Measuring photo-ionization rate and mean free path of HeII ionizing photons at $2.5\le z\le 3.6$: Evidence for late and rapid HeII reionization Part-II

Abstract

We present measurements of the spatially averaged HeII photo-ionization rate ($⟨{\Gamma }_{\mathrm{HeII}}⟩$), mean free path of HeII ionizing photons (${\lambda }_{\mathrm{mfp},\mathrm{HeII}}$), and HeII fraction ($f_{\mathrm{HeII}}$) 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 (${\tau }_{\mathrm{eff},\mathrm{HeII}}$) 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 ${\lambda }_{\mathrm{mfp},\mathrm{HeII}}$ and $⟨{\Gamma }_{\mathrm{HeII}}⟩$. We employ Anderson-Darling test for the cumulative distribution of ${\tau }_{\mathrm{eff},\mathrm{HeII}}$ to simultaneously measure ${\lambda }_{\mathrm{mfp},\mathrm{HeII}}$ and $⟨{\Gamma }_{\mathrm{HeII}}⟩$. 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 $⟨{\Gamma }_{\mathrm{HeII}}⟩$ and ${\lambda }_{\mathrm{mfp},\mathrm{HeII}}$ 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 (${\epsilon }_{228}$) and HeII ionizing photon emission rate ($\dot{n}$), 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 $z\sim 2.74$. 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 HeII reionization.

39 pages, 14 figures, 2 tables; Published in JCAP; Main results are summarized in Fig. 7, Fig. 8 and Table 2

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