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

Abstract

We present the analysis of the rest frame ultraviolet and optical spectra of 30 bright blue quasars at $z\sim 3$, selected to examine the suitability of AGN as cosmological probes. In our previous works, we found an unexpectedly high fraction ($\approx 25\%$) of X-ray weak quasars in the sample. The latter sources also display a flatter UV continuum and a broader and fainter CIV profile in the archival UV data with respect to their X-ray normal counterparts. Here we present new observations with the LBT in both the $zJ$ (rest-frame $\simeq$2300-3100 $\mathring{\mathrm{A}}$) and the $K_S$ ($\simeq$4750-5350 $\mathring{\mathrm{A}}$) bands. We estimated black hole masses ($M_{\mathrm{BH}}$) and Eddington ratios ($\hat{I}{»}_{\mathrm{Edd}}$) from the from the H${\hat{I}}^2$ and MgII emission lines, finding that our $z\sim 3$ quasars are on average highly accreting ($⟨\hat{I}{»}_{\mathrm{Edd}}⟩\simeq 1.2$ and $⟨M_{\mathrm{BH}}⟩\simeq 10^{9.7}{M}_{\odot }$), with no difference in $\hat{I}{»}_{\mathrm{Edd}}$ or $M_{\mathrm{BH}}$ between X-ray weak and X-ray normal quasars. From the $zJ$ spectra, we derive flux and equivalent width of MgII and FeII, finding that X-ray weak quasars display higher FeII/MgII ratios with respect to typical quasars. FeII/MgII ratios of X-ray normal quasars are instead consistent with other estimates up to $z\simeq 6.5$, corroborating the idea of already chemically mature BLRs at early cosmic time. From the $K_S$ spectra, we find that all the X-ray weak quasars present generally weaker [OIII] emission (EW<10 $\mathring{\mathrm{A}}$) than the normal ones. The sample as a whole, however, abides by the known X-ray/[OIII] luminosity correlation, hence the different [OIII] properties are likely due to an intrinsically weaker [OIII] 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.

25 pages, 17 figures, Accepted for publication on Astronomy & Astrophysics