A first direct measurement of the intergalactic medium temperature around a quasar at z = 6

The thermal state of the intergalactic medium (IGM) provides an indirect probe of both the H i and He ii reionization epochs. Current constraints on the IGM temperature from the Lyα forest are restricted to the redshift range 2 ≤ z ≤ 4.5, limiting the ability to probe the thermal memory of H i reionization towards higher redshift. In this work, we present the first direct measurement of the IGM temperature around a z = 6 quasar by analyzing the Doppler widths of Lyα absorption lines in the proximity zone of SDSS J0818+1722. We use a high-resolution (R = 40 000) Keck/HIRES spectrum in combination with detailed numerical modelling to obtain the temperature at mean density, T₀ = 23 600 ± ⁵⁰⁰⁰₆₉₀₀ K (±⁹²⁰⁰₉₃₀₀ K) at 68 (95) per cent confidence assuming a prior probability following H i and He ii reionization. This enables us to place an upper limit on the redshift of H i reionization, z_H, within 33 comoving Mpc of SDSS J0818+1722. If the quasar reionizes the He ii in its vicinity, then in the limit of instantaneous reionization we infer z_H < 9.0(11.0) at 68 (95) per cent confidence assuming photoheating is the dominant heat source and that H i reionization is driven by ionizing sources with soft spectra, typical of Population II stars. If the H i and He ii in the IGM around SDSS J0818+1722 are instead reionized simultaneously by a population of massive metal-free stars, characterized by very hard ionizing spectra, we obtain a tighter upper limit of z_H < 8.4(9.4). Initiating reionization at higher redshifts produces temperatures which are too low with respect to our constraint unless the H i ionizing sources or the quasar itself has spectra significantly harder than typically assumed.