The WISSH quasars project II. Giant star nurseries in hyper-luminous quasars

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 a ff ect 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 di ff erent 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 similar to 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 > 10(47) erg/s) and star formation rate (up to similar to 2000 M-circle dot = 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 proportional to L-QSO(0.73)) by combining several Herschel-detected quasar samples from z similar to 0 to similar to 4. WISSH quasars have masses (similar to 10(8) M-circle dot) and temperatures (similar to 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 e ff ect 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 e ff ect between quasar-driven outflows and on-going star formation, and reveal merger signatures in their host galaxies.