Near- and mid-infrared photometry of high-redshift 3CR sources

TL;DR

This study uses Spitzer Space Telescope data to compare mid-infrared properties of high-redshift radio galaxies and quasars, supporting orientation-based unified models of AGN with detailed spectral energy distribution analysis.

Contribution

It provides the first unbiased mid-infrared comparison of high-redshift radio-loud AGN, revealing orientation-dependent differences in their spectral energy distributions.

Findings

Quasars have similar, flat SEDs in the mid-infrared.

Radio galaxies show declining then rising SEDs, less luminous than quasars.

Results support the orientation-dependent unified scheme for AGN.

Abstract

Using the Spitzer Space Telescope, we have obtained 3.6--24 micron photometry of 38 radio galaxies and 24 quasars from the 3CR catalog at redshift 1<z<2.5. This 178 MHz-selected sample is unbiased with respect to orientation and therefore suited to study orientation-dependent effects in the most powerful active galactic nuclei (AGN). Quasar and radio galaxy subsamples matched in isotropic radio luminosity are compared. The quasars all have similar spectral energy distributions (SEDs), nearly constant in nu F_nu through the rest 1.6-10 micron range, consistent with a centrally heated dust distribution which outshines the host galaxy contribution. The radio galaxy SEDs show larger dispersion, but the mean radio galaxy SED declines from rest 1.6 to 3 micron and then rises from 3 to 8 micron. The radio galaxies are on average a factor 3-10 less luminous in this spectral range than the quasars. These characteristics are consistent with composite emission from a heavily reddened AGN plus starlight from the host galaxy. The mid-infrared colors and radio to mid-infrared spectral slopes of individual galaxies are also consistent with this picture. Individual galaxies show different amounts of extinction and host galaxy starlight, consistent with the orientation-dependent unified scheme.