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

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 nearinfrared (NIR) light, which ensures that the composite spectrum covers the full rest-frame range from Lyβ to 11 350 Å without any significant host contamination. Assuming a power-law continuum for the composite we find a spectral slope of α λ = 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.

Discovery of a Perseus-like cloud in the early Universe

“…This, together with the detection of neutral and molecular species strongly suggests that the dust reddening is caused by the absorber. In the following analysis, we used the quasar template of Selsing et al (2016) to infer the amount of reddening.…”

“…Extinction curve as function of inverse wavelength at the restframe of the DLA normalised to the V-band extinction A V . The grey line shows the full X-shooter spectrum divided by the template of Selsing et al (2016). The black regions indicate the wavelength regions used in the fit, and the two black squares indicated the photometry in the K and W1 bands.…”

Determining the fraction of reddened quasars in COSMOS with multiple selection techniques from X-ray to radio wavelengths

“…We show the K s imaging from KPNO since this (and not the UltraVISTA imaging) is available at the NASA/IPAC-IRSA web page. Overplotted is a composite quasar template by Selsing et al (2016) with and without reddening (solid red and dashed blue lines, respectively). The composite was constructed from luminous blue quasars at 1 < z < 2.1 selected from SDSS and has a slightly steeper spectral slope than other existing templates (α λ = 1.70 ± 0.01 assuming a power-law continuum).…”

“…The composite was constructed from luminous blue quasars at 1 < z < 2.1 selected from SDSS and has a slightly steeper spectral slope than other existing templates (α λ = 1.70 ± 0.01 assuming a power-law continuum). This is explained for instance by the broader spectral wavelength coverage because the slope can more easily be determined without strong contamination quasar emission lines or by intrinsic host galaxy emission (see Selsing et al 2016, for further details). The template was matched to Extracted 1D spectra and images in four wavelength bands of the four candidate quasars observed for the purpose of this study.…”

“…Bottom panels: The solid black lines show the observed spectra from the NOT. Overplotted are the photometric data points shown by the purple dots from the merged optical/nearinfrared photometric COSMOS catalog together with the composite quasar template spectrum from Selsing et al (2016) with (red solid line) and without (blue dashed line) reddening, respectively. The redshift and amount of reddening, A V , for each of the objects are shown in the upper left corner, while the name and selection method (from Col. 3 in Table 1) by which they were identified are shown in the upper right corner.…”