The WISSH quasars project

II. Giant star nurseries in hyper-luminous quasars

¹ Dipartimento di Matematica e Fisica, Università Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
e-mail: federica.duras@oa-roma.inaf.it
² INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
³ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy
⁴ Centro de Astronomia e Astrofísica da Universidade de Lisboa, Observatório Astronómico de Lisboa, 1349-017 Lisbona Tapada da Ajuda, Portugal
⁵ Instituto de Astrofísica e Ciencias do Espaço, Universidade de Lisboa, OAL, Tapada da Ajuda, 1349-018 Lisboa, Portugal
⁶ INAF–Osservatorio Astronomico di Trieste, via G. Tiepolo 11, 34124 Trieste, Italy
⁷ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁸ Dipartimento di Fisica Università di Roma La Sapienza, 00185 Roma, Italy
⁹ Dipartimento di Fisica e Astronomia, Università di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
¹⁰ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy

Received: 26 April 2017
Accepted: 13 June 2017

Abstract

Context. Studying the coupling between the energy output produced by the central quasar and the host galaxy is fundamental to fully understand galaxy evolution. Quasar feedback is indeed supposed to dramatically affect the galaxy properties by depositing large amounts of energy and momentum into the interstellar medium (ISM).

Aims. In order to gain further insights on this process, we study the spectral energy distributions (SEDs) of sources at the brightest end of the quasar luminosity function, for which the feedback mechanism is assumed to be at its maximum, given their high efficiency in driving powerful outflows.

Methods. We modelled the rest-frame UV-to-far-IR SEDs of 16 WISE-SDSS Selected Hyper-luminous (WISSH) quasars at 1.8 < z < 4.6 based on SDSS, 2MASS, WISE and Herschel/SPIRE data. Through an accurate SED-fitting procedure, we separate the different emission components by deriving physical parameters of both the nuclear component (i.e. bolometric and monochromatic luminosities) and the host galaxy (i.e. star formation rate, mass, and temperature of the cold dust). We also use a radiative transfer code to account for the contribution of the quasar-related emission to the far-IR fluxes.

Results. Most SEDs are well described by a standard combination of accretion disc plus torus and cold dust emission. However, about 30% of SEDs require an additional emission component in the near-IR, with temperatures peaking at ~750 K, which indicates that a hotter dust component is present in these powerful quasars. We measure extreme values of both AGN bolometric luminosity (L_BOL > 10⁴⁷ erg/s) and star formation rate (up to ~2000 M_⊙/yr) based on the quasar-corrected, IR luminosity of the host galaxy. A new relation between quasar and star formation luminosity is derived (L_{SF ∝} L^0.73_QSO) by combining several Herschel-detected quasar samples from z ~ 0 to ~4. WISSH quasars have masses (~10⁸M_⊙) and temperatures (~50 K) of cold dust in agreement with those found for other high-z IR luminous quasars.

Conclusions. Thanks to their extreme nuclear and star formation luminosities, the WISSH quasars are ideal targets to shed light on the feedback mechanism and its effect on the evolution of their host galaxies, as well as on the merger-induced scenario that is commonly assumed to explain these exceptional luminosities. Future observations will be crucial to measure the molecular gas content in these systems, probe the effect between quasar-driven outflows and on-going star formation, and reveal merger signatures in their host galaxies.