2023/05/12 by Bartolomeo Trefoloni, Elisabeta Lusso, Trefoloni, Bartolomeo +29 · 1 citation
Mathematics · Physics and Astronomy · #Astrophysical Phenomena and Observations #Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #Galaxies: Formation, Evolution, Phenomena #Statistics Education and Methodologies
paper · pdf · doi:10.48550/arxiv.2305.07699
openalex publication_date 2023/05/12 · openalex created_date 2023/05/17 · openalex updated_date 2026/07/28
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 AGN as cosmological probes. In our previous works, 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 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 ≃2300-3100 \rm \mathringA) and the KS (≃4750-5350 \rm \mathringA) bands. We estimated black hole masses (M\rm BH) and Eddington ratios (λ\rm Edd) from the from the Hβ and MgII emission lines, finding that our z∼3 quasars are on average highly accreting (⟨ λ\rm Edd ⟩≃ 1.2 and ⟨ M\rm BH ⟩≃ 109.7M_\odot), with no difference in λ\rm Edd or M\rm 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≃6.5, corroborating the idea of already chemically mature BLRs at early cosmic time. From the KS spectra, we find that all the X-ray weak quasars present generally weaker [OIII] emission (EW<10 \rm \mathringA) 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.