The most luminous blue quasars at 3.0 < z < 3.3. III. LBT spectra and accretion parameters

We present the analysis of the rest frame ultraviolet and optical spectra of 30 bright blue quasars at z ∼ 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 (≈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 zJ (covering the rest frame ≃2300-3100 Å) and the K_S (≃4750-5350 Å) bands. We estimated black hole masses (M_BH) and Eddington ratios (λ_Edd) from the available rest frame optical and UV emission lines (Hβ, Mg II), finding that our z ∼ 3 quasars are on average highly accreting (⟨λ_Edd⟩≃1.2 and ⟨M_BH⟩≃10^9.7 M_⊙), with no difference in λ_Edd or M_BH 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 ≃ 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 Å) than the normal ones. The sample as a whole, however, abides by the known X-ray-[O III] luminosity correlation, hence the different [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.