The WISSH quasars project

XI. The mean spectral energy distribution and bolometric corrections of the most luminous quasars^⋆

¹ Dipartimento di Matematica e Fisica, Universitá Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
e-mail: ivano.saccheo@uniroma3.it
² INAF, Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Italy
³ INAF, Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy
⁴ Dipartimento di Fisica, Sezione di Astronomia, Universitá di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
⁵ INAF – Istituto di Astrofisica Spaziale e Fisica cosmica Milano, Via Alfonso Corti 12, 20133 Milano, Italy
⁶ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Rome, Italy
⁷ INAF – Osservatorio Astrofisco di Arcetri, Largo E. Fermi 5, 50127 Firenze, Italy
⁸ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁹ Dipartimento di Fisica e Astronomia, Universitá di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino (Firenze), Italy
¹⁰ ESO, Karl-Schwarschild-Strasse 2, 85748 Garching bei München, Germany
¹¹ Kavli Institute for Cosmology, University of Cambridge, Cambridge CB3 0HE, UK
¹² Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, UK
¹³ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹⁴ Department of Physics, University of Rome ‘Tor Vergata’, Via della Ricerca Scientifica 1, 00133 Rome, Italy
¹⁵ Dipartimento di Fisica ‘G. Occhialini’, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
¹⁶ Dipartimento di Fisica e Astronomia “Augusto Righi”, Università degli Studi di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
¹⁷ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via P. Gobetti 93/3, 40129 Bologna, Italy

Received: 17 June 2022
Accepted: 10 November 2022

Abstract

Context. Hyperluminous quasi-stellar objects (QSOs) are ideal laboratories to investigate active galactic nucleus (AGN) feedback mechanisms. Their formidable energy release causes powerful winds at all scales, and thus the maximum feedback is expected.

Aims. Our aim is to derive the mean spectral energy distribution (SED) of a sample of 85 WISE-SDSS selected hyperluminous (WISSH) quasars. Since the SED provides a direct way to investigate the AGN structure, our goal is to understand if quasars at the bright end of the luminosity function have peculiar properties compared to the bulk of the QSO population.

Methods. We collected all the available photometry, from X-rays to the far-infrared (FIR); each WISSH quasar is observed in at least 12 different bands. We then built a mean intrinsic SED after correcting for the dust extinction, absorption and emission lines, and intergalactic medium absorption. We also derived bolometric, IR band, and monochromatic luminosities together with bolometric corrections at λ = 5100 Å and 3 μm. We define a new relation for the 3 μm bolometric correction.

Results. We find that the mean SED of hyperluminous WISSH QSOs shows some differences compared to that of less luminous sources (i.e., a lower X-ray emission and a near- and mid-IR excess which can be explained assuming a larger dust contribution. WISSH QSOs have stronger emission from both warm (T ∼ 500 − 600 K) and very hot (T ≥ 1000 K) dust, the latter being responsible for shifting the typical dip of the AGN SED from 1.3 μm to 1.1 μm. We also derived the mean SEDs of two subsamples created based on their spectral features (presence of broad absorption lines and equivalent width of CIV line). We confirm that broad absorption lines (BALs) are X-ray weak and that they have a reddened UV-optical continuum. We also find that BALs tend to have stronger emission from the hot dust component. For sources with a weaker CIV line, our main result is the confirmation of their lower X-ray emission. By populating the LIR vs. z diagram proposed by Symeonidis & Page (MNRAS, 503, 3992), we found that ∼90% of WISSH QSOs with z ≥ 3.5 have their FIR emission dominated by star-forming activity.

Conclusions. This analysis suggests that hyperluminous QSOs have a peculiar SED compared to less luminous objects. It is therefore critical to use SED templates constructed exclusively from very bright quasar samples (such as this one) when dealing with particularly luminous sources, such as high-redshift QSOs.