Giant radio quasars: composite optical spectra

TL;DR

This study constructs and analyzes the composite optical spectrum of giant radio quasars, revealing their continuum properties, dependencies on luminosity and size, and consistency with AGN models through spectral energy distribution fitting.

Contribution

It provides the first composite optical spectrum for the largest sample of giant radio quasars and explores their spectral and physical properties in detail.

Findings

GRQ's continuum is flatter than smaller radio quasars.

Continuum slope depends on radio luminosity and size.

Spectral energy distribution fitting confirms larger dust luminosity for redder spectra.

Abstract

We present the composite optical spectrum for the largest sample of giant radio quasars (GRQs). They represent a rare subclass of radio quasars due to their large projected linear sizes of radio structures, which exceed 0.7 Mpc. To construct the composite spectrum, we combined 216 GRQ's optical spectra from Sloan Digital Sky Survey (SDSS). As a result, we obtained the composite spectrum covering the wavelength range from 1400 {\AA} to 7000 {\AA}. We calculated the power-law spectral slope for GRQ's composite, obtaining $\alpha_{\lambda}=-1.25$ and compared it with that of the smaller-sized radio quasars, as well as with the quasar composite spectrum obtained for large sample of SDSS quasars. We obtained that the GRQ's continuum is flatter (redder) than the continuum of comparison quasar samples. We also show that the continuum slope depends on core and total radio luminosity at 1.4 GHz, being steeper for higher radio luminosity bin. Moreover, we found the flattening of the continuum with an increase of the projected linear size of radio quasar. We show that $\alpha_{\lambda}$ is orientation-dependent, being steeper for a higher radio core-to-lobe flux density ratio which is consistent with AGN unified model predictions. For two GRQs, we fit the spectral energy distribution using X-CIGALE code to compare the consistency of results obtained in the optical part of the electromagnetic spectrum with broad-band emission. The parameters obtained from the SED fitting confirmed the larger dust luminosity for the redder optical continuum.