The most luminous blue quasars at 3.0 < z < 3.3

We present the analysis of the rest frame ultraviolet and optical spectra of 30 bright blue quasars at z similar to 3, selected to examine the suitability of active galactic nuclei as cosmological probes. In our previous works, based on pointed XMM-Newton observations, we found an unexpectedly high fraction ( approximate to 25%) of X-ray weak quasars in the sample. The latter sources also display a flatter UV continuum and a broader and fainter C iv profile in the archival UV data with respect to their X-ray normal counterparts. Here we present new observations with the Large Binocular Telescope in both the (z)J (covering the rest frame similar or equal to 2300-3100 A) and the KS ( similar or equal to 4750-5350 angstrom) bands. We estimated black hole masses ( MBH) and Eddington ratios (lambda(Edd)) from the available rest frame optical and UV emission lines (H beta, Mg II), finding that our z similar to 3 quasars are on average highly accreting (< h lambda(Edd)> similar or equal to 1:2 and < M-BH > similar or equal to 10(9.7) M-circle dot), with no di fference in lambda(Edd) or MBH between X-ray weak and X-ray normal quasars. From the zJ spectra, we derived the properties (e.g. flux, equivalent width) of the main emission lines (Mg II, Fe II), finding that X-ray weak quasars display higher Fe ii/Mg ii ratios with respect to typical quasars. Fe II/Mg II ratios of X-ray normal quasars are instead consistent with other estimates up to z similar or equal to 6:5, corroborating the idea of already chemically mature broad line regions at early cosmic time. From the K-S spectra, we find that all the X-ray weak quasars present generally weaker [O III] emission (EW < 10 angstrom) than the normal ones. The sample as a whole, however, abides by the known X-ray-[O III] luminosity correlation, hence the di fferent [O III] properties are likely due to an intrinsically weaker [O iii] emission in X-ray weak objects, associated to the shape of the spectral energy distribution. We interpret these results in the framework of accretion-disc winds.