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Astron. Astrophys. 334, 799-804 (1998)

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1. Introduction

Thermal emission from cool circumnuclear dust is widely accepted as an important mechanism for producing far-infrared radiation in radio-quiet and radio-loud AGN (Sanders et al. 1986, Chini et al. 1989, Antonucci et al. 1990). This emission - provided isotropic - gives the opportunity to test the orientation unification model of radio-loud quasars (QSRs) and powerful radio galaxies (RGs) (Barthel 1989, Urry & Padovani 1995), since matched samples should yield similar far-infrared output for both classes. However, from IRAS observations it appears that QSRs are stronger 60µm emitters than Fanaroff & Riley class II RGs (Heckman et al. 1992, 1994, Hes, Barthel & Hoekstra 1995). This would imply that the simple unification model does not hold or that the assumption of isotropic 60µm radiation is incorrect. In support of the latter, Pier & Krolik (1992) drew attention to moderate anisotropy effects in the thermal far-infrared radiation from optically thick tori surrounding AGN. Furthermore, Hoekstra, Barthel & Hes (1997) demonstrated that beamed nonthermal far-infrared radiation is not insignificant in radio sources having prominent nuclei, and that the 60µm differences might be attributed to a stronger, beamed, nonthermal component in the QSR class. Within the unified model framework, QSRs would - at decreasing angle to the line of sight - naturally lead into the radio-loud blazar class, objects in which the dominance of the beamed component is without doubt. Indeed, both blazars and core-dominated QSRs display a smooth, single component spectral energy distribution, extending from radio to X-rays, indicating that all continuum radiation is of synchrotron origin (Landau et al. 1986). This causes blazars and core-dominated QSRs to be the most luminous IRAS AGN (Impey & Neugebauer 1988).

IRAS detected three [FORMULA], lobe-dominated 3CR QSRs at 60µm, which corresponds to [FORMULA] m emitted wavelength. Comparable radio galaxies were not detected - contrary to the expectation within the simple unification model. The QSRs, 3C 47, 3C 207, and 3C 334, have relatively bright radio cores and large double-lobed structures. Two of these, 3C 47 and 3C 334, had been observed to display superluminal motion - a clear sign of beamed emission (e.g., Zensus & Pearson 1987) - thus raising the suspicion that their far-infrared brightness could be (partly) due to a beamed non-thermal component.

The goal of the present research is to investigate to what extent (beamed) nonthermal radiation can account for the infrared emission in lobe-dominated QSRs. We have determined the cm-mm core spectra of 3C 47, 3C 207, and 3C 334 in order to assess their nonthermal 60µm radiation by means of extrapolation. Given the possibility of flaring submm components (observed in blazars - e.g., Brown et al. 1989) this research is a first attempt to isolate nonthermal and thermal FIR radiation. Observations over a wider frequency range are forthcoming.

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© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998