Optical and near-infrared spectroscopy of quasars at $z>6.5$: public data release and composite spectrum

We present optical and near-infrared (NIR) spectroscopic observations for a sample of $45$ quasars at 6.50 < z ≤ 7.64 with absolute magnitudes at $1450$ {} in the range -28.82 ≤ M₁₄₅₀ ≤ -24.13 and their composite spectrum. The median redshift and M₁₄₅₀ of the quasars in the sample are zmedian=6.71 and M1450,median -26.1, respectively. The NIR spectra are taken with echelle spectrographs, complemented with additional data from optical long slit instruments, and then reduced consistently using the open-source Python-based spectroscopic data reduction pipeline PypeIt. The median value of the mean signal-to-noise ratios of the spectra in J, H, and K band (median SNRλ) is: median SNRJ =9.7, median SNRH =10.3, and median SNRK =11.7; demonstrating the good data quality. This work presents the largest medium/moderate-resolution sample of quasars at $z>6.5$ from ground-based instruments. Its homogeneity and reproducibility make it ideally suited for several scientific goals, i.e., the study of the quasar proximity zones and damping wings, the Lyα forest, the intergalactic medium's metal content, as well as other properties such as the distribution of SMBH masses and Eddington ratios. Our composite spectrum is compared to others at both high and low-z from the literature, showing differences in the strengths of many emission lines, probably due to differences in luminosity among the samples, but a consistent continuum slope, which proves that the same spectral features are preserved in quasars at different redshift ranges.