Composite spectra of quasars with different UV spectral index

Abstract

TThe composite spectra of quasars are widely used as templates for redshift determination, as well as for measurements of the mean transmission (the mean ratio of observed and emitted flux within the Lya forest part of quasar spectra) in Lyα-forest studies, and for investigation of general spectral properties of quasars. Possibility of composite spectra utilization in these fields is related to remarkable similarity of quasar spectra in UV-optical range. But despite of general similarity in spectral shapes, they differ in several parameters, one of which is the spectral index. In this paper, we study the possible effects related to neglect of this difference. We compiled 16 composite spectra from subsamples of individual Sloan Digital Sky Survey (SDSS) DR7 quasar spectra with different spectral indices αλ within the wavelength range 1270-1480 Å. Using them we show that (i) the uncertainties of redshifts measured for a test sample of high signal-to-noise ratio quasar spectra using these composites as templates appear to be 1.5 times smaller than of those calculated with a traditional template, compiled from spectra with different a?; (ii) the difference in αλ in individual spectra used for compilation of composites can yield the mean transmission uncertainty up to 20 per cent; (iii) a number of emission lines indistinguishable in ordinary composites, but seen in individual high-resolution spectra, can be detected in such composites. It is also shown that there is no dependence of a? on quasar luminosity in SDSS u, g, r and i bands, and monochromatic luminosity at 1450 Å. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.