we efficiently explore a large parameter space ($sim 15000$ models) by varying $λ_rm mfp, HeII$ and $langle Γ_rm HeII rangle$. We employ Anderson-Darling test for the cumulative distribution of $τ_rm eff, HeII$ to simultaneously measure $λ_rm mfp, HeII$ and $langle Γ_rm HeII rangle$. 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 $langle Γ_rm HeII rangle$ and $λ_rm mfp, HeII$ decreasing by factors of $sim 4.32$ and $ sim 3.27$, respectively, from $z = 2.88$ to $z = 3.16$. Based on these measurements, we constrain the emissivity at the HeII ionization frequency ($ε_228$) and HeII ionizing photon emission rate ($dotn$), finding consistency with results from galaxy and QSO surveys. Comparison of our measured parameters with widely used uniform UVB models supports a scenario where HeII reionization is not completed before $zsim2.74$. 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 HeII reionization.
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