The spectral energy distribution of the redshift 7.1 quasar ULAS J1120+0641 ^⋆

¹ Astrophysics Group, Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2AZ, UK
e-mail: rhys.barnett09@imperial.ac.uk
² Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
³ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁴ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁵ Department of Mathematics, Imperial College London, London SW7 2AZ, UK
⁶ Astrophysics Research Institute, Liverpool John Moores University, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁷ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁸ Cavendish Laboratory, University of Cambridge, 19 J.J. Thomson Avenue, Cambridge CB3 0HE, UK
⁹ Institute for Cosmic Ray Research, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, 277-8582 Chiba, Japan

Received: 13 October 2014
Accepted: 20 November 2014

Abstract

We present new observations of the highest-redshift quasar known, ULAS J1120+0641, redshift z = 7.084, obtained in the optical, at near-, mid-, and far-infrared wavelengths, and in the sub-mm. We combine these results with published X-ray and radio observations to create the multiwavelength spectral energy distribution (SED), with the goals of measuring the bolometric luminosity L_bol, and quantifying the respective contributions from the AGN and star formation. We find three components are needed to fit the data over the wavelength range 0.12−1000 μm: the unobscured quasar accretion disk and broad-line region, a dusty clumpy AGN torus, and a cool 47K modified black body to characterise star formation. Despite the low signal-to-noise ratio of the new long-wavelength data, the normalisation of any dusty torus model is constrained within ±40%. We measure a bolometric luminosity L_bol = 2.6 ± 0.6 × 10⁴⁷ erg s^-1 = 6.7 ± 1.6 × 10¹³ L_⊙, to which the three components contribute 31%,32%,3%, respectively, with the remainder provided by the extreme UV < 0.12 μm. We tabulate the best-fit model SED. We use local scaling relations to estimate a star formation rate (SFR) in the range 60−270 M_⊙/yr from the [C ii] line luminosity and the 158 μm continuum luminosity. An analysis of the equivalent widths of the [C ii] line in a sample of z> 5.7 quasars suggests that these indicators are promising tools for estimating the SFR in high-redshift quasars in general. At the time observed the black hole was growing in mass more than 100 times faster than the stellar bulge, relative to the mass ratio measured in the local universe, i.e. compared to M_BH/M_bulge ≃ 1.4 × 10^-3, for ULAS J1120+0641 we measure Ṁ_BH/Ṁ_bulge ≃ 0.2.