Revealing reionization with the thermal history of the intergalactic medium: new constraints from the Lyman-$\alpha$ flux power spectrum

TL;DR

This study uses the Lyman-$eta$ flux power spectrum from high-resolution spectra to constrain the thermal history of the intergalactic medium during reionization, providing new insights into the timing and energy input of reionization.

Contribution

It presents the first observational constraints on the IGM's thermal history at high redshifts using small-scale flux power spectrum measurements.

Findings

Temperatures at mean density are around 7000-8000 K with no significant evolution from z~4.2 to 5.0.

Total energy input per unit mass increases from 4.6 eV to 7.3 eV from z~6 to 4.2.

Results are consistent with reionization occurring around z~8.5, aligning with Planck constraints.

Abstract

We present a new investigation of the thermal history of the intergalactic medium (IGM) during and after reionization using the Lyman-$\alpha$ forest flux power spectrum at $4.0\lesssim z\lesssim5.2$. Using a sample of 15 high-resolution spectra, we measure the flux power down to the smallest scales ever probed at these redshifts ($-1\lesssim \log(k/$km$^{-1}$s)$\lesssim -0.7$). These scales are highly sensitive to both the instantaneous temperature of the IGM and the total energy injected per unit mass during and after reionization. We measure temperatures at the mean density of $T_{0}\sim7000$-8000 K, consistent with no significant temperature evolution for redshifts $4.2\lesssim z\lesssim5.0$. We also present the first observational constraints on the integrated IGM thermal history, finding that the total energy input per unit mass increases from $u_{0}\sim4.6$ ${\rm eV}$ $m_{\rm p}^{-1}$ to 7.3 eV $m_{\rm p}^{-1}$ from $z\sim 6$ to $4.2$ assuming a $\Lambda$-CDM cosmology. We show how these results can be used simultaneously to obtain information on the timing and the sources of the reionization process. Our first proof of concept using simplistic models of instantaneous reionization produces results comparable to and consistent with the recent Planck constraints, favoring models with $z_{\rm rei}\sim 8.5^{+1.1}_{-0.8}$.