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-VIKING surveys .
Our data-set consists of new VLT/X-Shooter and Magellan/FIRE observations .
These are the best optical/NIR spectroscopic data that are likely to be obtained for the z > 6.5 sample using current 6 - 10 m facilities .
We estimate the black hole mass , the Eddington ratio , and the Si iv /C iv , C iii ] /C iv , and Fe ii /Mg ii emission-line flux ratios .
We perform spectral modeling using a procedure that allows us to derive a probability distribution for the continuum components and to obtain the quasar properties weighted upon the underlying distribution of continuum models .
The z > 6.5 quasars show the same emission properties as their counterparts at lower redshifts .
The z > 6.5 quasars host black holes with masses of \sim 10 ^ { 9 } M _ { \odot } that are accreting close to the Eddington luminosity ( \langle { log } ( L _ { Bol } / L _ { Edd } ) \rangle = -0.4 \pm 0.2 ) , in agreement with what has been observed for a sample of 4.0 < z < 6.5 quasars .
By comparing the Si iv /C iv and C iii ] /C iv flux ratios with the results obtained from luminosity-matched samples at z \sim 6 and 2 \leq z \leq 4.5 , we find no evidence of evolution of the line ratios with cosmic time .
We compare the measured Fe ii /Mg ii flux ratios with those obtained for a sample of 4.0 < z < 6.4 sources .
The two samples are analyzed using a consistent procedure .
There is no evidence that the Fe ii /Mg ii flux ratio evolves between z = 7 and z = 4 .
Under the assumption that the Fe ii /Mg ii traces the Fe/Mg abundance ratio , this implies the presence of major episodes of chemical enrichment in the quasar hosts in the first \sim 0.8 Gyr after the Big Bang .