Abstract
The unification model for powerful radio galaxies and radio-loud quasars postulates that these objects are intrinsically the same but viewed along different angles. Herschel Space Observatory data permit the assessment of that model in the far-infrared spectral window. We analyze photometry from Spitzer and Herschel for the distant 3CR hosts, and find that radio galaxies and quasars have different mid-infrared, but indistinguishable far-infrared colors. Both these properties, the former being orientation dependent and the latter orientation invariant, are in line with expectations from the unification model. Adding powerful radio-quiet active galaxies and typical massive star-forming galaxies to the analysis, we demonstrate that infrared colors not only provide an orientation indicator, but can also distinguish active from star-forming galaxies.
Highlights
Active galactic nuclei (AGNs) show a large variety of observational properties despite being universally driven by accretion of matter onto supermassive black holes
Far- to mid-infrared colors represent a useful diagnostic of the orientation of powerful radio-loud AGNs, and support the unification model for these objects
At wavelengths longer than 20 μm, where obscuration is a small effect, these colors are indicators of the relative contributions of star formation and nuclear activity in galaxies. These diagnostics may prove useful in future studies of large samples of dusty SF galaxies/AGNs to investigate the duty cycle of nuclear activity, feedback, and AGN and galaxy growth
Summary
Active galactic nuclei (AGNs) show a large variety of observational properties despite being universally driven by accretion of matter onto supermassive black holes. Their ubiquitous nature and complex interplay with their host galaxies make AGNs essential elements of any model of galaxy evolution. Since their discovery in the 1950s, AGNs have been observationally classified into a number of different types depending on the particular wavelength range used for the observations. The validity of the unification model for powerful radio-loud AGNs has been examined using a variety of orientation-dependent properties (recently reviewed by Antonucci 2012), in the radio (Barthel 1989; Singal 1993), mid-infrared (MIR) (Ogle et al 2006; Cleary et al 2007; Haas et al 2008; Leipski et al 2010), and X-ray (Wilkes et al 2013) domains
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