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

TL;DR

This paper provides a comprehensive dataset of optical and near-infrared spectra for 45 quasars at redshifts above 6.5, including a composite spectrum, enabling detailed studies of early universe quasar properties.

Contribution

It introduces the largest uniformly processed medium/moderate-resolution quasar sample at z>6.5, with a publicly available dataset and a composite spectrum for comparative analysis.

Findings

High-quality spectra with median SNR around 10-12 across bands.

The composite spectrum shows consistent continuum slopes across redshifts.

Differences in emission line strengths suggest luminosity effects.

Abstract

We present optical and near-infrared (NIR) spectroscopic observations for a sample of $45$ quasars at $6.50 < z \leq 7.64$ with absolute magnitudes at $1450$ \r{A} in the range $-28.82 \leq M_{1450} \leq -24.13$ and their composite spectrum. The median redshift and $M_{1450}$ of the quasars in the sample are $z_{\rm{median}}=6.71$ and $M_{1450,\rm{median}} \simeq -26.1$, 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 of the mean signal-to-noise ratios per $110$ km s$^{-1}$ pixel in the J, H, and K band [median $\langle \rm{SNR}_{\lambda} \rangle$] is: median $\langle \rm{SNR}_{J} \rangle=9.7$, median $\langle \rm{SNR}_{H} \rangle=10.3$, and median $\langle \rm{SNR}_{K} \rangle=11.7$; demonstrating the good data quality. This work presents the largest medium/moderate-resolution sample of quasars at $z>6.5$ from ground-based instruments. Despite the diversity in instrumental setups and spectral quality, the data set is uniformly processed and well-characterized, making it ideally suited for several scientific goals, including the study of the quasar proximity zones and damping wings, the Ly$\alpha$ 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.