Improved measurements of the intergalactic medium temperature around quasars: possible evidence for the initial stages of He ii reionization at z ≃ 6

We present measurements of the intergalactic medium (IGM) temperature within ~5 proper Mpc of seven luminous quasars at z~6. The constraints are obtained from the Doppler widths of Lyman-alpha absorption lines in the quasar near-zones and build upon our previous measurement for the z=6. 02 quasar SDSS J0818+1722. The expanded data set, combined with an improved treatment of systematic uncertainties, yields an average temperature at the mean density of log (T₀/K) = 4. 21 \ 0. 03 (\⁰. 06₀. 07) at 68 (95) per cent confidence for a flat prior distribution over 3. 2 < log (T₀/K) < 4. 8. In comparison, temperatures measured from the general IGM at z~5 are ~0. 3 dex cooler, implying an additional source of heating around these quasars which is not yet present in the general IGM at slightly lower redshift. This heating is most likely due to the recent reionisation of He-II in vicinity of these quasars, which have hard, non-thermal ionising spectra. The elevated temperatures may therefore represent evidence for the earliest stages of He-II reionisation in the most biased regions of the high-redshift Universe. The temperature as a function of distance from the quasars is consistent with being constant, log (T₀/K) ~4. 2, with no evidence for a line-of-sight thermal proximity effect. However, the limited extent of the quasar near-zones prevents the detection of He-III regions larger than ~5 proper Mpc. Under the assumption the quasars have reionised the He-II in their vicinity, we infer the data are consistent with an average optically bright phase of duration in excess of 10⁶. 5 yr. These measurements represent the highest-redshift IGM temperature constraints to date, and thus provide a valuable data set for confronting models of H-I reionisation.