Measurements of the thermal and ionization state of the intergalactic medium during the cosmic afternoon

Abstract

Abstract We perform the first measurement of the thermal and ionization state of the intergalactic medium (IGM) across 0.9 < z < 1.5 using 301 Lyα absorption lines fitted from 12 archival HST STIS quasar spectra. We employ the machine-learning-based inference method that uses joint Doppler parameter - column density ($b-N_{{\rm {H\,{\small I}}}{}}$) distributions obtained from Lyα forest decomposition. Our results show that the H i photoionization rates, $\Gamma _{{\rm{H\,{\small I}}}{}}$, agree with recent UV background synthesis models, with $\log (\Gamma _{{\rm{H\,{\small I}}}}/\text{s}^{-1})={-11.79}^{+0.18}_{-0.15}$, ${-11.98}^{+0.09}_{-0.09}$, and ${-12.32}^{+0.10}_{-0.12}$ at z = 1.4, 1.2, and 1 respectively. We obtain the IGM temperature at the mean density, T0, and the adiabatic index, γ, as [log (T0/K), γ] = [${4.13}^{+0.12}_{-0.10}$, ${1.34}^{+0.10}_{-0.15}$], $[{3.79}^{+0.11}_{-0.11}$, ${1.70}^{+0.09}_{-0.09}]$ and $[{4.12}^{+0.15}_{-0.25}$, ${1.34}^{+0.21}_{-0.26}]$ at z = 1.4, 1.2 and 1. Our measurements of T0 at z = 1.4 and 1.2 are consistent with the trend predicted from previous z < 3 temperature measurements and theoretical expectations, where the IGM cools down after He ii reionization in the absence of any non-standard heating. However, our T0 measurement at z = 1 shows unexpectedly high IGM temperature. Given the relatively large uncertainty in these measurements, where $\sigma _{T_0} \sim 5000$ K, mostly emanating from the limited size of our dataset, we can not conclude whether the IGM cools down as expected. Lastly, we generate mock datasets to test the constraining power of future measurement with larger datasets. The results demonstrate that, with redshift pathlength Δz ∼ 2 for each redshift bin, three times the current dataset, we can constrain the T0 of IGM within 1500K, which would be sufficient to constrain the IGM thermal history at z < 1.5 conclusively.