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 (M₁₄₅₀ = −29.0 to −24.4) quasars at 5.78 < z < 7.54, which have complementary [C ii]_{158 µm} 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 Fe ii/Mg ii flux ratio suggests that the broad line regions of all quasars in the sample are already enriched in iron. We also find the Mg ii line to be on average blueshifted with respect to the [C ii] redshift with a median of −391 km s⁻¹. A significant correlation between the Mg ii-[C ii]_{158 µm} and C iv-[C ii]_{158 µm} velocity shifts indicates a common physical origin. Furthermore, we frequently detect large C iv-Mg ii emission line velocity blueshifts in our sample with a median value of −1848 km s⁻¹. While we find all other broad emission line properties not to be evolving with redshift, the median C iv-Mg ii blueshift is much larger than found in low-redshift, luminosity-matched quasars (−800 km s⁻¹). Dividing our sample into two redshift bins, we confirm an increase of the average C iv-Mg ii 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.