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تسجيل مستخدم جديدThe imprint of inhomogeneous HeII reionization on the HI and HeII Ly-alpha forest
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Michele Compostella
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We use a set of AMR hydrodynamic simulations post-processed with the radiative-transfer code RADAMESH to study how inhomogeneous HeII reionization affects the intergalactic medium (IGM). We propagate radiation from active galactic nuclei (AGNs) considering two scenarios for the time evolution of the ionizing sources. We find that HeII reionization takes place in a very inhomogeneous fashion, through the production of well separated bubbles of the ionized phase that subsequently percolate. Overall, the reionization process is extended in time and lasts for a redshift interval Delta z>1. At fixed gas density, the temperature distribution is bimodal during the early phases of HeII reionization and cannot be described by a simple effective equation of state. When HeII reionization is complete, the IGM is characterized by a polytropic equation of state with index gamma~1.20. This relation is appreciably flatter than at the onset of the reionization process (gamma=1.56) and also presents a much wider dispersion around the mean. We extract HI and HeII Ly-alpha absorption spectra from the simulations and fit Voigt profiles to them. We find that the regions where helium is doubly ionized are characterized by different probability density functions of the curvature and of the Doppler b parameters of the HI Ly-alpha forest as a consequence of the bimodal temperature distribution during the early phases of HeII reionization. The column-density ratio in HeII and HI shows a strong spatial variability. Its probability density function rapidly evolves with time reflecting the increasing volume fraction in which ionizing radiation is harder due to the AGN contribution. Finally we show that the number density of the flux-transmission windows per unit redshift and the mean size of the dark gaps in the HeII spectra have the potential to distinguish between different reionization scenarios. (abridged)
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101 -
Prakash Gaikwad
, Raghunathan Srianand
, Martin G. Haehnelt andn Tirthankar Roy Choudhury
2020
We characterise the thermal state of the intergalactic medium (IGM) in ten redshift bins in the redshift range $2 leq z leq 4$ with a sample of 103 high resolution, high S/N Ly$alpha$ forest spectra using four different flux distribution statistics.
Our measurements are calibrated with mock spectra from a large suite of hydrodynamical simulations post-processed with our thermal IGM evolution code CITE, finely sampling amplitude and slope of the expected temperature-density relation. The thermal parameters inferred from our measurements of the flux power spectrum, Doppler parameter distribution, as well as wavelet and curvature statistics agree well within their respective errors and all clearly show the peak in temperature and minimum in slope of the temperature density relation expected from HeII reionization. Combining our measurements from the different flux statistics gives $T_0=(14750 pm 1322)$K for the peak temperature at mean density and a corresponding minimum slope $gamma = 1.225 pm 0.120$. The peak in the temperature evolution occurs at $z approx 3$, in agreement with previous measurements that had suggested the presence of such a peak, albeit with a large scatter. Using CITE, we also calculate the thermal state of the IGM predicted by five widely used (spatially homogeneous) UV-background models. The rather rapid thermal evolution inferred by our measurements is well reproduced by two of the models, if we assume (physically well motivated) non-equilibrium evolution with photo-heating rates that are reduced by a moderate factor of $sim 0.7-0.8$. The other three models predict HeII reionization to be more extended with a somewhat earlier as well as higher temperature peak than our measurements suggest.
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