An X-shooter composite of bright 1 < z < 2 quasars from UV to infrared

TL;DR

This paper presents a new composite spectrum of luminous blue QSOs at 1 < z < 2.1, covering UV to NIR wavelengths with X-shooter, providing a more accurate spectral slope and a tool for assessing reddening in QSOs.

Contribution

The authors constructed a broad-wavelength composite spectrum from SDSS QSOs using X-shooter, reducing systematic effects and host contamination, and determined a steeper spectral slope than previous studies.

Findings

Composite spectrum covers Lyβ to 11350 Å without host contamination.

Spectral slope found to be α_λ = 1.70±0.01, steeper than previous results.

Demonstrated use of the spectrum for evaluating reddening in QSOs.

Abstract

Quasi-stellar object (QSO) spectral templates are important both to QSO physics and for investigations that use QSOs as probes of intervening gas and dust. However, combinations of various QSO samples obtained at different times and with different instruments so as to expand a composite and to cover a wider rest frame wavelength region may create systematic effects, and the contribution from QSO hosts may contaminate the composite. We have constructed a composite spectrum from luminous blue QSOs at 1 < z < 2.1 selected from the Sloan Digital Sky Survey (SDSS). The observations with X-shooter simultaneously cover ultraviolet (UV) to near- infrared (NIR) light, which ensures that the composite spectrum covers the full rest-frame range from Ly$\beta$ to 11350 $\AA$ without any significant host contamination. Assuming a power-law continuum for the composite we find a spectral slope of $\alpha_\lambda$ = 1.70+/-0.01, which is steeper than previously found in the literature. We attribute the differences to our broader spectral wavelength coverage, which allows us to effectively avoid fitting any regions that are affected either by strong QSO emissions lines (e.g., Balmer lines and complex [Fe II] blends) or by intrinsic host galaxy emission. Finally, we demonstrate the application of the QSO composite spectrum for evaluating the reddening in other QSOs.