2024MNRAS.527.3912L

Bright quasar samples at high redshift are useful for investigating active galactic nuclei evolution. In this study, we describe XQz5, a sample of 83 ultraluminous quasars in the redshift range 4.5 < z < 5.3 with optical and near-infrared spectroscopic observations, with unprecedented completeness at the bright end of the quasar luminosity function. The sample is observed with the Southern Astrophysical Research Telescope, the Very Large Telescope, and the Australian National University 2.3 m Telescope, resulting in a high-quality, moderate-resolution spectral atlas of the brightest known quasars within the redshift range. We use established virial mass relations to derive the black hole masses by measuring the observed Mg II λ2799 Å emission line and we estimate the bolometric luminosity with bolometric corrections to the ultraviolet continuum. Comparisons to literature samples show that XQz5 bridges the redshift gap between other X-shooter quasar samples, XQ-100 and XQR-30, and is a brighter sample than both. Luminosity-matched lower redshift samples host more massive black holes, which indicate that quasars at high redshift are more active than their counterparts at lower redshift, in concordance with recent literature.