The X-SHOOTER/ALMA sample of Quasars in the Epoch of Reionization. I.
  NIR spectral modeling, iron enrichment and broad emission line properties
  We present X-SHOOTER near-infrared spectroscopy of a large sample of 38
luminous ($M_{1450}=-29.0$ to $-24.4$) quasars at $5.78<z<7.54$, which have
complementary CII observations from ALMA. This X-SHOOTER/ALMA sample provides
us with the most comprehensive view of reionization-era quasars to date,
allowing us to connect the quasar properties with those of its host galaxy. In
this work we introduce the sample, discuss data reduction and spectral fitting,
and present an analysis of the broad emission line properties. The measured
FeII/MgII flux ratio suggests that the broad line regions of all quasars in the
sample are already enriched in iron. We also find the MgII line to be on
average blueshifted with respect to the CII redshift with a median of
$-391\,\rm{km}\,\rm{s}^{-1}$. A significant correlation between the MgII-CII
and CIV-CII velocity shifts indicates a common physical origin. Furthermore, we
frequently detect large CIV-MgII emission line velocity blueshifts in our
sample with a median value of $-1848\,\rm{km}\,\rm{s}^{-1}$. While we find all
other broad emission line properties not to be evolving with redshift, the
median CIV-MgII blueshift is much larger than found in low-redshift,
luminosity-matched quasars ($-800\,\rm{km}\,\rm{s}^{-1}$). Dividing our sample
into two redshift bins, we confirm an increase of the average CIV-MgII
blueshift with increasing redshift. Future observations of the rest-frame
optical spectrum with the James Webb Space Telescope will be instrumental in
further constraining the possible evolution of quasar properties in the epoch
of reionization.
