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We present the analysis of optical and near-infrared spectra of the only four$z>6.5$ quasars known to date, discovered in the UKIDSS-LAS and VISTA-VIKINGsurveys. Our data-set consists of new VLT/X-Shooter and Magellan/FIREobservations. These are the best optical/NIR spectroscopic data that are likelyto be obtained for the $z>6.5$ sample using current $6$ - $10$ m facilities. Weestimate the black hole mass, the Eddington ratio, and the SiIV/CIV, CIII]/CIV,and FeII/MgII emission-line flux ratios. We perform spectral modeling using aprocedure that allows us to derive a probability distribution for the continuumcomponents and to obtain the quasar properties weighted upon the underlyingdistribution of continuum models. The $z>6.5$ quasars show the same emissionproperties as their counterparts at lower redshifts. The $z>6.5$ quasars hostblack holes with masses of $\sim 10^9$ M$_{\odot}$ that are accreting close tothe Eddington luminosity ($\langle{\rm log} (L_{\rm Bol}/L_{\rm Edd})\rangle=-0.4\pm0.2$), in agreement with what has been observed for a sample of$4.0<z<6.5$ quasars. By comparing the SiIV/CIV and CIII]/CIV flux ratios withthe results obtained from luminosity-matched samples at $z\sim6$ and $2\leqz\leq4.5$, we find no evidence of evolution of the line ratios with cosmictime. We compare the measured FeII/MgII flux ratios with those obtained for asample of $4.0<z<6.4$ sources. The two samples are analyzed using a consistentprocedure. There is no evidence that the FeII/MgII flux ratio evolves between$z=7$ and $z=4$. Under the assumption that the FeII/MgII traces the Fe/Mgabundance ratio, this implies the presence of major episodes of chemicalenrichment in the quasar hosts in the first $\sim0.8$ Gyr after the Big Bang.

Black hole mass estimates and emission-line properties of a sample of redshift z>6.5 quasars