The most luminous blue quasars at 3.0 &lt; <i>z</i> &lt; 3.3

Abstract

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 (H β , 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 &lt; 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.

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