Letter to Nature

The intergalactic medium was not completely reionized until approximately a billion years after the Big Bang, as revealed by observations of quasars with redshifts of less than 6.5. It has been difficult to probe to higher redshifts, however, because quasars have historically been identified in optical surveys, which are insensitive to sources at redshifts exceeding 6.5. Here we report observations of a quasar (ULAS J112001.48+064124.3) at a redshift of 7.085, which is 0.77 billion years after the Big Bang. ULAS J1120+0641 had a luminosity of 6.3 × 10LSun and hosted a black hole with a mass of 2×10MSun (where LSun and MSun are the luminosity and mass of the Sun). The measured radius of the ionized near zone around ULAS J1120+0641 was 1.9 megaparsecs, a factor of three smaller than typical for quasars at redshifts between 6.0 and 6.4. The near zone transmission profile is consistent with a Lyα damping wing, suggesting that the neutral fraction of the intergalactic medium in front of ULAS J1120+0641 exceeded 0.1. ULAS J1120+0641 was first identified in the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Eighth Data Release, which took place on 3 September 2010. The photometry from UKIDSS, the Sloan Digital Sky Survey (SDSS) and follow-up observations on UKIRT and the Liverpool Telescope (listed in Fig. 1) was consistent with a quasar of redshift z ∼ 6.5. Hence, a spectrum was obtained using the Gemini MultiObject Spectrograph on the Gemini North Telescope on the night beginning 27 November 2010. The absence of significant emission blueward of a sharp break at λ = 0.98 μm confirmed ULAS J1120+0641 as a quasar with a preliminary redshift of z = 7.08. Assuming a fiducial flat cosmological model (that is, cosmological density parameters Ωm = 0.26, Ωb = 0.024, ΩΛ = 0.74 and current value of the Hubble parameter H0 = 72 km s −1 Mpc−1), ULAS J1120+0641 is seen as it was 12.9 billion years (Gyr) ago, when the Universe was 0.77 Gyr old. While three sources have been spectroscopically confirmed to have even higher redshifts, two are faint JAB ∼ 26 galaxies 10,11 and the other is a γ-ray burst which has since faded. Indeed, it has not been possible to obtain high signal-to-noise ratio spectroscopy of any sources beyond the most distant quasars previously known: CFHQS J0210−0456 (z = 6.44), SDSS 1148+5251 (z = 6.42) and CFHQS J2329+0301 (z = 6.42). Follow-up measurements of ULAS J1120+0641 will provide the first opportunity to explore the 0.1 Gyr between z = 7.08 and z = 6.44, a significant cosmological epoch about which little is currently known. Further spectroscopic observations of ULAS J1120+0641 were made using the FOcal Reducer/low dispersion Spectrograph 2 (FORS2) on the Very Large Telescope (VLT) Antu and the Gemini Near-Infrared Spectrograph (GNIRS) on the Gemini North Telecope and the results combined into the spectrum shown in Fig. 1. The spectrum of ULAS J1120+0641 is similar to those of lower redshift quasars of comparable luminosity, and comparison to a rest-frame template spectrum over the wavelength range including the strong Si iii]+C iii] and Mg ii emission features gives an accurate systemic redshift of z = 7.085± 0.003. The most unusual feature of the spectrum is the 2800±250 km s−1 blueshift of the C iv emission line, which is greater than that seen in 99.9 % of redshift z ∼ 2 quasars . There is associated absorption (visible through the N v doublet at λ = 0.999 μm and the C iv doublet at λ = 1.249 μm), indicating the presence of material in front of the quasar flowing out at 1100 ± 200 km s−1. There is also a narrow absorption line at the Lyα emission wavelength that is consistent with a cloud of H i close to the quasar. If ULAS J1120+0641 is not significantly magnified by gravitational lensing, the GNIRS spectrum gives an absolute magnitude (measured at 0.1450 μm in the rest-frame) of M1450,AB = −26.6 ± 0.1 and, applying a fiducial bolometric correction of 4.4, a total luminosity of L = (6.3 ± 0.6) × 10LSun. ULAS J1120+0641 has not been detected at radio wavelengths, with a measured flux