2010MNRAS.406..612B

The thermal state of the intergalactic medium (IGM) provides an indirect probe of both the HI and HeII 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 HI 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 = 40000) Keck/HIRES spectrum in combination with detailed numerical modelling to obtain the temperature at mean density, T₀= 23600±⁵⁰⁰⁰₆₉₀₀ K (±⁹²⁰⁰₉₃₀₀K) at 68 (95) per cent confidence assuming a prior probability following Hi and HeII reionization. This enables us to place an upper limit on the redshift of HI reionization, z_H, within 33 comoving Mpc of SDSS J0818+1722. If the quasar reionizes the HeII 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 HI reionization is driven by ionizing sources with soft spectra, typical of Population II stars. If the HI and HeII 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 HHI ionizing sources or the quasar itself has spectra significantly harder than typically assumed.