Some spectral properties of the quasar ultraviolet bump

TL;DR

This study analyzes the ultraviolet bump of quasars using composite spectra from SDSS data, revealing weak emission lines and examining their relation to spectral indices, which is important for accurate redshift measurements.

Contribution

It introduces a method of dividing quasar spectra by spectral index to detect weak emission lines previously unseen in composite spectra.

Findings

Weak emission lines were detected in the UV bump.

The equivalent width of emission lines does not depend on spectral index.

Spectral index differences must be considered in composite spectrum generation.

Abstract

In the present work the part of the quasar UV-optical bump within the wavelength range 1210-1450\AA\ was studied with the help of composite spectra compiled from the samples of SDSS DR7 spectra with the similar spectral index \alpha_{\lambda} within 1270-1480 \AA. This division allowed to see weak emission lines, which were not detected in previous studies of the quasar composite spectra, but were known from individual optical or composite UV spectra from the Hubble Space Telescope. Although the physical explanation of the difference in spectral indices between quasars and their dependence on quasar parameters is still not clear, it is obvious that this difference has to be taken into account when generating composite spectra, e.g. for redshift measurements. It was also shown that the equivalent width of the emission lines does not depend on the spectral index.